Naturally occurring amino acids | MD MS Entrance MCQs

True about naturally occurring amino acids: 

A. All are racemic mixtures 
B. Have specific genetic code 
C. 20 in number 
D. All are D-isomers 
E. All are L-isomers

Ans. B. Have specific genetic code ; 

• Of the over 300 naturally occurring amino acids., 20 constitute the monomer units of proteins.

• Both D-amino acids and non a anion acids occur in nature, but only L α — amino acids arepresent in proteins. D-amino acids that occur naturally inclued D-serine and D-aspartate in brain tissue, D-alanine and D-glutamate in cell walls of Gram +ve bacteria and D-amino acids in some non-mammalian peptides and certain antibiotics.

• The genetic code is universal, specific, non-overlapping and degenerate. This universal nature of genetic code has lead to incorporation of 20 amino acids during protein synthesis in cells. 

• Racemic mixture has both D and L isomers in equal concentration, thereby optically neutral.