| Abstract
| - Phosphoglucose isomerase (PGI, EC 5.3.1.9) catalyzes the interconversion of d-glucose6-phosphate (G6P) and d-fructose 6-phosphate (F6P) and plays important roles in glycolysis andgluconeogenesis. Biochemical characterization of the enzyme has led to a proposed multistep catalyticmechanism. First, the enzyme catalyzes ring opening to yield the open chain form of the substrate. Thenisomerization proceeds via proton transfer between C2 and C1 of a cis-enediol(ate) intermediate to yieldthe open chain form of the product. Catalysis proceeds in both the G6P to F6P and F6P to G6P directions,so both G6P and F6P are substrates. X-ray crystal structure analysis of rabbit and bacterial PGI haspreviously identified the location of the enzyme active site, and a recent crystal structure of rabbit PGIidentified Glu357 as a candidate functional group for transferring the proton. However, it was not clearwhich active site amino acid residues catalyze the ring opening step. In this paper, we report the X-raycrystal structure of rabbit PGI complexed with the cyclic form of its substrate, d-fructose 6-phosphate, at2.1 Å resolution. The location of the substrate relative to the side chains of His388 suggest that His388promotes ring opening by protonating the ring oxygen. Glu216 helps to position His388, and a watermolecule that is held in position by Lys518 and Thr214 accepts a proton from the hydroxyl group at C2.Comparison to a structure of rabbit PGI with 5PAA bound indicates that ring opening is followed by lossof the protonated water molecule and conformational changes in the substrate and the protein so that ahelix containing amino acids 513−520 moves in toward the substrate to form additional hydrogen bondswith the substrate.
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