tailieunhanh - Báo cáo khoa học: Donor substrate regulation of transketolase

The influence of substrates on the interaction of apotrans-ketolase with thiamin diphosphate was investigated in the presence of magnesium ions. It was shown that the donor substrates, but not the acceptor substrates, enhance the affinity of the coenzyme either to only one active center of transketolase or to both active centers, but to different degrees in each, resulting in a negative cooperativity for coenzyme binding. In the absence of donor substrate, neg-ative cooperativity is not observed. . | Eur. J. Biochem. 271 4189-4194 2004 FEBS 2004 doi Donor substrate regulation of transketolase Olga A. Esakova1 Ludmilla E. Meshalkina1 Ralph Golbik2 Gerhard Hiibner2 and German A. Kochetov1 1A. N. Belozersky Institute of Physico-Chemical Biology Moscow State University Moscow Russia 2 Martin-Luther-University Halle-Wittenberg Halle Saale Germany The influence of substrates on the interaction of apotransketolase with thiamin diphosphate was investigated in the presence of magnesium ions. It was shown that the donor substrates but not the acceptor substrates enhance the affinity of the coenzyme either to only one active center of transketolase or to both active centers but to different degrees in each resulting in a negative cooperativity for coenzyme binding. In the absence of donor substrate negative cooperativity is not observed. The donor substrate did not affect the interaction of the apoenzyme with the inactive coenzyme analogue N3 -pyridyl-thiamin diphosphate. The influence of the donor substrate on the coenzymeapotransketolase interaction was predicted as a result of formation of the transketolase reaction intermediate 2- a b-dihydroxyethyl -thiamin diphosphate which exhibited a higher affinity to the enzyme than thiamin diphosphate. The enhancement of thiamin diphosphate s affinity to apotransketolase in the presence of donor substrate is probably one of the mechanisms underlying the substrate-affected transketolase regulation at low coenzyme concentrations. Keywords 2- a b-dihydroxyethyl -thiamin diphosphate regulation of enzyme activity spectrophotometric titration thiamin diphosphate transketolase. Transketolase TK EC containing divalent cations and thiamin diphosphate ThDP as cofactors catalyzes one of the key reactions of the pentosephosphate pathway in carbohydrate transformation namely the cleavage of a carbon-carbon bond adjacent to a carbonyl group in ketoses donor substrates with subsequent transfer of a .