Enzyme catalyzed reactions that are similar to reductive amination are a very important class in biochemistry. If the substrate for substitution of a carbonyl group for an amine is an α-keto acid, such as many important glycolytic or citric acid cycle intermediates (pyruvate, oxaloacetate, etc) the product of the reaction is an α-amino acid (alanine, aspartate, etc.). The conversion of uracil to cytosine (uridine triphosphate to cytidine triphosphate) is an enzyme catalyzed amination. A subclass of amination reactions begins with the conversion of the carbonyl oxygen into a phosphoryl ester, making the oxygen more apt to depart following nucleophilic addition of the amine. This latter class is particularly important in purine biosynthesis.