Figure 1

An example of isotope exchange in one of the reactions of non-oxidative pentose phosphate pathway catalyzed by transketolase: xu5p + r5p <-> g3p + s7p. The catalytic cycle consists of a series of reversible elementary steps: binding of donor substrate (xu5p) and formation (k₁, k_-1) of a covalent enzyme-substrate complex (E*xu5p); splitting (k₂, k_-2) of donor substrate and formation of a covalently bound intermediate (the α-carbanion of α, β-dihydroxyethyl-ThDP, the so-called 'active glycolaldehyde') and an aldose (g3p); both are localized in the active site of the enzyme (EG*g3p). This complex dissociates (k₃, k_-3) into the complex of the enzyme with active glycolaldehyde (EG) and the first product, free aldose (g3p). In the second half-reaction, active glycolaldehyde interacts with the other aldose (r5p) available in the reaction mixture (k₄, k_-4). The new ketose (s7p) is released from the enzyme-substrate complex after passing through the same reaction steps in reverse order (k₅, k_-5 and k₆, k_-6). Large circles represent the protein molecule, while small linked circles represent the carbon skeleton of the metabolites. Two dark circles represent the part of substrate attached to the enzyme during whole catalytic cycle and to be transferred between ketoses. The gray circles are the parts released after ketose splitting. Stared circles are labeled carbon atoms. The scheme presents, as an example, formation of non-labeled g3p and double labeled s7p from xu5p labeled in first position and r5p labeled in third position.