FUNCTION OF THE AMINOPYRIMIDINE PART IN THIAMINE PYROPHOSPHATE ENZYMES

To answer the question on the mechanistic significance of the pyrimidine moiety of thiamine pyrophosphate (TPP), the two pyridine analogs of TPP (N1-pyridyl-TPP and N3-pyridyl-TPP), as well as 4′-deamino-TPP, have been resynthesized and incubated with the apoenzymes of pyruvate decarboxylase, pyruvate dehydrogenase complex, and transketolase. By comparison of activity and binding properties of the three TPP analogs it is shown that only N1-pyridyl-TPP causes catalytic activity (between 65 and 100%) with all the enzymes tested. N3-Pyridyl-TPP as well as 4′-deamino-TPP proved inactive generally. The binding experiments demonstrate that both analogs with the N1-atom preserved in the structure (N1-pyridyl-TPP and 4′-deamino-TPP) offer practically the same affinity as TPP to the three apoenzymes tested. A mechanism is proposed that explains the essential function of the amino group and the pyrimidine-Ni in TPP catalysis. © 1991.