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=Aldose Reductase (2IKH)= | |||
<Structure load='2ikh' size='500' frame='true' align='right' caption=' | <Structure load='2ikh' size='500' frame='true' align='right' caption='Human aldose reductase with bound ligand and NADPH ' scene='Sandbox_Reserved_339/Default_scene/1' /> | ||
[http://en.wikipedia.org/wiki/Aldose_reductase Wikipedia] | Information on aldose reductase is also available at [http://en.wikipedia.org/wiki/Aldose_reductase Wikipedia] | ||
==Introduction== | ==Introduction== | ||
<scene name='Sandbox_Reserved_339/Default_scene/1'>Aldose reductase</scene> is an enzyme that 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.<ref name="Steuber"> PMID 17368668 </ref><ref name="review"> PMID 15094999 </ref> Aldose reductase is most well known in the first step of the polyol pathway of glucose metabolism (Figure 1).<ref name="Steuber"/><ref name="review"/> | |||
[[Image:Polyol_pathway.jpg|left|400px|thumb|Figure 1. Polyol pathway of glucose metabolism. Adapted from Petrash JM. All in the family: aldose reductase and closely related aldo-keto reductases. Cell Mol Life Sci. 2004 Apr;61(7-8):737-49. PMID:15094999 doi:10.1007/s00018-003-3402-3]] | |||
==Polyol Pathway and Diabetes== | |||
As figure 1 shows, the polyol pathway involves the synthesis of fructose from glucose.<ref name="Steuber"/><ref name="review"/> The first step of the pathway is the production of sorbitol from glucose, catalyzed by aldose reductase and using NADPH as a reducing cofactor.<ref name="Steuber"/><ref name="review"/> The second step in the pathway is the production of fructose from sorbitol, catalyzed by sorbitol dehydrogenase using NAD+.<ref name="Steuber"/><ref name="review"/> 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.<ref name="review"/> Under the hyperglycemic conditions of diabetes the flux of glucose through the polyol pathway is increased.<ref name="Steuber"/><ref name="review"/> This causes osmotic and oxidative stress, which can cause pathological interferences with cytokine signalling, regulation of apoptosis, and activation of kinase cascades.<ref name="Steuber"/> 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.<ref name="Steuber"/> This explains estimates that 75-80% of adults with diabetes die from complications of atherosclerosis.<ref name="Steuber"/> Aldose reductase is also linked to long-term diabetic complications such as retinopathy, nephropathy, neuropathy, cataracts, and angiopathy.<ref name="Steuber"/> Aldose reductase inhibitors are possible beneficial treatment options for diabetes.<ref name="Steuber"/> | |||
==Structure== | ==Structure== | ||
Aldose reductase is a 36kDa aldo-keto reductase made of a single 315 amino acid residue polypeptide chain.<ref name="Steuber"/><ref name="review"/> It has a (β/α)8-TIM-barrel structural motif made of 8 parallel β-strands connected to 8 peripheral α-helices running anti-parallel to the β-strands.<ref name="Steuber"/><ref name="review"/> Including the β-strands and α-helices of the TIM barrel, aldose reductase has a total of 10 helices and 13 β-strands. The catalytic active site is located at the C-terminal loop of the enzyme deeply buried inside the barrel core.<ref name="Steuber"/><ref name="review"/> This site | Aldose reductase is a 36kDa aldo-keto reductase made of a single 315 amino acid residue polypeptide chain.<ref name="Steuber"/><ref name="review"/> It has a (β/α)8-<scene name='Sandbox_Reserved_339/Tim_barrel/2'>TIM-barrel</scene> structural motif made of 8 parallel β-strands connected to 8 peripheral α-helices running anti-parallel to the β-strands.<ref name="Steuber"/><ref name="review"/> Including the β-strands and α-helices of the TIM barrel, aldose reductase has a total of 10 <scene name='Sandbox_Reserved_339/Helices_and_strands/1'>α-helices and 13 β-strands</scene>. The catalytic active site is located at the C-terminal loop of the enzyme deeply buried inside the barrel core.<ref name="Steuber"/><ref name="review"/> This site consists of residues that are most likely involved in the catalytic reaction (including residues Tyr48, Lys77, His110).<ref name="Steuber"/> The <scene name='Sandbox_Reserved_339/Nadph/2'>NADPH</scene> 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.<ref name="review"/> Trp111 and the nicotinamide moiety of NADPH interact with the head group of most <scene name='Sandbox_Reserved_339/Ligand/1'>ligands</scene>.<ref name="Steuber"/> Hydrophobic contacts can be formed by the side-chains of Trp20, Val47, Trp79, and Trp219.<ref name="Steuber"/> | ||
===Aldose Reductase Structure and Inhibitors=== | |||
Most inhibitors that bind tightly to aldose reductase have a polar group, which is usually a carboxylate, that is attached to a hydrophobic core.<ref name="review"/> Inhibitors bind with their polar head group oriented close to the pyridine ring and usually form hydrogen bonds with Tyr48, His110, and Tyr111.<ref name="review"/> Hydrophobic interactions between the inhibitor and the residues that line the active site help to stabilize the ternary enzyme-coenzyme-inhibitor complex. <ref name="review"/> | |||
==Mechanism== | ==Mechanism== | ||
The exact mechanism of the operation of the enzyme is under discussion.<ref name="Steuber"/> NADPH binds to the polypeptide first, followed by the substrate.<ref name="review"/> 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.<ref name="review"/> NADPH donates a hydride ion to the carbonyl carbon of the aldehyde. | The exact mechanism of the operation of the enzyme is under discussion.<ref name="Steuber"/> NADPH binds to the polypeptide first, followed by the substrate.<ref name="review"/> 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.<ref name="review"/> NADPH donates a hydride ion to the carbonyl carbon of the aldehyde.<ref name="Steuber"/> The hydride transferred from NADPH to glucose comes from C-4 of the nicotinamide ring at the base of the hydrophobic cavity.<ref name="review"/> Most likely then the transfer of a proton from one of the neighbouring acidic residues to the intermediately formed substrate ion occurs.<ref name="Steuber"/> Tyr48, His110, and Cys298 are all within a proper distance of C-4 to be potential proton donors.<ref name="review"/> Evolutionary, thermodynamic, and molecular modeling evidence predicted that Tyr48 was the proton donor, which was later confirmed by mutagenesis studies.<ref name="review"/> Hydrogen-bonding interactions between the phenolic hydroxyl group of Tyr48 and the ammonium side chain of Lys77 are thought to help facilitate hydride transfer.<ref name="review"/> Lys 77 is salt linked to the carboxylate of Asp43.<ref name="review"/> After the reaction occurs and the alcohol product has been released another conformational change occurs to release the NADP+.<ref name="review"/> Kinetic studies have shown that the reorientation of the loop to release the NADP+ may be the rate-limiting step.<ref name="review"/> Thus, disturbing the interactions that stabilize the coenzyme binding can have dramatic effects on the maximum rate of the reaction.<ref name="review"/> | ||
==References== | ==References== | ||
<references/> | <references/> | ||