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