Mechanistic studies on thiamin formation and degradation: Thiaminase I and thiamin phosphate synthase

Mechanistic studies on two enzymes catalyzing substitution reactions on the pyrimidirie moiety of thiamin will be described. Thiamin phosphate synthase catalyzes the displacement of the pyrophosphate of 2-methyl-4-amino5-hydroxymethylpynmidine pyrophosphate by 4 methyl-5-hydroxyethylthiazole phosphate. A mechanism for this reaction which is consistent with substituent effects and with the X ray crystal structure of the enzyme/thiamin phosphate/pyrophosphate ternary complex will be described. Thiaminase I catalyzes the displacement of the thiazole moiety of thiamin by a variety of nucleophiles. A mechanistic proposal for this enzyme will be described. This proposal is consistent with the stereochemistry of the reaction, with the inactivation of the enzyme with 2-methyl-4-amino-6 chloropyrimidine and with the X ray crystal structure of the mechanism based inactivated enzyme.