6-Phosphogluconolactonase

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6-phosphogluconolactonase catalyzes the hydrolysis of 6-phosphogluconolactone to 6-phosphogluconate in the oxidation phase of the pentose phosphate pathway, and is a cytoplasmic enzyme found in all organisms. The tertiary structure of 6PGL is folded by α/β hydrolase, and the active site residues are gathered on the loop of the α-helix. According to the crystal structure of the enzyme, this mechanism is thought to rely on the proton transfer of histidine residues in the active site. 6PGL selectively catalyzes the hydrolysis of δ-6-phosphogluconolactone and has no activity on the γ isomer.

Enzyme structure

The 6PGL in Homo sapiens exists as a monomer under cytoplasmic physiological conditions, consisting of 258 amino acid residues, with a total molecular weight of about 30 kDa. The tertiary structure of the enzyme uses α/β hydrolase to fold, and the parallel and anti-parallel β-sheet layers are surrounded by 8 α helices and 5 310 helices. The stability of the tertiary structure of the protein is enhanced by the salt bridge between aspartic acid. Acid and arginine residues, as well as from the stacking interaction of aromatic side chains.

Figure 1. Structure of 6-phosphogluconolactonase.

Mechanism

It has been proposed that the hydrolysis of 6-phosphogluconolactone from 6PGL to 6-phosphogluconate is carried out by transferring protons to the O5 epoxy atom, similar to xylose isomerase and ribose-5-phosphate isomerase. This reaction is initiated by the hydroxide ion attacking the C5 ester. This is followed by the formation of tetrahedral intermediates and the elimination of ester bonds, which is achieved by donating protons from the active site histidine residues. The specific residues involved in proton transfer have troubled researchers until 2009, because previous structural studies have shown that there are two possible substrate conformations at the active site, positioning the O5 epoxy at arginine or histidine. The proximal end of the acid residue. Molecular dynamics simulations were used to find that the proton-donating residue is histidine, while the arginine residue is only related to the electrical stability of the negatively charged phosphate group. The electrical stabilization of the enzyme-substrate complex also occurs between the product carboxylate and the backbone amine of the surrounding glycine residues.

Function

6-phosphogluconolactonase catalyzes the conversion of 6-phosphogluconolactone to 6-phosphogluconate, which are two intermediates in the oxidation phase of the pentose phosphate pathway, where glucose is converted to 5-phosphoribose. The oxidative phase of the pentose phosphate pathway releases carbon dioxide and results in the production of two equivalents of NADPH from NADP+. The final product, ribose 5-phosphate, is further processed by organisms in the non-oxidative stage of the pentose phosphate pathway to synthesize biomolecules including nucleotides, ATP and coenzyme A. The enzyme glucose-6-phosphate dehydrogenase that precedes 6PGL in the pentose phosphate pathway exclusively forms the delta-isomer of 6-phosphogluconolactone. However, if accumulated, the compound may undergo intramolecular rearrangement to isomerize to a more stable γ-form, which cannot be hydrolyzed by 6PGL and cannot continue to the non-oxidative phase of the pentose phosphate pathway. By rapidly hydrolyzing the δ-isomer of 6-phosphogluconolactone, 6PGL can prevent its accumulation and subsequent formation of γ-isomer, which wastes the glucose resources available to the cell. 6-phosphogluconolactone is also vulnerable to the attack of intracellular nucleophiles, which is confirmed in the α-N-6-phosphogluconate reaction of His-tagged proteins expressed in E. coli, and 6PGL is effectively hydrolyzed 6-phosphogluconolactone can prevent lactone accumulation and toxic reaction between lactone intermediates and cells

Clinical significance

It has been shown that Plasmodium berghei and Plasmodium falciparum express a bifunctional enzyme, which has the activities of both glucose 6 phosphate dehydrogenase and 6 phosphate gluconolactonase, which enables it to catalyze the front of the pentose phosphate pathway. Two steps. This bifunctional enzyme has been identified as a treatable target for malaria parasites, and high-throughput screening of small molecule inhibitors has led to the discovery of novel compounds that can potentially be transformed into effective antimalarials.