Elementary plant Biochemistry and Biotechnology (2+1)

ii. Conjugated or compound proteins

• These are simple proteins combined with some non-protein substances known as prosthetic groups.The nature of the non-protein or prosthetic groups is the basis for the sub classification of conjugated proteins

Nucleoproteins

• Nucleoproteins are simple basic proteins (protamines or histones) in salt combination with nucleic acids as the prosthetic group.They are the important constituents of nuclei and chromatin

Mucoproteins

• These proteins are composed of simple proteins in combination with carbohydrates like mucopolysaccharides, which include hyaluronic acid and chondroitin sulphates.
• On hydrolysis, mucopolysaccharides yield more than 4% of amino-sugars, hexosamine and uronic acid e.g., ovomucoid from egg white.
• Soluble mucoproteins are neither readily denatured by heat nor easily precipitated by common protein precipitants like trichloroacetic acid or picric acid. The term glycoproteins is restricted to those proteins that contain small amounts of carbohydrate usually less than 4% hexosamine

Chromoproteins

• These are proteins containing coloured prosthetic groups e.g., haemoglobin, flavoprotein and cytochrome.

Lipoproteins
• These are proteins conjugated with lipids such as neutral fat, phospholipids and cholesterol

Metalloproteins

• These are metal-binding proteins. A ß-globulin, termed transferrin is capable of combining with iron, copper and zinc. This protein constitutes 3% of the total plasma protein. Another example is ceruloplasmin, which contains copper

Phosphoproteins

• These are proteins containing phosphoric acid. Phosphoric acid is linked to the hydroxyl group of certain amino acids like serine in the protein e.g., casein of milk