Classification of Proteins

On the basis of their structure, proteins are grouped into three classes:

• Simple proteins, which yield only amino acids on hydrolysis. Simple proteins are soluble in one or more solvents; many, such as histones, are soluble in water and are not coagulated by heat. Albumins, another group of simple proteins, are also water-soluble but do coagulate on heating — egg albumin, serum albumin and the legumelin of pulses are all albumins.
• Conjugated proteins, in which a simple protein is combined with a non-protein component called a prosthetic group. Haemoglobin is a classic example: its protein part is globin, and its prosthetic group is the iron-containing pigment haem. Nucleoproteins similarly combine nucleic acid with histone protein. Conjugated proteins are further named by the nature of their prosthetic group — glycoproteins/mucoproteins carry a carbohydrate component (e.g. the mucin of saliva, the heparin of blood), while lipoproteins carry a lipid component (e.g. the conjugated proteins of brain tissue, the plasma membrane …