Hydron transfer catalyzed by triosephosphate isomerase. products of isomerization of dihydroxyacetone phosphate in D2O

The product distributions for the reactions of dihydroxyacetone phosphate (DHAP) in D2O at pD 7.9 catalyzed by triosephosphate isomerase (TIM) from chicken and rabbit muscle were determined by H-1 NMR spectroscopy using glyceraldehyde 3-phosphate dehydrogenase to trap the first-formed products of the thermodynamically unfavorable isomerization reaction, (R)-glyceraldehyde 3-phosphate (GAP) and [2(R)-H-2]-GAP (d-GAP). Three products were observed from the reactions catalyzed by TIM: GAP from isomerization with intramolecular transfer of hydrogen (18% of the enzymatic products), d-GAP from isomerization with incorporation of deuterium from D2O into C-2 of GAP (43% of the enzymatic products), and [1(R)-H-2]-DHAP (d-DHAP) from incorporation of deuterium from D2O into C-1 of DHAP (40% of the enzymatic products). The ratios of the yields of the deuterium-labeled products d-DHAP and d-GAP from partitioning of the intermediate of the TIM-catalyzed reactions of GAP and DHAP in D2O are 1.48 and 0.93, respectively. This provides evidence that the reaction of these two substrates does not proceed through a single, common, reaction intermediate but, rather, through distinct intermediates that differ in the bonding and arrangement of catalytic residues at the enediolate O-1 and O-2 oxyanions formed on deprotonation of GAP and DHAP, respectively.