Introduction to chemistry of proteins

I. Classification base on Composition: 

A. Simple protein:

Yields only amino acids and no other major organic or inorganic hydrolysis products i.e. most of the elemental compositions. 

B. Conjugated Proteins

These are proteineous molecules that yields amino acids and other organic and inorganic components

E.g., Nucleoprotein (a protein containing Nuclei acids)

 Lipoprotein (a protein containing lipids)

 Phosphoprotein (a protein containing phosphorous)

 Metalloprotein (a protein containing metal ions of Fe²⁺)

 Glycoprotein (a protein containing carbohydrates).

II. Classification base on Solubility

a) Albumins: These proteins such as egg albumin and serum albumin are readily soluble in water and coagulated by heat.

b) Globulins: These proteins are present in serum, muscle and other tissues and are soluble in dilute salt solution but sparingly in water.

c) Histones: Histones are present in glandular tissues (thymus, pancreas etc.) soluble in water; they combine with nucleic acids in cells and on hydrolysis yield basic amino acids

III. Classification base on shape

A. Fibrous proteins

In this protein, the molecule is constituted by several coiled cross-linked polypeptide chains, they are insoluble in water and highly resistant to enzyme digestion. The ratio of length to breath (axial ratio) is more than 10 in such protein. A few sub groups are listed below.

1. Collagens: the major protein of the connective tissue, insoluble in water, acids or alkalis. But they are convertible to water-soluble gelatin which is easily digestible by enzymes.

2. Elastins: present in tendons, arteries and other elastic tissues, but not convertible to gelatin.

3. Keratins: protein of hair, nails etc.

B. Globular proteins: These are globular or ovoid in shape, soluble in water and constitute the enzymes, oxygen carrying proteins, hormones etc.

Subclasses include: - Albumin, globulins and histones.

IV. Classification base on biological functions:

Proteins are sometimes described as the "workhorses" of the cell because they perform various biological roles.

Enzymes: kinases, transaminases etc.

Storage proteins: myoglobin, ferritin

Regulatory proteins: peptide hormones DNA binding proteins

Structural protein: collagen, proteoglycan

Protective proteins: blood clotting factors, Immunoglobins,

Transport protein: Haemoglobin, plasma lipoproteins

Contractile or motile Proteins: Actin, tubulin