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Aldose Reductase (2IKH)
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IntroductionAldose reductase is an oxidoreductase/dehydrogenase enzyme.[1] Aldose reductase can reduce the aldehyde group of aldoses, aliphatic, aromatic aldehydes and some keto groups from aromatic and aliphatic ketones to their corresponding alcohol products using NADPH as a cofactor.[2][3] Aldose reductase is most well known in the first step of the polyol pathway of glucose metabolism.[2][3] Polyol Pathway and DiabetesThe polyol pathway involves the synthesis of fructose from glucose, but does not require energy from ATP like glycolysis does.[2][3][1] The first step of the pathway is the production of sorbitol from glucose, catalyzed by aldose reductase and using NADPH as a reducing cofactor.[2][3] The second step in the pathway is the production of fructose from sorbitol, catalyzed by sorbitol dehydrogenase using NAD+.[2][3] Under normal blood glucose levels most glucose is metabolized through glycolysis or the pentose phosphate pathway while only a small amount of glucose is metabolized through the polyol pathway.[3] Under the hyperglycemic conditions of diabetes the flux of glucose through the polyol pathway is increased.[2][3] This causes osmotic and oxidative stress, which can cause pathological interferences with cytokine signalling, regulation of apoptosis, and activation of kinase cascades.[2] For example, under increased glucose flux through the polyol pathway protein kinase C activivty increases, which causes smooth muscle cell proliferation of blood vessels in agreement with atherosclerosis.[2] This explains estimates that 75-80% of adults with diabetes die from complications of atherosclerosis.[2] Aldose reductase is located in the cornea, retina, lens, kidneys, and myelin sheath.[1] This correlates with long-term complications such as retinopathy, nephropathy, neuropathy, cataracts, and angiopathy.[2] Aldose reductase inhibitors are possible beneficial treatment options for diabetes.[2] StructureAldose reductase is a 36kDa aldo-keto reductase made of a single 315 amino acid residue polypeptide chain.[2][3] It has a (β/α)8- structural motif made of 8 parallel β-strands connected to 8 peripheral α-helices running anti-parallel to the β-strands.[2][3] Including the β-strands and α-helices of the TIM barrel, aldose reductase has a total of 10 . The catalytic active site is located at the C-terminal loop of the enzyme deeply buried inside the barrel core.[2][3] This site consists of residues that are most likely involved in the catalytic reaction (including residues Tyr48, Lys77, His110).[2] The cofactor is situated at the top of the barrel with the nicotinamide ring projecting down the center of the barrel and the pyrophosphate straddling the lip of the barrel.[3] Trp111 and the nicotinamide moiety of NADPH interact with the head group of most .[2] Hydrophobic contacts can be formed by the side-chains of Trp20, Val47, Trp79, and Trp219.[2] Aldose Reductase Structure and InhibitorsMost inhibitors that bind tightly to aldose reductase have a polar group, which is usually a carboxylate, that is attached to a hydrophobic core.[3] Inhibitors bind with their polar head group oriented close to the pyridine ring and usually form hydrogen bonds with Tyr48, His110, and Tyr111.[3] Hydrophobic interactions between the inhibitor and the residues that line the active site help to stabilize the ternary enzyme-coenzyme-inhibitor complex. [3] MechanismThe exact mechanism of the operation of the enzyme is under discussion.[2] NADPH binds to the polypeptide first, followed by the substrate.[3] The binding of NADPH induces a conformational change that involves a hinge-like movement of the surface loop (residues 213-217) so it covers part of the NADPH like a safety belt.[3] NADPH donates a hydride ion to the carbonyl carbon of the aldehyde.[1][2] The hydride transferred from NADPH to glucose comes from C-4 of the nicotinamide ring at the base of the hydrophobic cavity.[3] Most likely then the transfer of a proton from one of the neighbouring acidic residues to the intermediately formed substrate ion occurs.[2] Tyr48, His110, and Cys298 are all within a proper distance of C-4 to be potential proton donors.[3] Evolutionary, thermodynamic, and molecular modeling evidence predicted that Tyr48 was the proton donor, which was later confirmed by mutagenesis studies.[3] Hydrogen-bonding interactions between the phenolic hydroxyl group of Tyr48 and the ammonium side chain of Lys77 are thought to help facilitate hydride transfer.[3] Lys 77 is salt linked to the carboxylate of Asp43.[3] After the reaction occurs and the alcohol product has been released another conformational change occurs to release the NADP+.[3] Kinetic studies have shown that the reorientation of the loop to release the NADP+ may be the rate-limiting step.[3] Thus, disturbing the interactions that stabilize the coenzyme binding can have dramatic effects on the maximum rate of the reaction.[3] References
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