Acceptor Substrate Inhibits Transketolase Competitively with Respect to Donor Substrate

O. N. Solov'eva¹, L. E. Meshalkina¹, M. V. Kovina¹, V. A. Selivanov^1,2, I. A. Bykova¹, and G. A. Kochetov^1*

¹Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 939-3181; E-mail: kochetov@bac.genebee.msu.su

²Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences, Pushchino, Moscow Region, 142292 Russia

^* To whom correspondence should be addressed.

Received February 28, 2000; Revision received June 15, 2000

---

Two substrates of the transketolase reaction are known to bind with the enzyme according to a ping-pong mechanism [1]. It is shown in this work that high concentrations of ribose-5-phosphate (acceptor substrate) compete with xylulose-5-phosphate (donor substrate), suppressing the transketolase activity (K_i = 3.8 mM). However, interacting with the donor-substrate binding site on the protein molecule, the acceptor substrate, unlike the donor substrate, 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 transketolase activity and the structure of the enzyme-substrate complex.

---

KEY WORDS: transketolase, thiamine pyrophosphate, protein conformation, competitive inhibition, substrate inhibition

---