Pyruvate decarboxylase III. Specificity restrictions for thiamine pyrophosphate in the protein association step, sub-unit structure.

Pyruvate decarboxylase dissociates into sub-units of one half the molecular weight at alkaline pH. At the same conditions the cofactors thiamine pyrophosphate and Mg2+ are released and can be separated from the protein. Thiamine pyrophosphate is an obligatory cofactor for reconstitution to the oligomer [1]. In this study the effect of thiamine pyrophosphate derivatives (thiamine monophosphate, thiamine, and thiazole pyrophosphate) upon the reconstitution procedure was evaluated. The complete association of sub-units to form active oligomer was attained only when thiamine pyrophosphate was present. It is concluded that both the pyrimidine ring and the pyrophosphate group are required for productive co-enzyme binding and it is proposed that this interaction effects a conformational change which promotes protomer aggregation to form the enzymatically active holoenzyme. In addition data are presented which indicate that the monomer unit is 60 000 +/- 3000 daltons and that the N-terminal amino acid is histidine. Since the molecular weight of the active oligomer is 230 000 it is proposed that pyruvate decarboxylase is a tetramer comprised of four identical or nearly identical monomer units.