Protein is by far the most important organic substance known so far in a living cell.
The name protein was suggested by Berzelius (1938).
It is derived from proteios, a Greek word, meaning ‘first rank’.
Proteins are complex substances of high molecular weight ranging up to several millions and contain nitrogen in addition to carbon, hydrogen, and oxygen. Sometimes elements like phosphorus, sulphur, iron, zinc, and iodine may also be present.
However, elementary composition of most proteins is very similar (approximate percentage is C = 50 – 55, H = 6- 8, O = 20 – 23, N = 15 – 18 and S = 0 – 4).
Proteins are made up of several simple nitrogen containing organic molecules called amino acids, i.e., proteins are polymerised forms or polymers of amino acids. Thus, amino acid is the basic unit of protein.
In nature large number of them are found in free and combined forms. A partial list of amino acids identified at least from one biological source includes 28 amino acids. However, the names and structures of 20 amino acids which combine to form one or other proteins are listed following, all other but cysteine and hydroxyproline are commonly found in proteins. These two are rarely known.
The general formula for majority of amino acids is, H
H2N – C- COOH
Where R represents any one of the great variety of structures, eg. In glycine R=H, In serine R = OH and in alanine R=CH3.
Types of natural amino acids groupwise arrangement –
1. Glycine (Neutral)
Monoaminodicarboxylic 6. Glutamic acid (Acidic) 7. Aspartic acid
8. Arginine (Basic)
11. Threonine (Neutral)
13. Cysteine (Neutral)
19. Hydroxyproline (Basic)
Amino acids —
These are organic compounds with at least one amino group and one carboxyl group. Other functional groups may also be present. The chemical properties of amino acids reflect the chemical properties of amino, carboxyl or other functional groups. Theoretically limitless number of amino acids are possible.
Amino acids are amphoteric compounds, or they possess both acidic and basic groups. The electrical charges on the amino acids influence their properties markedly. After losing water molecules different kinds of amino acids link up with peptide bonds to form long chains i.e. different kinds of proteins.
Amino acids are the principal building blocks for proteins. Some amino acids like tyrosine is converted into hormones thyroxin and adrenaline, glycine is involved in formation of heme and tryptophan in the formation of vitamin nicotinamide and hormone IAA.
Peptides are primarily made up of amino acids. As intermediates in the hydrolysis of proteins, peptides are formed. Theoretically it is possible to combine quite many amino acids into enormous number of peptides. These compounds are found in many biological materials but even a rough estimate of naturally occurring peptides is not yet available due to shortcomings in isolation and separation techniques. Some investigators believe that the proteins are formed directly from amino acids without intervening peptides, however, others do not agree with this concept.
As already stated these are chains of amino acids still larger than peptides. These compound always per a giant macromolecule with molecular weights in the range of 104 to 107. Although the list of 20 amino acids in protein molecule is limited, the possible variety of protein is not. The number of each kind of amino acids per protein molecule is widely variable. The sequence of amino acids in the peptide chain is specific for each protein and potentially capable of enormous diversity. Proteins may differ in number of peptide chains per protein molecule. Amino acids form peptides and peptides form proteins. Two peptides may be held together with S bonds.
Structure of Proteins
Four basic structural levels of proteins are recognised by modern biochemists. They are called the primary, secondary, tertiary, and quaternary structures.
1. Primary structure –
The primary structure of protein refers to the linear sequence of amino acid residues making up its polypeptide chain. Disulphide (s-s-) bond is the other important characteristic of the primary structure. It may be found among one or between two polypeptide chains. It does not make protein functional.
2. Secondary structure –
It refers to the spirally coiled structure of the polypeptide chain called α- helical form. It is most common structure. It is stabilised by extensive hydrogen bonding. Hydrogen bonds between the carboxyl and the amino acid groups of the peptide bonds. (It is formed between H atom attached to peptide N, and O atom attached to peptide C). salt links and van der Waals forces help in maintaining the helical structure. The second type is β pleated sheet composed of two or more segments of fully extended peptide chains. In β sheets, the hydrogen bonds are formed between the neighbouring segments of polypeptide chains.
3. Tertiary structure –
The term tertiary structure refers to the arrangement of secondary structures into a three-dimensional (folded and super folded) structure. Folding normally occurs from interactions between amino acid residues relatively far apart in the sequence. It makes the protein functional.
4. Quaternary structure –
It refers to the association of more than one polypeptide chain to form a stable unit. For example, enzyme phosphorylase contains two identical subunits. Each subunit when alone is catalytically inactive but when joined as a dimer forms the active enzyme. If in the quaternary structure participating units are similar, it is called homogeneous quaternary structure and if dissimilar, it is called heterogeneous quaternary structure. A subunit may also be called protomer, and a protein made up of more than one protomer would be an oligomeric protein.