(v) β-alanine, a derivative of asparate, takes part in the formation of coenzyme A and pantothenic acid (a vitamin).
(vi) Histamine is derived from amino acid histidine through decarboxylation.
(vii) Histamine is required for optimum functioning muscles, blood capillaries and gastric secretion but in excess it causes anaphylaxis.
PROTEINS
Proteins are large-sized heteropolymeric macromolecules having one or more polypeptides (chains of amino acids). The term
polypetide is often used interchangeably with protein. They are the most abundant and most varied of the macromolecules of the
cells which constitute about 50% of their dry weight. Closely related species have many similar proteins while unrelated species
have fewer common proteins. Thus, protein proximity shows evolutionary relationship. Being macromolecules, proteins are not freely
soluble in water but form colloidal complex with the same. Chemically a protein is made of carbon, hydrogen, oxygen, nitrogen
and sulphur. Some proteins additionally contain phosphorus, iron and other elements.
Proteins are variously folded linear heteropolymers of amino acids. The amino acids are linked serially by peptide bonds
(–CONH–) formed between amino group (–NH 2 ) of one amino acid and carboxylic group (–COOH) of the adjacent one. Collagen
is most abundant protein of animal world. RuBisCo (ribulose biphosphate carboxylase - oxygenase) is not only the most abundant
protein in plants but also the whole biosphere.
Structure of Protein
(i) Primary structure is the description of basic structure of a protein i.e., number of polypeptides, number and sequence of amino
acids in each polypeptide.
(ii) Secondary structure is the development of new stearic relationships of amino acids present in the linear sequence inside the
polypeptides. There are three types of secondary structure-α-helix, β-pleated and collagen helix. Secondary structures depend on
hydrogen bonds between – C = O and –NH groups.
(iii) In tertiary structure, there is bending and folding of secondary strand of polypeptide of various types to form spheres, rods or
fibres. Tertiary structure is stabilised by several types of bonds – hydrogen bonds, ionic bonds, van der Waals interactions, disulphide
bonds, hydrophobic bonds, etc.
(iv) Quaternary structure, is found only in multimeric proteins. Each polypeptide develops its own tertiary structure and functions
as subunits of protein, e.g., haemoglobin.
Classification of Proteins
Proteins are classified in several ways. Three major ways are discussed below.
Based on Composition
Simple Proteins Conjugated Proteins Derived Proteins
These proteins are made up of
amino acids only. Additional
non-amino groups are absent,
e.g., histones, keratin.These proteins have non-amino prosthetic groups.
Depending upon the type of prosthetic group, conjugate
proteins are of several types, i.e., nucleoproteins,
chromoproteins, glycoproteins, lipoproteins.They are derived from proteins
through denaturation, coagulation
and breakdown, e.g., metaproteins,
proteoses, peptones, peptides, fibrin.
Based on Structure
Fibrous Proteins Globular Proteins
They are thread like proteins which may occur singly
or in groups. They are tough, non-enzymatic and
structural proteins. Fibrous proteins generally possess
secondary structure. They are insoluble in water.
Keratin of skin and hair are fibrous proteins. Some of
the fibrous proteins are contractile, e.g., myosin of
muscles and elastin of connective tissue.They are rounded in outline. Contractibility is absent. Final structure
is tertiary or quaternary. Globular proteins may be enzymatic
or non-enzymatic. Smaller globular proteins are mostly soluble in
water. They are usually not coagulated by heat, e.g., histones. Egg
albumin, serum globulins and glutelins (wheat, rice) are examples
of large globular proteins which get coagulated by heat.Based on Quality
Complete Proteins Incomplete Proteins
Complete proteins are first class proteins. They are the
proteins which contain all the essential amino acids
required by humans, e.g., milk, meat, egg, fish, soya.Incomplete proteins are second class proteins which lack one or more
essential amino acids.,e.g., plant proteins.