In this article we will discuss about:- 1. Meaning of Proteins 2. Properties of Proteins 3. Reaction 4. Types.
Meaning of Proteins:
Proteins are large-sized heteropolymeric macromolecules having one or more polypeptides (chains of amino acids). The term polypeptide is often used interchangeably with protein. However, a single polypeptide must be at least 50 amino acid long in order to qualify for the term. Proteins are body builders of organisms.
They are the most abundant and most varied of the macromolecules of the cells which constitute about 50% of their dry weight. A bacterial cell has 1000-2000 types of proteins. Each type of cell has some unique proteins. Closely related species have many similar proteins while unrelated species have fewer common proteins. Thus protein proximity shows evolutionary relationship.
The minimum molecular weight of a protein is that of adrenocorticotropin hormone (4500), insulin (bovine insulin – 5733) and bacterial ferredoxin (about 6000). Other common proteins are human haemoglobin (66500), enzyme urease (483000), iso-citrate dehydrogenase (1,000,000) and pyruvate dehydrogenase complex (4,600,000).
Being macromolecules proteins are not freely soluble in water but form colloidal complex with the same. Chemically a protein is made of carbon, hydrogen, nitrogen, oxygen and sulphur. Some proteins additionally contain phosphorus, iron and other elements.
Proteins are variously folded linear heteropolymers of amino acids. The linear polymers of amino acids are called polypeptides.
A protein may have one polypeptide (monomeric proteins, e.g., myoglobin, ribonuclease), two polypeptides (e.g., insulin), four polypeptides (2α, 2β in haemoglobin) or more (24 in ribulose bi-phosphate carboxylase and 72 in pyruvate dehydrogenase complex). A protein having two or more polypeptides is called multimer or multimeric protein.
The common multimers are dimers, trimers, tetramers, pentamers and decamers. A polypeptide contains from a few (21 and 30 in the two polypeptides of human insulin, 97 in spinach ferredoxin) to a few hundred amino acid residues (582 in human serum albumin).
The amino acids are linked serially by peptide bonds (—CONH—) formed between amino group (—NH2) of one amino acid and carboxylic group (—COOH) of the adjacent one. The sequence of amino acids present in a polypeptide is specific for a particular protein.
The distinctive sequence of amino acid units is governed by the codon sequence of the gene or cistron that controls its formation. Only some 20 amino acids are used in the synthesis of all types of proteins. This is similar to the formation of innumerable words from a limited number of alphabets. In a polypeptide of only 100 amino acid residues, there is possibility of 20100 arrangements or types of polypeptides.
This accounts for thousands of specific proteins found in each of the living species as well as those of the past species. Collagen is most abundant protein of animal world. Rubisco (ribulose bi-phosphate carboxylase- oxygenase) is not only the most abundant protein in plants but also the whole biosphere.
Properties of Proteins:
There are thousands of proteins present in each organism. This large variety is due to specific arrangement of amino acids in their chains. For example, 100 amino acids can form 20100 types of polypeptides.
Each species has certain specific proteins not found in others. Closely related species share several common proteins. Number of common proteins decreases with the increase in dissimilarity between species. The principle is used in bringing out evolutionary relationships amongst various groups of plants and animals.
3. Large-Sized Molecules:
Proteins are large-sized molecules with a minimum molecular weight of 4500 (adrenocorticotropin) to a maximum molecular weight of 4,600,000 (pyruvate dehydrogenase).
Being large-sized, many proteins function as colloids and form colloidal solution.
Due to intra-chain and inter-chain bonding and folding, a protein comes to have a particular surface configuration with specific reactive groups. This specificity provides functional diversity to proteins for performing different cellular activities.
Enzymes are specific due to their proteinaceous nature. Antibodies are complex glycoproteins which attach to particular pathogens and their toxins for their immobilisation. Certain bacterial toxins, which affect specific tissues or organs are also proteins. Similarly membrane based proteins have reactive sites for certain nutrients.
Cell membranes do not allow permeation to proteins. They can, however, pass outwardly and inwardly through exocytosis and endocytosis. Normally, every cell synthesizes its own proteins from amino acids.
7. Amphoteric Nature:
Like amino acids, proteins are amphoteric.
Functional three-dimensional form of a protein is called native state. The state is maintained by specific bonds that form its quaternary (4°), tertiary (3°) and secondary (2°) structure. These bonds are easily broken by high temperature, high energy radiations, soap, disinfectants, detergents, alcohol, etc. The phenomenon is called denaturation.
Reaction of Proteins:
In the aqueous medium a protein possesses both cationic and anionic groups. There are several of such groups on the same molecule. A chemical, like protein, carrying both positive and negative charges is called amphoteric.
Ionization of cationic or anionic groups depends on the pH of the medium. At a specific pH, a protein may be electrically neutral because the number of positive charges is exactly balanced by the number of negative charges.
This pH is known as isoelectric point. At physiological pH of 7.4, a protein may have more of negative charges. Such proteins are called basic proteins. They are rich in basic amino acids like lysine and arginine. Histones associated with DNA are basic proteins.
Acidic proteins have more of positive charges. They are rich in acidic amino acids like aspartic acid and glutamic acid. Most of the blood proteins are acidic in nature. There is a third category of neutral proteins which have their isoelectric point at 7.4 pH.
Types of Proteins:
Proteins are classified on the basis of their shape, function and constitution:
(a) On the Basis of Shape:
Depending upon their shape, proteins are divided into two categories, fibrous and globular.
1. Fibrous 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 is such a fibrous protein. Some of the fibrous proteins are contractile, e.g., myosin of muscles and elastin of connective tissue.
2. Globular Proteins:
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 not coagulated by heat, e.g., histones.
Water solubility decreases but heat coagulability increases with the increase in size of globular proteins. Egg albumin, serum globulins and glutelins (wheat, rice) are examples of large globular proteins which get coagulated by heat.
(b) On the Basis of Function:
Depending upon the function, proteins are classified into enzymatic and non-enzymatic types. The non-enzymatic proteins may be— structural, storage, protective, hormonal, transport, types.
1. Enzymatic Proteins:
They are proteins which function as enzymes, either directly (e.g., amylase, pepsin) or in conjunction with a non-protein part called cofactor (e.g., dehydrogenases). Enzymatic proteins are usually globular in shape.
2. Structural or Protoplasmic Proteins:
They form part of cellular structures and their products, e.g., colloidal complex of protoplasm, cell membranes, contractile proteins, structural proteins of hair and nails. In shape, structural proteins can be globular or fibrous.
3. Reserve or Storage Proteins:
They occur as food reserve mostly in seeds, eggs or milk. Storage proteins are usually globular. Depending upon their solubility, reserve proteins are of four types— albumins, globulins, prolamines and glutelins.
(c) On the Basis of Constitution:
On the basis of their constitution, proteins are of 3 types- simple, conjugate and derived.
1. Simple Proteins:
The proteins are made up of amino acids only. Additional non-amino groups are absent, e.g., histones, keratin.
2. Conjugated Proteins:
The proteins have non-amino prosthetic groups. Depending upon the type of prosthetic group, conjugate proteins are of several types.
3. Derived Proteins:
They are got from proteins through denaturation, coagulation and breakdown, e.g., meta-protein, proteoses, fibrin. Precursor of fibrin is an elongated but asymmetric soluble plasma protein called fibrinogen.
(d) On the Basis of Quality:
1. Complete Proteins:
They are proteins which contain all the essential amino acids required by humans, e.g., milk, egg, meat, fish. Soya also possesses a nearly complete protein. Complete proteins are also called first class proteins.
2. Incomplete Proteins:
They are proteins which lack one or more essential amino acids. Most of the plant proteins are incomplete proteins. Plant proteins, being incomplete, are also called second class proteins.