Explain Classification of Protein based on amino acid content?

Nutritionally, amino acids are classified on the basis of the body's ability to synthesize them - as essential (indispensable and not synthesized in the body) and non-essential (dispensable and that can be synthesized in the body) amino acids. Indispensable amino acids must be a part of the diet while dispensable amino acids need not be present in food. These definitions, however, become blurred at the metabolic level. Lysine and threorzine are perhaps the only metabolically indispensable amino acids because they are not transaminated to any nutritionally significant extent. This is a crucial point because lysine and threonine are the first and second limiting amino acids in cereal protcins. Lysine is the first limiting amino acid in human milk.

Table: Classification of amino acids

Glutamic acid and probably serine are the only truly dispeilsable amino acids since these are the only amino acids which can be synthesized by the reductive amination of the appropriate keto acid. There is a third group of amino acids which are 'conditionally essential', and are characterized by two features. Firstly, their synthesis uses other amino acids as carbon precursors and is confined to specific organs. This is an important metabolic distinction from the dispensable amino acids. For' some conditionally essential amino acids, e.g., tyrosine, the precursor is a dispensable amino acid; while for others such as cysteine both an essential amino acid, methionine as sulphur donor and a non essential amino acid, serine are required. At the metabolic level, the organism's ability to synthesize a conditionally essential amino acid is constrained by the availability of a suitable amino acid precursor. Secondly, the maximum rate at which their synthesis proceeds may be limited and potentially restricted by developmental or pathophysiological factors. Thus, very low birth weight infants are unable to synthesize cysteine and proline and lack the ability to synthesize glycine. These factors are important because human milk proteins have low glycine content. Amino acids are classified based on the amino acid content as given in the above Table.