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STRUCTURE OF PROTEINS
Proteins are considered to be biopolymers containing a large number of amino acids joined to each other by peptide linkages having three dimensional (3 D) structures. Protein structure and shape can be studied at four different levels, i.e., primary, secondary, tertiary and quarternary structures, each level being more complex than the previous one.
Primary structure of Proteins
Proteins may have one or more polypeptide chains. Each polypeptide in a protein has amino acids linked with each other in a specific sequence of amino acids that is said to have the primary structure of that protein. Any change in this primary structure i.e., the sequence of amino acids creates a different protein.
A protein containing a total of 100 amino acid residues is a very small , yet 20 different amino acids can be combined at one time in (20)100 different ways.
Secondary structure
The secondary structure of a protein refers to shape in which a long polypeptide chain can exist. There are two different conformations of the peptide linkage present in proteins viz., a-helix and b-conformation. The a-helix model postulated by Linus Pauling in 1951 purely on theoretical considerations was later verified experimentally. In order to understand this, let us look at the nature of the peptide bond, which shows resonance as shown below . Hydrogen bonding between –NH- and –C=O groups on different peptide linkages is shown in Fig .
Resonance structure of amide linkage
Due to partial double bond character of the C–N bond in peptide linkage, the amide part , i.e., –CO–NH–
Model of a tripeptide showing peptide bonds in boxes
and Ramachandran angles of rotation(F and y )
around a-carbon.
nearly planar amide groups to the a-carbons. The angles F and y in the figure are known as Ramachandran angles. The C=O and –NH groups of the peptide bond are trans to each other.
Hydrogen bonds between –N-H and –C=O groups of peptide bonds give stability to the structure. Thus, a structure having maximum hydrogen bonds shall be favoured. a-helix is one of the most common ways in which a polypeptide chain forms all possible hydrogen bonds by twisting into a right handed screw (helix) with the –NH group of each amino acid residue hydrogen bonded to the –C=O of an adjacent turn of helix as shown in Fig (b)
a-Helix structure of protein
The a-helix is also known as 3.6 13 helix, since each turn of the helix was approximately 3.6 amino acids and 13 member ring is formed by hydrogen bonding.
It may be noted that in proteins, the helix always has a right handed arrangement. If you hold your hand so that the thumb points in the direction of travel along the axis of the helix, the curl of your fingers describes the direction in which the hellix rotates, Fig (a) . All amino acids in a polypeptide chain have L-configuration and therefore, it can result in a stable helix if it is right handed. The ball and stick model of the a a-helix present is shown in Fig c.
b-Structures was also proposed by Linus Pauling and co-workers in 1951. In this conformation all peptide chains are stretched out nearly maximum extension and then laid side by side, held together by intermolecular hydrogen bonds. The structure resembles the pleated folds of drapery and therefore is known as b-pleated sheet. The polypeptide chains may run parallel, i.e., in the same direction or may be antiparallel i.e., run opposite directions ( Fig ) N-termini are aligned head to head i.e., on the same side in parallel b-conformation and are aligned head to tail i.e., N-terminus of one chain and C-terminus of another chain are on the same side in antiparallel conformation. The b-sheet is parallel in keratin , the protein present in hair and antiparallel in silk fibroin.
Tertiary Structure of Proteins
The tertiary structure of proteins represents overall folding of the polypeptide chains, i.e., further folding of the secondary structure. Two major molecular shapes are found viz., fibrous and globular. The fibrous proteins e.g. silk, collagen a-keratins have large helical content and have rod like rigid shape and are insoluble in water. The structure of the collagen triple helix is shown in Fig.
Collagen triple helix
In globular proteins e.g., hemoglobin the polypeptide chains consist partly of helical sections which are folded about the random cuts to give it a spherical shape.
DENATURATION OF PROTEINS
Proteins are very sensitive to the action of heat , mineral acids, alkalies etc. On heating or on treatment with mineral acids, soluble forms of proteins such as globular proteins often undergo coagulation or precipitation to give fibrous proteins which are insoluble in water. This coagulation also results in the loss of the biological activity of the protein. That is why the coagulated proteins so formed are called denatured proteins. Chemically, denaturation does not change the primary structure but brings about changes in secondary and tertiary structures (Fig).
The most common example of denaturation of proteins is coagulation of albumin present in the white of an egg. When the egg is boiled hard, the soluble globular protein present in it is denatured resulting in the formation of insoluble fibrous proteins.
Another example of denaturation is the coagulation that occurs when milk is heated with an acid (lemon juice) leading to the formation of cheese. During this denaturation, the globular milk protein , lactalbumin becomes fibrous.
Denaturation of globular proteins.
It was believed that denaturation was irreversible. However, it has been shown now that in some cases , the process is actually reversible. The reverse process is called renaturation. In such cases, when temperature and pH of denatured protein are brought back to conditions under which the native protein is stable, secondary and tertiary structures of the protein are restored. Consequently, renaturation is accompanied by recovery of the biological activity particularly in the case of enzymes.
ENZYMES
Enzymes are naturally occurring simple or conjugate proteins acting as specific catalysts in cell processes. Some enzymes can be non-proteins also. The enzyme facilitates a biochemical reaction by providing alternative lower activation energy pathways thereby increasing the rate of the reaction.
At present about 3000 enzymes have been recognized by International Union of Biochemistry. However, only about 300 are commercially available.
An enzyme molecule may contain a non-protein component known as prosthetic group. The prosthetic group which is covalently attached with the enzyme molecule is known as cofactor. The prosthetic groups which get attached to the enzyme at the time of reaction are known as coenzymes.
Specificity and mechanism of Enzyme Action
In case of enzymatic reaction the enzyme is so built that it binds to the substrate in a specific manner. The enzymatic reaction may proceed through the following four stages.
(i) The formation of complex between enzyme and substrate (ES).
(ii) The conversion of this complex to an enzyme – intermediate complex (EI).
(iii) Further conversion to complex between enzyme and product (EP) and
(iv) The dissociation of the enzyme-product complex, leaving the enzyme unchanged.
NUCLEIC ACIDS
Every living cell contains nucleoproteins, substances made up of proteins combined with biopolymers , the nucleic acids. These are mainly of two types, the deoxyribonucleic acids (DNA) and ribonucleic acids (RNA). Nucleic acids are long chain polymers (polynucleotides) of nucleotides. While proteins have a polyamide chain, nucleic acids contain a polyphosphate ester chain.
In higher cells, DNA is localized mainly in the nucleus, within the chromosome. A small amount of DNA is present in the cytoplasm also where it is contained in mitochondria and chloroplasts. RNA is also present in the nucleus as well as cytoplasm, which is copied into RNA molecules (transcription). The sequence of nucleotides contain the code for specific amino acid sequence. Proteins are also synthesized in a process involving translation of RNA.
Chemical Composition of Nucleic Acids
Complete hydrolysis of DNA or RNA yields a pentose sugar (ribose in RNA and deoxyribose in DNA), two types of heterocyclic bases viz., purines and pyrimidines along with phosphoric acid.
Deoxyribose differs from ribose in not having an –OH on C–2 (Fig)
b-D-ribose b-D-deoxyribose
Structure of b-D-ribose and b-D-deoxyribose
Purine and Pyrimidine Bases
As shown in the figure the pyrimidines have a single heterocyclic ring while purines have two fused rings. DNA contains the purine bases, adenine (A), and guanine (G) and pyrimidine bases, thymine (T) and cytosine (C) while RNA has uracil (U) in place of thymine(T). It can be noted that there are two main structural difference between DNA and RNA :
(i) DNA has deoxyribose while RNA has ribose sugar.
(ii) DNA contains thymine while RNA has uracil.
NUCLEOSIDES
The N-glycosides of purine or pyrimidine bases with pentose sugars are known as nucleosides :
Base + Sugar = Nucleoside ( Fig)
Base sugar linkage
Base | Abbreviation | Nucleoside |
Adenine | A | Adenosine |
Guanine | G | Guanosine |
Cytosine | C | Cytidine |
Thymine | T | Thymidine |
Uracil | U | uridine |
In nucleosides the sugar carbons are primed e.g. 1’, 2’ , 3’ etc in order to distinguish these from the bases. The purine or pyrimidine bases are attached to position 1 of pentoses through N-glycoside linkages.
NUCLEOTIDES
A nucleotide is a phosphate ester of nucleoside and consists of a purine or pyrimidine base, the 5-carbon sugar and one or more phosphate groups (Fig)
Base + Sugar + Phosphate = Nucleotide
The nucleotides are abbreviated by three capital letters , preceded by d- in case of deoxy series e.g
AMP = adenosine monophosphate
dAMP = deoxyadenosine monophosphate
ATP = adenosine triphosphate
UDP = uridine diphosphate etc.
Nucleotides are joined together by phosphatediester linkages between 5’ and 3’ carbon atoms of the pentose sugar. The formation of a typical dinucleotide is shown in Fig.
Formation of dinucleotide.
A nucleic acid chain is commonly abbreviated by a one letter code with the 5’ end of the chain written at the left e.g. a tetranucleotide having adenine , cytosine, guanine and thymine bases from 5’ end to 3’ end is represented as ACGT .
The backbone is composed of alternating sugar and phosphate bonds. It is extremely time consuming to write the complete structure of these oligonucleotides. In order to simplify , the bases are represented by their respective symbols, the phosphate bond is represented by symbol ‘P’ and sugar is drawn by simple Fischer projection. The the tetranucleotide ACGT can be drawn as follows :
DNA – A DOUBLE HELIX
E. Chargaff found that the base composition in DNA varied from one species to other species , but in all cases the amount of adenine was equal to that of thymine ( A = T) and amount of cytosine and guanine were found to be equal (G = C) . Therefore , the total amounts of purines was equal to the total amount of pyrimidines ( A + G = C + T) . However, the A T / C G ratio varies considerably between species e.g. this ratio is 1.52 in man while in E. coli it is 0.93.
In 1953 , based on the X-ray diffraction studies of DNA J.D Watson and F.H.C. Crick proposed a double helical structure for DNA which explained not only the base equivalence ( A = T ; G = C ) but also other properties of DNA, specially its duplication in a living cell (replication) . The double helical structure of DNA is shown in the Fig.
Double helical structure of DNA
The double helix is composed of two right handed helical polynucleotide chains coiled around the central axis. The two strands are antiparallel , i.e., their (5’ ® 3’) phosphodiester linkages run in opposite directions. The bases are stacked inside the helix in planes perpendicular to the helical axis. It is like a stack of flat plates held together by two ropes of sugar-phosphate polymeric backbone running along outside of stack.
The two strands are held together by hydrogen bonds, shown as black rods in the figure. Only two base pairs i.e., AT and CG fit into the structure. Two hydrogen bonds are formed between A and T ( A = T) and three hydrogen bonds are formed between C and G ( CºG) . Therefore, CG base pair has more stability as compared to A T base pair. The two complimentary base pairs of DNA , i.e., ( T – A) and (C–G ) with their hydrogen bonds are shown in Fig.
Hydrogen bonds are formed between complimentary base pairs.
In addition to hydrogen bonds, other forces e.g. hydrophobic interactions between stacked bases are responsible for stability and maintenance of double helix.
The diameter of double helix is 2 nm, as shown in Fig. , the double helical structure repeats at interval of 3.4 nm (one complete turn) , which corresponds to ten base pairs. Two kinds of grooves, one major and one minor are evident in the structure. DNA helices can be right handed as well as left handed. The b-conformation of DNA having the right-handed helices is most stable. On heating the two two strands of DNA separate from each other ( known as melting ) and on cooling these two hybridise (annealing). The temperature at which the two strands separate completely is known as melting temperature (Tm) which is specific for each specific sequence. In secondary structure of RNA, helices are present but only single stranded.
Hereditary – The Genetic Code
Nucleic acids control heredity on molecular level. The double helix of DNA is store house of hereditary information of the organism. This information is in the coded form as the sequence of bases along the polynucleotide chain. Since DNA has only four different bases, the genetic message can be compared to a language which has only four letters A , C, G and T.
DNA must preserve this information and use it in the following manner :
1. DNA molecules can duplicate themselves i.e., can synthesise other DNA molecules identical with the original ; this process is known as replication.
2. A single strand of DNA can act as a template on which a molecule of RNA is synthesized in a specific manner ; this known as transcription.
3. The RNA molecule in turn directs synthesis of specific proteins which are characteristic of each kind of organism, this is known as translation.
BIOLOGICAL FUNCTIONS OF NUCLEIC ACIDS
The important biological functions of nucleic acids are :
1. Replication
It is the property of a molecule to synthesis another molecule. DNA has a unique property to duplicate or replicate itself i.e., it can bring about the synthesis of another DNA molecule. Replication of DNA is an enzyme catalysed process. In this process, the two strands of DNA helix unwind and each strand serves as a template or pattern for synthesis of a new strand. Due to unique specificity of base pairing, the newly synthesised completementary strand in each case is an exact copy of the originally separated from it. As a result, the two double stranded DNA molecules are formed called two daughter DNA molecules. One of the strand comes from the parent DNA molecule and the other is newly synthesised. Each DNA is exact replication of the parent. In this way hereditary effects are transmitted from one cell to another. This is shown in Fig.
This replication can be easily be undersood . Suppose a segment along a double helix is :
When this double helix uncoils, then it forms two strands as :
Each strand can act as a template to build identical double helices. The complements to the two strand are :
These two double helices are identical to each other and to the first double helix. Thus, the original double helix is repeated itself.
2. Protein Synthesis
DNA molecules also perform an important function of synthesing proteins, which serves as a machinery of the living cell. In this process, the genetic information coded in DNA in the form of specific base sequences is translated and expressed in the form of sequence of amino acids wich result in the synthesis of specific proteins which perform various functions in the cell. This is brought about in two steps :
(i) transcription (ii) translation
Transcription : The transcription involves copying of DNA sequences into a complimentary RNA molecule called messenger RNA ( mRNA). A portion of DNA double helix strand is unwound and one of the two DNA strands acts as the template for synthesis of RNA molecule called mRNA. This process is similar to replcation process. However, it differs in the following respects.
(i) In mRNA synthesis, ribose nucleotide assemble along the uncoiled template instead of deoxyribose nucleotide which assemble in replication of DNA.
(ii) In this case , the base uracil (U) is substituted for the base thymine (T) .
For example, the sequence of bases in new mRNA from unwound region of DNA are shown below :
Therefore, the m-RNA will have sequence of bases which is complimentary to the sequence of bases in DNA strand. m-RNA is synthesised by cells whenever it is needed. This process is also catalysed by an enzyme called RNA polymerase which recognises certain base sequences as the starting point of transcription and binds to the DNA near the site where unwinding occurs. Thus DNA, transfers its genetic code to mRNA. After transcription mRNA detaches from the DNA molecule and moves from the nucleus of the cell to a ribosome in cytoplasm where it serves as template for protein synthesis. The DNA returns to its original double helix structure. DNA transfers its genetic code to mRNA by replication.
(ii) Translation
During translation , mRNA directs protein synthesis in the cytoplasm of cell with the involvement of another type of RNA molecule namely transfer RNA (i.e., tRNA) and the ribosomal particles (RNA-protein complex). This process is called translation.
The process occurs with the attachment of mRNA to the very small ribosome particles in cytoplasm. The mRNA then gives the message of the DNA and dictates the specific amino acid sequence for the synthesis of protein. The four bases in mRNA (A, C, G and U) acts as a code for particular amino acid. Each triplet of nucleotides is called a codon and it specifies one amino acid. It may be noted that there may be more than one triplet combination code for the same amino acid. For example, the amino acid methionine has the code AUG while glycine has for four codes CGU, GGC , GGA , GGG.
The code expressed in mRNA is tRNA and is translated into an amino acid sequence. Each triplet tRNA calls the desired amino acid and transfers it to the proper position on the ribosome. This process is repeated again and again and thus proteins are synthesised. When the synthesis of a specific protein is completed , it is released from ribosome.
The synthesis of protein may be represented as :
Thesemay be represented below:
Protein synthesis is afast process and about 20 amino acids are added in one second. For example, silk has the major component fibroin protein. A single fibroin gene makes 104 copies of its mRNA and each mRNA produces 105 molecules of fibroin protein amounting to a total of 109 molecules of protein per cell in a period of 4 days.
GENETIC CODE AND GENE
The sequence of bases in mRNA are read in a serial order in groups of three at a time. Each triplet of nucleotides is called a codon and it specifies one amino acid. For example, a segment of mRNA is shown below. In this, the bases are read three at a time. Each set of three bases represents a codon.
Each codon (set of three) designates one amino acid. For example, A U G is codon for methionine. CCU is codon for proline, etc.
The sequence of amino acids in proteins is determined by the sequence of nucleotide bases on mRNA , which in turn is determined by the sequence of bases in DNA molecule. The DNA sequence that codes for a specific protein or polypeptide is called gene. Therefore, every protein in the cell has corresponding gene. The relation between the nucleotide triplets and the amino acids is called genetic code.
MUTATION
It is a chemical change in a DNA molecule that could yield to synthesis of proteins with different amino acid sequence. The changes in DNA molecule can occur spontaneously or it may be caused by radiation, chemical agents or viruses. The damage caused by mutation is repaired by special enzymes in the cell. The altered proteins caused by mutation is repaired by special enzymes in the cell. The altered proteins caused by mutation may lose their biological activities and thus causing the death of the cell. The defective genes can also cause abnormalities or diseases.
Distinction between DNA and RNA
The important differences between DNA and RNA are :
DNA | RNA |
i) It occurs in the nucleus of the cell. ii) It has double stranded structure in which two strands are coiled spirally in opposite directions. iii) The sugar molecule is 2-deoxyribose. iv) Nitrogeneous base uracil is not present. v) It is responsible for transmission for heredity character. vi) Alkaline hydrolysis is quite slow. | i) It occurs in the cytoplasm of the cell ii) It has single stranded structure. iii) The sugar molecule is ribose iv) Nitrogeneous base thymine is not present. v) Helps in protein synthesis. vi) Alkaline hydrolysis can readily take place. |
Problems
01. The two samples of DNA , A and B have melting temperatures (Tm) 340 and 350 K respectively. Can you draw any conclusion from this data regarding their base content ?
02. In E.coli DNA the AT/GC is 0.93. If the number of moles of adenine in its DNA sample are 465,000, calculate the number of moles of guanine present.
03. If one strand of a DNA has the sequence –ATCGTCCA- what is the sequence of the complementary strand ?
04. What will be the sequence of bases on mRNA molecule that synthesised on the following strand of DNA ?
T A T G T A C C T G
05. What will be the sequence of bases on the strand of DNA that would be complementary to strand having the following sequence of bases :
A A T C G T A G G C
LIPIDS
Lipids are naturally occurring compounds related to fatty acids and include fats, oils, waxes and other related compounds. The lipids are important constituents of diet. In the body these fats serve as an efficient source of energy and are stored in the adipose tissues. These are hydrophobic in nature and dissolve in organic solvents. Phospholipids (lipids containing phosphorus) are important constituents of cell membranes.
Classification
Based on their composition, lipids are classified as follows :
(A) Simple lipids (homolipids) : Simple lipids are alcohol esters of fatty acids which include ,
(i) Neutral fats (glycerides) : Also known as triglycerides, these are triesters of fatty acids and glycerol.
(ii) Waxes : These are esters of fatty acids with long chain monohydroxy alcohols. These have higher melting points than neutral fats.
(B) Compound lipids (heterolipids) : Lipids with additional groups are called compound lipids. These include :
(i) Phospholipids : These contain additional groups e.g., a phosphoric acid, nitrogen containing bases and other substituents.
(ii) Glycolipids : These are esters of fatty acids with carbohydrates and may contain nitrogen but no phosphorus.
(C) Derived lipids : These are substances derived from simple and compound lipids by hydrolysis. These include fatty acids, fatty alcohols, mono and diglycerides, steroids, terpenes and carotenoids. These are sometimes present as waste products of metabolism. Glycerides and cholosterol esters are also called neutral lipids since these do not carry any charge.
SIMPLE LIPIDS
Oils and fats are simple lipids. These are most abundant lipids. These are esters of glycerol and three fatty acids. These fatty acids always have an even number of carbons.These are also called triglycerides. The three fatty acid may be identical or different. The naturally occurring fatty acids may be saturated or unsaturated.
For example,
Saturated fatty acids
Lauric acid : CH3(CH2)10COOH
Myristic acid : CH3(CH2)12COOH
Palmitic acid : CH3(CH2)14COOH
Stearic acid : CH3(CH2)16COOH
Unsaturated (contains one or more double bonds) fatty acids
Oleic acid : C17H31COOH
CH3(CH2)7CH = CH(CH2)7COOH
(one double bond)
Linoleic acid : C17H31COOH
CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
(two double bonds)
Linolenic acid : C17H29COOH
CH3 CH2CH=CH CH2CH=CH CH2CH=CH(CH2)7COOH
(Three double bonds)
Oils and fats being glyceryl esters of fatty acids are also called triglycerides. These fatty acids may be same or different. Fats are glycerides of saturated fatty acids e.g. tripalmitin and tristearin. Oils contain unsaturated fatty acids e.g. triolein. a-Oleo-b-palmito-a’-stearin is an example of mixed triglyceride.
The presence of double bonds with less stable ‘cis’ stereochemistry in unsaturated fatty acids e.g. at C-9 in oleic acid (C17H31COOH) is of vital biological significance. In solid state, the molecules of saturated fatty acids fit closely together in zig-zag tetrahedral structure. The cis-unsaturated acid chains have a bend at the double bond and do not fit closely resulting in lowering of the melting point of the fat.
Problems
06. An unsaturated fatty acid on ozonolysis yields one aldehyde CH3(CH2)7CHO and an aldehyde monocarboxylic acid OHC(CH2)7COOH . Write down the structure and name of the acid.
07. One mole of naturally occurring fat on hydrolysis with NaOH gave one mole of glycerol together with sodium palmitate and sodium stearate in 1 : 2 molar ratio. The molecule of the fat is symmetric. Write down the structure of the fat.
2. COMPOUND LIPIDS
The compound lipids on hydrolysis give other substances in addition to alcohols and fatty acids. Phospholipids are examples of compound lipids.
Phospholipids are mixed glycerides of higher fatty acids and phosphoric acid in which two OH groups of glycerol are esterified by fatty acids and third by some derivatives of phosphoric acid.
The common examples of phospholipids are lecithins and cephalins which are found principally in the brain, nerve cells and liver of animals. These are also found in egg yolks, yeast, soyabeans and other foods.
Lecithin contains a quarternary N whereas cephalin contains only primary N.
The phosphate group forms a polar hydrophilic head (a dipolar ion group) on the molecule and two fatty acid chains constitute the non-polar hydrophobic (water repelling) tail.
As a result, phospholipids are good neutral surfactants. They have excellent emulsifying and membrane forming properties. 3. DERIVED LIPIDS
Steroids like cholestrol, fat soluble vitamins like vitamin A, D, E and K are the examples of derived lipids.
Biological Functions and other Applications of Lipids
Oils and fats are one of the main sources of food for living organisms. The main functions are :
1. Simple lipids act as important sources of energy in our food supply. They are even richer sources of energy than carbohydrates. For example 1g of fat on oxidation gives 37 kJ of energy whereas 1 g of carbohydrates provides only 17 kJ of energy. Inside the systems, fats and oils are hydrolysed to glycerol and fatty acids which slowly gets oxidised to carbon dioxide and water releasing a large amount of energy. The excess of glycerol and fatty acids recombine to form body fat which is absorbed and stored in the tissues of human beings for use in emergency periods such as illness and during fast. However, it is to be noted that plants store their energy in the form of polysaccharides (particularly starch) . Thus, fats are richest source of energy to our body.
2. Phospholipids serve as structural materials of cells and tissues such as cell membrane.
3. Simple lipids can act as heat insulators and shock absorbers for living organisms.
4. Lipids are essential for the absorption of fat soluble vitamins such as A, D, E and K.
5. Some fats supply essential fatty acids.
WAXES
Waxes are esters and they are simple lipids. They are fatty acid esters of long chain monohydric alcohols and may be represented by the general formula RCOOR’ where R and R’ are long hydrocarbon chains. For example,
They occur as mixtures. These waxes are widely spread in nature and play an important role as a protective coatings on fruits, leaves and animals. They have the properties of water insolubility , flexibility and non-reactivity and therefore , they act as excellent coatings.
Uses
Waxes are used
(i) in cosmetics, ointments and polishes for floors, furniture etc.
(ii) as thin coatings on fruits, leaves , skin and protect the surface from the loss of water and attack of micro-organisms.
(iii) In making candles.
HORMONES
Hormones are the chemical substances which are produced in ductless glands and are carried to different parts of body by blood stream where they control the various body functions. Because of the action of hormones as communication among cells , they are called chemical messengers. The word hormone is derived from a Greek verb meaning ‘’ to stir up or excite’’. They are secreted only in trace amounts by one type of tissue and are carried by blood to a target tissue to stimulate a specific biochemical or physiological activity. These hormones not only control different aspects of metabolism but they also perform many other functions such as : cell and tissue growth, heart rate, blood pressure, kidney function, secretion of digestive enzymes, lactation , the reproductive system etc. The deficiency of hormones cause metabolic disturbances in the body. In mammals, the secretion of hormones is controlled by the anterior lobe of the pituitary gland present at the base of the brain. These hormones are transported to other glands such as adrenal cortex, thyroid and sex glands to stimulate the production of other hormones.
Classification and Functions of Hormones
Hormones may be classified on the basis of (i) their structure or (ii) their site of activity in the cell. Classification based on structure is given in Fig.
Steroid nucleus is found in some vitamins, drugs and bile acids also. The steroid nucleus and a few sex hormones are given in Fig.
Functions of steroid hormones
1. Sex hormones : Sex hormones are divided into three groups (i) the male sex hormones , or androgens (ii) the female sex hormones or estrogens and (iii) pregnancy hormones or progestines. Testosterone is the major sex hormone produced by testes. It is responsible for male characteristics (deep voice, facial hair, general physical constitution) during puberty. Synthetic testosterone analogs are used in medicine to produce muscle and tissue growth, for example, in patients with muscular atrophy. Estradiol is the main female sex hormone. It is responsible for development of secondary female characteristics and participates in control of menstural cycle. An example of progestin is progesterone, responsible for preparing the uterus for implantation of fertilized egg. Many steroid hormones, for example, progesterone itself, have played important role as birth control agents.
2. Corticosteroids (adrenal cortical hormones)
(a) Mineralo corticoids , made by different cells in the adrenal cortex are concerned with water-salt balance in the body. These regulate NaCl content of blood and cause excretion of potassium in urine.
(b) Glucocorticoids , made by adrenal cortex , modify certain metabolic reactions and have an anti-inflammatory effect.
Functions of non-steroid hormones
1. Peptide hormones : Insulin has profound influence on carbohydrate metabolism. It facilitates entry of glucose and other sugars into the cells, by increasing penetration of cell membranes and augmenting phosphorylation of glucose. This decreases glucose concentration in blood and therefore insulin is commonly known as hypoglycemic factor. It promotes anabolic processes and inhibits catabolic ones. Its deficiency in human beings causes diabetes mellitus. Insulin isolated from islets of Langerhans or islet tissue of pancreas was first hormone to be recognised as protein. Sanger got Nobel prize in 1985 for determining the structure of insulin. Bovine insulin hormone has two polypeptide chains with 21 and 30 aminoacids. They are joined by sulphur bridges connecting cysteine amino acid groups on two chains.
2. Amino acid derivatives : The thyroidal hormones e.g. thyroxin and triiodothyronine affect the general metabolism, regardless of the nature of their specific activity. It is for this reason why thyroid gland is known as pace setter of the endocrine system.
Based upon the site of activity in the cell, hormones may be divided into two categories. Hormones in the first category affect the properties of the plasma membranes. These include all peptide hormones, e.g insulin and hormones of pituitary gland. In the other category, hormones are taken into the cell and transported to the nucleus where they influence the nature and rate of gene expression.
VITAMINS
These are organic compounds which cannot be produced by the body and must be supplied in small amounts in diet for the normal health , growth and maintanance of body. These are essential to us for proper functioning of different organisms. They are chemically different from the main nutrients; fats, carbohydrates and proteins. The absence or deficiency of a vitamin can cause specific diseases.
The actual formula for vitamins are complicated. For the sake of simplicity, these are designated as A, B, C, D, E and K. These are classified as water soluble or fat soluble.
Water soluble vitamins : The vitamins which are soluble in water are called water soluble vitamins. For example, vitamin B and C.
Fat soluble vitamins : The vitamins which are soluble in fats are called fat soluble vitamins. For example, vitamins, A, D, E and K.
Sources of Vitamins
Most of the vitamins cannot be made by our body, Vitamin D can be formed in our body by the action of sunlight on fat under the skin. All the vitamins are prepared in plants. Almost all the food items contain more than one vitamin in varying amounts. These days vitamins are also synthesised in the laboratories.
Functions of Vitamins
The common vitamins, their sources and important functions are given below :
1. Vitamin A
The chemical name of vitamin A is retinol. It is a fat soluble vitamin.
Functions :
(i) It helps in proper growth and normal skeletal development of the body.
(ii) It plays an important role in maintainining proper vision.
(iii) It is also essential for healthy tooth structure.
(iv) It helps in the maintanance of healthy , glowing soft skin.
Sources : Butter, liver, carrots, spinach, ghee, kidney, yellow pumpkin, mustard leaves, whole milk, fish liver oil, tomatoes, coriander leaves, curd, mangoes, egg yolk, cheese , papaya.
Vitamine A is present in the form of carotenes in vegetables and fruits which are converted to vitamin A in the body. In general , the darker the colour of the green vegetables, greater the carotene content.
Deficiency of vitamin A causes night blidness and xerosis.
2. Vitamin B
Vitamin B is a water soluble vitamin. It consists of eleven substances. Out of these vitamins, B1, B2, B4 and B12 are important.
3. Vitamin B1 : The chemical name of vitamin B1 is thiamine. It is a water soluble vitamin.
Functions
(i) Vitamin B1 helps in carbohydrate metabolism.
(ii) It helps in functioning of heart , nerves and and muscles.
(iii) It sharpens our appetite and is sometimes referred to as an ‘appetite vitamin’
Sources : Milk, pulses, wheat bran, sea food, yest, whole grain cereals, green vegetables, soyabean, dairy products (except butter).
The deficiency of vitamin B1 causes beri-beri.
4. Vitamin B2
The chemical name of vitamin B2 is riboflavin. It is water soluble vitamin.
Functions
(i) It helps in oxidation and utilisation of oxygen.
(ii) It helps in carbohydrate and protein metabolism.
(iii) It is necessary to keep the skin healthy.
(iv) It helps in normal functioning of the eye.
Sources
meat, whole grains and pulses, milk, yeast , liver, peas , egg, green vegetables.
The richest source of riboflavin is dried yeast and liver.
The deficiency of vitamin B2 causes cracking of skin, lips , corners of mouth , photophobia (rough eye lids).
5. Vitamin B4
Vitamin B4 is also called niacin. It is a water soluble vitamin belonging to B-complex group. It is one the stable of vitamins.
Functions
(i) It is needed for the metabolism of carbohydrates, fats and proteins.
(ii) It keeps the skin healthy.
(iii) It gives sound mental health
(iv) It has most important positive contribution to good nutrition.
Sources : Milk, fish, legumes, potatoes, green leafy vegetables, meat, egg, whole grains.
The deficiency of vitamin B4 causes pellagra.
6. Vitamin B12
The chemical name of vitamin B12 is cyanocobalamine. It is also a water soluble vitamin.
Functions
(i) It is essential for the metabolism of nervous tissue.
(ii) It is necessary for the formation of healthy blood and proper growth of the body.
(iii) It is essential for preventing the disease called pernicious anaemia.
Sources : Liver, cheese, milk, eggs, kidney, fish, meat.
The deficiency of vitamin B12 causes pernicious anaemia, inflammation of tongue and mouth.
7. Vitamin C
The chemical name of vitamin C is ascorbic acid. Of all vitamins it is the most highly soluble in water.
Functions
(i) It is necessary for keeping teeth, gums and joints healthy.
(ii) It places an important role in normal metabolism of the amino acids.
(iii) It helps in healing of cuts and wounds.
(iv) It gives resistance to our body against diseases and infections.
Sources : Amla, tomatoes, mangoes, orange, peas, pineapple, cabbage, apples, lemon, lime, green chillies, guava.
Amla is a good source of vitamin C. The deficiency causes scurvy.
8. Vitamin D
It is also called calciferol. It is a fat soluble vitamin. It is formed in the skin under the action of sunshine.
Functions
(i) It keeps the bones and teeth healthy.
(ii) It helps in the utilisation of calcium and phosphorus.
Sources : Cod liver oil, butter, milk, egg yolk, fish, ghee, cheese.
Exposure to sunlight provides a cheap method of production of vitamin D in the body itself.
The deficiency of vitamin D causes rickets.
9. Vitamin E
The chemical name of vitamin E is tochoferol. It is a fat soluble vitamin.
Functions
(i) It place an important role in the protection of vitamin A, carotene and ascorbic acid.
(ii) It is necessary for the normal reproduction and protection of liver.
Sources : Vegetable oils, milk, tomatoes, dark green leafy vegetables, eggs, kidney, whole grain cereals, nuts liver.
The deficiency of vitamin E causes loss of sexual power of reproduction.
10. Vitamin K
It is also known as phylloquinone. It is a fat soluble vitamin. It is also known as coagulation vitamin.
Functions : It helps in clotting of blood and prevents haemorrhage.
Sources : Green leafy vegetables, soyabean, cabbage, vegetable oils , spinach, tomatoes.
The deficiency of vitamin K causes haemorrahage, lengthens the time of blood clotting.
Physiological Functions of Vitamins
Vitamins catalyze biological reactions in very low concentration, therefore the daily requirement of any vitamin for any individual is extremely small. However, the daily dossage of any vitamin for any individual is not a fixed quantity and varies according to size, age and rate of metabolism of individual. Youngsters need higher quantity of vitamins than elders and their requirement increase when a person perform exercise. The need of growing children and pregnant mothers for vitamins is more. The intestinal organisms may synthesize vitamins in significant amounts and play an important role in regulating the quantity of vitamins available to the organism. Most of the vitamins of B-complex group and vitamin K are some vitamins synthesized by intestinal organisms. These may be absorbed in variable amounts and utilized.
A lack of one or more vitamins leads to characteristic deficiency symptoms in man. Multiple deficiencies caused by lack of more than one vitamin are common in human beings. This condition for vitamin deficiency is known as avitaminoses.
Differences between Hormones and Vitamins
Hormones | Vitamins |
1. These are chemical substances produced in the ductless glands in the body. 2. These are not stored in the body but are continuosly produced. | 1. These are not produced in the body (except vitamin D) but have to be supplied in deit and are essential for proper functioning of different organisms. 2. These may be stored in the body to fight out diseases. |
QUESTIONS
1. What are the two stages of photosynthesis in gree plant ? Give the basic equation of photosynthesis.
2. What are reducing and non-reducing sugars? What is the structural feature characterizing reducing sugars ?
3. Draw open chain structure of aldopentose and aldohexose. How many asymmetric carbons are present in each ?
4. Draw the simple Fischer projections of D- and L-glucose. Are these enantiomers ?
5. Draw the Fischer projections of L-galactose and L-mannose.
6. Write down the structures and names of the products obtained when D-glucose is treated with (i) acetic anhydride (ii) hydrocyanic acid (iii) bromine (iv) con HNO3 and (v) HI.
7. Enumerate the reactions of glucose which cannot be explained by its open chain structure.
8. Explain mutarotation. Give its mechanism in case of D-glucose.
9. Amylose and cellulose are both straight chain polysaccharides containing only D-glucose units. What is the structural difference between the two ?
10. What are essential and non-essential amino acids ? Give two examples of each.
11. Give reasons for the following :
(i) Amino acids have relatively higher melting point as compared to corresponding haloacids.
(ii) Amino acids are amphoteric in nature.
(iii) On electrolysis in acidic solution amino acids migrate towards cathode while in alkaline solution these migrate towards anode.
(iv) The monoamino monocarboxylic acids have two pK values.
12. If three amino acids viz., glycine , alanine and phenyl alanine react together , how many possible tripeptides can be formed ? Write down the structures and names of each one. Also write their names using three and one letter abbreviations for each amino acid.
13. What type of linkages are responsible for the formation of :
(i) Primary structure of proteins.
(ii) Cross linking of polypeptide chains
(iii) a- Helix formation.
(iv) b-Sheet structure.
14. Which forces are responsible for the stability of a- Helix ? Why is it named 3.613 helix ?
15. What is denaturation and renaturation of proteins ?
16. Define enzymes. How do enzymes differ from ordinary chemical catalysts ?
17. What are the products obtained on complete hydrolysis of DNA ?
18. Write down the structure of pyrimidine and purine bases present in DNA.
19. Enumerate the structural differences between RNA and DNA .
20. Write down the structure of a nucleoside, which is present only in RNA.
21. What are complementary bases ?
22. Draw structure to show that hydrogen bonding between adenine and thymine and between guanine and cytosine.
23. What is the melting temperature (Tm ) of DNA ?
24. A DNA molecule with more number of GC base pairs than AT base pairs has higher Tm than one with lesser number of GC base pairs than AT base pairs. Explain why ?
25. When RNA is hydrolysed there is no relationship among the quantities of four bases obtained unlike DNA . What does this fact indicate about the structure of RNA ?
26. How does DNA replicate ? Give mechanism of replication. How is the process responsible for preservation of heridity ?
27. Genetic code is degenerate. Comment
28. How are lipids classified ? Give example of each class.
29. Hormones are ‘chemical messengers’. Explain.
30. Define and classify vitamins. Name the deficiency dieases caused by the lack of vitamins.