Dissecting the total transition state stabilization provided by amino acid side chains at orotidine 5′-monophosphate decarboxylase: A two-part substrate approach

Kinetic analysis of decarboxylation catalyzed by S154A, Q215A, and S154A/Q215A mutant yeast orotidine 5′-monophosphate decarboxylases with orotidine 5′-monophosphate (OMP) and with a truncated nucleoside substrate (EO) activated by phosphite dianion shows (1) the side chain of Ser-154 stabilizes the transition state through interactions with the pyrimidine rings of OMP or EO, (2) the side chain of Gln-215 interacts with the phosphodianion group of OMP or with phosphite dianion, and (3) the interloop hydrogen bond between the side chains of Ser-154 and Gln-215 orients the amide side chain of Gln-215 to interact with the phosphodianion group of OMP or with phosphite dianion.