Zinc(II) and cadmium(II) metal complexes of thiamine pyrophosphate and 2-(alpha-hydroxyethyl)thiamine pyrophosphate: Models for activation of pyruvate decarboxylase

Metal complexes of thiamine pyrophosphate (IPP) of the general formula [M-2 (TPPH)(2)Cl-2]. 4H(2)O (M = Zn2+, Cd2+) were isolated from methanolic solut ions and characterized by elemental analysis, FT-IR, and multinuclear NMR s pectroscopies. The data provide evidence for the bonding of the metals to t he N(1') atom of the pyrimidine ring and to the pyrophosphate group. The st ability constant measurements of TPP and 2-(alpha-hydroxyethyl)thiamine pyr ophosphate (HETPP) metal complexes in aqueous solution imply the formation of dimeric complex species similar to the isolated solid products. They ind icate also that HETPP forms more stable metal complexes than does TPP. To e valuate the coenzyme action of TPP and HETPP metal complexes, enzymic studi es have been done using pyruvate decarboxylase apoenzyme. TPP metal complex es do not bind to the apoenzyme, unlike the Zn(II)-HETPP complex which can act as coenzyme. Considering these results, possible functional implication s for thiamine involvement in catalysis are discussed.