Cooperative binding of substrates to transketolase from <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Catalytic activity of two active sites of transketolase and their affinity towards the substrates (xylulose-5-phosphate and ribose-5-phosphate) has been studied in the presence of Ca²⁺ and Mg²⁺. In the presence of Ca²⁺, the active sites exhibit negative cooperativity in binding both xylulose-5-phosphate (donor substrate) and ribose-5-phosphate (acceptor substrate) and positive cooperativity in the catalytic transformation of the substrates. In the presence of Mg²⁺, nonequivalence of the active sites is not observed.