查看文章

Two circular dichroism signals observed on thiamin diphosphate (ThDP)-dependent enzymes, a positive band in the 300−305 nm range and a negative one in the 320−330 nm range, were investigated on yeast pyruvate decarboxylase (YPDC) and on the E1 subunit of the Escherichia coli pyruvate dehydrogenase complex (PDHc-E1). Addition of the tetrahedral ThDP−acetaldehyde adduct, 2-α-hydroxyethylThDP, to PDHc-E1 generates the positive band at 300 nm, consistent with the formation of the 1‘,4‘-iminopyrimidine tautomer, as also demonstrated for phosphonolactylthiamin diphosphate, a stable analogue of the tetrahedral ThDP−pyruvate adduct 2-α-lactylThDP (Jordan, F. et al. (2003) J. Am. Chem. Soc. 125, 12732−12738). Therefore, we suggest that all tetrahedral ThDP-bound covalent complexes will also prefer this tautomer, and that the 4‘-aminopyrimidine of ThDP participates in multiple steps of acid …