Effect of the substrate on the binding of coenzyme and coenzyme analogues to 6-phosphogluconate dehydrogenase from Candida utilis

6-Phosphogluconate dehydrogenase from Candida utilis is a dimeric enzyme with apparently identical subunits. Two coenzyme analogues, periodate-oxidized NADP+ and 3-amino pyridine adenine dinucleotide phosphate, bind to the protein with Kdiss of 42 microM and 6.2 microM, respectively. NADPH binds to the enzyme with a Kdiss of 0.42 microM. Both coenzyme and coenzyme analogues show "all-of-the sites reactivity." In the presence of the substrate 6-phosphogluconate, the Kdiss of the coenzyme analogues is lowered to 3.5 microM and 2.4 microM, respectively, whereas the Kdiss of NADPH is increased to 2.7 microM. Coenzyme analogues, but not NADPH, show an half of-the sites reactivity in the presence of 6-phosphogluconate. The enzymatic activity is inhibited by high NADP+ concentrations in presence of low concentrations of substrate, whereas at high substrate concentrations no inhibition by NADP+ is showed. These data suggest that: 1) the enzyme exhibits an half-of-the-sites reactivity during the catalytic cycle; 2) the recycle of the enzyme occurs mainly through an abortive ternary complex enzyme-substrate-NADPH