Protein word is originated from Greek word ‘Proteios’ meaning ‘holding primary place‘. Berzelius thought them to be the most important biological substance. Proteins are dehydrated polymers of amino acids with each amino acid residue joined to its neighbour by a special type of covalent bond. The term residue reflects the loss of elements of H2O by combination of 2 amino acids (when 1 amino acid is joined to another).
The first amino acid to be discovered was asparagine in 1806 and the last was threonine in 1938. All the 20 amino acids have trivial or common names. For e.g. asparagines were first found in asparagus, glutamate in wheat gluten, tyrosine first isolated from cheese (cheese Greek tyros), glycine got its name because of its sweet taste (Greek word glykos means sweet).
Proteins are complex organic molecules of large molecular weight ranging from a few thousand to a million or above. Proteins contain elements C, H, O, N and in some cases S. These biopolymers are built up of fundamental units of amino acids joined by peptide bonds. Amino acids are class of organic molecules which contains 2 functional groups; basic –NH2 group attached to C containing –COOH group, it is called α-amino acid. All 20 standard amino acid found in proteins are α-amino acids. These amino acids differ from each other in their side chains or R groups, which vary in structure, size and electric charge which influence the solubility of amino acid in water. The 20 amino acids of proteins are often referred to as standard amino acids to distinguish them from less common amino acids present in living organisms known as non-standard amino acids.
Classification of Proteins
Classification based on solubility and physical properties
Simple Proteins Those proteins, which on complete hydrolysis yield only amino acids. These proteins depend on size and shape of the molecule. This class is further sub-divided based on solubility and heat combination. Major sub- classes are histones, albumins, globulins, gliadins, glutenins, prolamines and structural proteins (keratin, collagen, elastin).
Conjugated Proteins These proteins, in addition to amino acids, contain a non-protein group called prosthetic group. Protein part is called apoprotein and whole protein is called holoprotein e.g. nucleoprotein, mucoprotein, glycoprotein, chromoprotein, phosphoproteins, lipoproteins and metalloproteins.
Derived Proteins These proteins are formed from native proteins by action of heat, physical forces or chemical factor. Derived proteins are of 2 types
i. Primary Proteins
o Proteins obtained by the action of acid or enzyme.
o Metaproteins obtained by the action of acid or alkali.
o Denatured or conjugated proteins.
ii. Secondary Proteins
o These are obtained by progressive hydrolysis, e.g. proteases, peptones and peptides.
On the basis of Size or Shape
- Fibrous Proteins: Protein in which ratio of length to width of a molecule is more than 10 e.g., Collagen, Elastin
- Globular Proteins: Protein in which ratio of length to width of a molecule is around 2 e.g., Myoglobin, Hemoglobin
On the Basis of Functional Properties
- Defense Proteins: Immunoglobulins involved in defense mechanism
- Contractile Proteins: Contractile proteins of skeletal muscle involved in muscle contraction and relaxation
- Respiratory Proteins: Involved in function of respiration e.g. Hemoglobin, Myoglobin
- Structural Proteins: Proteins of skin, nails, cartilage
- Catalytic proteins (Enzymes): Proteins acting as enzymes
- Hormonal Proteins: Proteins acting as hormones
- Genetic Proteins e.g. nucleoproteins
General Properties of Proteins
i. Taste: Proteins are tasteless. However hydrolytic proteins are bitter in taste.
ii. Odour: Odorless when heated to dryness, they turn brown and give off odour of burning feather.
iii. Molecular Weight: Proteins have large molecular weight and therefore are macromolecules. Molecular weight ranges from 1000 Daltons to millions of daltons.
iv. Viscosity: varies with kind of proteins and its concentration in solution. It is related to molecular shape, the long molecules being more viscous than globular proteins.
v. Hydration: Polar groups of proteins such as –NH2 and –COOH group become hydrated in presence of water and swell up when electrolytes, alcohols or sugars that form complexes with water are added to protein solution.
vi. Heat Coagulation: Several proteins coagulate forming an insoluble coagulum. Coagulation is max at isoelectric pH of protein. During coagulation, protein undergoes a change called denaturation.
vii. Amphoteric Nature (Zwitter ion concept as explained in amino acids)
viii. Precipitation: Proteins can be precipitated in solution by a variety of positive and negative ions. Precipitation is important in isolation of proteins from blood and fluids.
ix. Hydrolysis: It occurs with acid, alkali and enzymes. Complete hydrolysis is by heating proteins with 6N HCl for 8-10 hours at 120°C.