Acceptor substrate inhibits transketolase competitively with respect to donor substrate

Two substrates of the transketolase reaction are known to bind with the enz yme according to a ping-pong mechanism [1]. It is shown in this work that h igh concentrations of ribose-5-phosphate (acceptor substrate) compete with xylulose-5-phosphate (donor substrate), suppressing the transketolase activ ity (K-i = 3.8 mM). However, interacting with the donor-substrate binding s ite on the protein molecule, the acceptor substrate, unlike the donor subst rate, does not cause any change in the active site of the enzyme. The data are interesting in terms of studying the regulatory mechanism of the transk etolase activity and the structure of the enzyme-substrate complex.