(d) : Amino acids are mostly synthesized from α -ketoglutaric acid. These are the precursors of amino acids. A five carbon compound formed during Krebs' cycle is a α -ketoglutaric acid which is the first dicarboxylic acid formed. Pyruvic acid converted into alanine, α -ketoglutaric acid into glutamic acid, OAA into aspartic acid, polymerization of such amino acids results into formation of proteins.