NEET Biology Notes Biomolecules and Enzyme Proteins
Proteins
The term ‘protein’ was coined by Berzelius (1837) and Mulder (1838). Though approximately 300 amino acids occur in nature but only 20 make the composition of proteins. Proteins are polymers of amino acids.
General Structure
All amino acids have a common structure. The only difference between the different amino acids lies with F-groups in general formula. The fl-group have quite diverse chemical properties.
All amino acids, apart from the simplest one (glycine) show optical isomerism. This can result in two different arrangements as shown in the diagram. With a very few minor exceptions (e.g. bacterial cell wall contains D-amino acids) only the L-forms are found in living organisms.
- Gamma Amino Butyric Acid (GABA), histamine serotonin, ornithine, citruline and fralanine are the amino acids, which are not found in proteins.
- Glycine is the simplest amino acid with lowest molecular weight and absence of asymmetrical carbon atom. It is involved in the formation of heme.
- Tryptophan is the most complex amino acid containing indole ring.
- Methionine and cysteine are sulphur containing amino acids.
- In proline and hydroxyproline, instead of NH2 (amino) ‘ group, NH (imino) group is present. These amino
acids are called imino acids. - Proteins contain L-(Levorotatory) isomers of amino acids. D-(Dextrorotatory) isomers of amino acids, are found only in bacterial cell walls.
- Isoleucine is an amino acid with two asymmetrical carbon atoms. Tyrosine gives rise to dopamine, melanin, thyroxine, adrenaline and nor-adrenaline.
- Lysine and arginine are basic amino acids, which contain more than one amino groups. Glutamic acid and aspartic acid are acidic amino acids, which contain more than one acidic groups.
- Tryptophan amino acid forms the vitamin nicotinamide and a plant hormone Indole Acetic Acid (IAA). Amino acids that cannot be synthesized in the body are called essential amino acids, while those which can be synthesized in the body and need not be supplied in the diet are called non-essential or dispensable amino acids.
- For human beings eight amino acids are essential. Infants require arginine and histidine in addition.
Peptide Bond
Proteins are the linear sequence of amino acids linked together by peptide bonds. The peptide bond is chemically, a covalent
There are many peptide bonds in a single protein molecule. Therefore, proteins are also called polypeptide.
- Dipeptide When two amino acids are joined together via a peptide bond, a dipeptide is formed.
- Oligopeptide It is a long, unbranched chain of 2-25 amino acids residues, which are linked by peptide bonds.
- Polypeptide A long chain of many amino acids (>25 amino acid residues) linked end to end by peptide bond.
- Amino acids can combine to form peptide chains by a process called condensation reaction. Peptide chains can be broken down by hydrolysis to simple amino acids.
- Proteins constitute about 10 to 12% of the cell contents. Proteins are made up of carbon (51%), oxygen (25%), nitrogen (16%), hydrogen (7%), sulphur (0.4%) and sometimes phosphorus is also present in traces.
- Insulin (human) has 53 amino acids arranged in two polypeptide chains of 22 and 31 amino acids. Human serum albumin has 582 amino acids in its polypeptide chain.
- Proteins show enormous diversity because of different proportions and sequence of amino acids. The number and variety of proteins vary from species to species and within a species from cell to cell.
- A bacterium Escherichia coli {E. coli) may have about 3000 types of proteins. A human liver cell may have millions of proteins. However, all these proteins are synthesized from the same 20 amino acids. Proteins and amino acids are amphoteric in nature. It means they can react with both acids and bases. Proteins are oxidized by putrefaction process and produce bad smell.
Denaturation
It refers to the loss of three-dimensional structure of a protein. Strong acids and alkalis, heavy metals, heat, UV radiations, and detergents can denature a protein.
Collagen is the most abundant protein in animal world and ribulose biphosphate carboxylase oxygenase (Rubisco) is the most abundant protein in the whole of the biosphere. P-proteins are involved in the transport of organic compounds through phloem. Snake venom, ricin of castor, and bacterial toxins are proteinaceous in nature. Protamines are basic proteins associated with DNA of chromosomes, these are rich in lysine and arginine. Monellin, a protein is the sweetest chemical obtained from an African berry.
Structural Organisation in Proteins
- Primary structure The linear sequence of amino acid residues in a polypeptide chain. The enzyme ribonuclease and the protein myoglobin function only in their primary structure. Primary structure determines the higher levels of organisation in protein and its biological functions.
- Secondary structure It is regular folding patterns of continuous portions of the polypeptide chains, e.g., a-helix and (3-pleated sheets. Secondary structures are stabilized by hydrogen bonds. Most globular proteins contain region of a-helices together with p -sheet. Keratin (a fibrous protein found in skin) is composed almost entirely of a-helices, while fibrion (silk protein) is almost entirely composed of p-sheets.
- Tertiary structure Three dimensional structure formed by the folding of the secondary structure into a complex shape. The interactions involved in folding include weak ionic bonds, hydrogen bonds, hydrophobic interactions and strong disulphide bonds between neighbouring cysteine amino acids. Enzymes are functional with a tertiary structure only.
- Quarternary structure Proteins consist of two or more polypeptide chains. Haemoglobin, a globular protein composed of four polypeptide chains (i.e. two p-chains + two a-chains) each having a heme group at the centre of chain.
Proteins can be classified as follows :
- On the basis of their composition
- On the basis of their structure
- On the basis of their functions.
When amino group is free it is said to be basic and when carboxylic group is freest is acidic. Lysine and arginine are examples of basic amino acid, as they contain two amino group and one carboxylic group. Glutamic acid and aspartic acid bear one amino group and two carboxylic group hence they are acidic.
