Kinds of Protein: Notes on simple and conjugate proteins

The proteins can broadly be classi­fied into two types–I. Simple proteins and II. Conjugate proteins

(I) Simple proteins

These can conveniently be classi­fied into two categories, on the basis of their shape.

(A) Fibrous proteins

i. thread like, either single or in groups;

ii. generally possess secondary structure;

iii. insoluble in water;

iv. structural units or protec­tive structures. Ex., Keratin in hair and skin, some plant fibres, also in cuticles; be­sides some are contractile, such as myosin of muscles and elastin of connective tis­sue.

(B) Globular proteins

The globular proteins are divided into six categories and in general, these are:

(i). nearly rounded in outline;

(ii). with tertiary or quartenary structure;

(iii). mostly soluble, if small (solubility decreases and heat coagulability increases with increase in size), e.g., enzymes.

(iv). enzymatic or non-enzymatic function.

1. Albumins:

Large molecules, neutral, soluble in water and di­lute salt solution, coagulates on heating., e.g., ß-amylase, egg al­bumin, blood serum albumin, grains of wheat (Triticum), caster seeds (Ricinus communis)

2. Globulins:

Large molecules, neutral, soluble in water and di­lute salt solution, coagulates on heating at high temperatures, e.g., a-amylase, antibodies in blood, serum globulins, blood fi­brinogen, wheat grains, castor seeds, mustards, legumin and vicillin from peas, archin and cornarchin from peanuts , glycinin from soybeans.

3. Prolamins:

Insoluble in water, but soluble in salt solutions and 70-80% alcohol, e.g., gliadin from wheat, herdein from bar­ley and zein from maize. These are almost absent in dicotyle­dons.

4. Glutelins:

Insoluble in water, but soluble in weak acid or al­kali., e.g., oryzenin in paddy, hordenine in barley.

5. Histones:

Small molecules with more basic proteins, soluble in water but do not coagulate eas­ily on heating, generally found in association with nucleic acids, as in nucleoproteins.

6. Prolamines:

Contain basic amino acids, soluble in water and do not coagulate on heating.

(II) Conjugated proteins.

These complexes of proteins and various other molecules can be divided into seven types.

1. Nucleoproteins (protein + nu­cleic acid) are found in nucleus (mostly constituting the chromosomes). Ribosomes are in essence ribonucleoprotein particles.

2. Lipoproteins (protein+lipid) are found in membranes and mem­brane surfaces and take part in membrane organisation and function.

3. Glycoproteins (protein + carbo­hydrate) are found to play impor­tant roles in cell recognition sys­tems and cell defence mecha­nisms against microorganisms. They are found at membrane sur­faces and in cell walls.

4. Chromoproteins (protein + pig­ments) as found as flavoprotein, haemoglobin, chloroplastin (with chlorophylls in thylakoids).

5. Metalloproteins are complexes of protins with metallic elements (Zn, Mn, Cu, Fe) such as Fe in ferritin.

6. Mucoproteins (proteins+muoild) are present in saliva (e.g. mucin).

7. Phosphoproteins (protein +phosphate) are present in milk (e.g., casein), egg (e.g., vitellin), etc.