Session: Current Topics in Biological Chemistry:

The transition state stabilization imparted by the phosphodianion moiety on the reactions catalyzed by 6-phosphogluconate dehydrogenase, glucose 6-phosphate dehydrogenase, and glucose 6-phosphate isomerase were determined by comparing the k_cat/K_m of whole substrates (glucose 6-phosphate or 6-phosphogluconate) and k_cat/K_mK_HPi of the corresponding truncated substrates (D-xylose or D-xylonate) activated by phosphite. Of the 11–13 kcal/mol of stabilization arising from interactions of whole substrate phosphodianion with the enzyme, 4–6 kcal/mol of this binding energy is expressed at the transition state. These results are consistent with similar studies on β-phosphoglucomutase, triosephosphate isomerase, and glycerol 3-phosphate dehydrogenase. The preponderance of phosphodianion activation phenomena across enzymes implicated in sugar metabolism suggests that the utilization of phosphodianion binding energy to drive catalytically favorable conformational changes is a central theme in enzyme catalysis.

Richard Nagorski

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