Aldose reductase

In

Aldose reductase catalyzes the NADPH-dependent conversion of glucose to sorbitol, the first step in polyol pathway of glucose metabolism. The second and last step in the pathway is catalyzed by sorbitol dehydrogenase, which catalyzes the NAD-linked oxidation of sorbitol to fructose. Thus, the polyol pathway results in conversion of glucose to fructose with stoichiometric utilization of NADPH and production of NADH.^[1]

glucose + NADPH + H⁺ ${\displaystyle \rightleftharpoons }$ sorbitol + NADP⁺

Galactose is also a substrate for the polyol pathway, but the corresponding keto sugar is not produced because sorbitol dehydrogenase is incapable of oxidizing galactitol.^[2] Nevertheless, aldose reductase can catalyze the reduction of galactose to galactitol

galactose + NADPH + H⁺ ${\displaystyle \rightleftharpoons }$ galactitol + NADP⁺

Function

The aldose reductase reaction, in particular the sorbitol produced, is important for the function of various organs in the body. For example, it is generally used as the first step in a synthesis of fructose from glucose; the second step is the oxidation of sorbitol to fructose catalyzed by sorbitol dehydrogenase. The main pathway from glucose to fructose (glycolysis) involves phosphorylation of glucose by hexokinase to form glucose 6-phosphate, followed by isomerization to fructose 6-phosphate and hydrolysis of the phosphate, but the sorbitol pathway is useful because it does not require the input of energy in the form of ATP:

• Seminal vesicles: Fructose produced from sorbitol is used by the sperm cells
  .
• Liver: Fructose produced from sorbitol can be used as an energy source for glycolysis and glyconeogenesis.

Aldose reductase is also present in the

In Drosophila, CG6084 encoded a highly conserved protein of human Aldo-keto reductase 1B. dAKR1B in hemocytes, is necessary and sufficient for the increasement of plasma sugar alcohols after gut infection. Increased sorbitol subsequently activated Metalloprotease 2, which cleaves PGRP-LC to activate systemic immune response in fat bodies. Thus, aldose reductase provides a critical metabolic checkpoint in the global inflammatory response.[3]

Enzyme structure

Aldose reductase may be considered a prototypical enzyme of the aldo-keto reductase enzyme superfamily. The enzyme comprises 315 amino acid residues and folds into a β/α-barrel structural motif composed of eight parallel β strands.^[4] Adjacent strands are connected by eight peripheral α-helical segments running anti-parallel to the β sheet.^[5] The catalytic active site situated in the barrel core.^[5][6] The NADPH cofactor is situated at the top of the β/α barrel, with the nicotinamide ring projects down in the center of the barrel and pyrophosphate straddling the barrel lip.^[1]

Enzyme mechanism

The reaction mechanism of aldose reductase in the direction of aldehyde reduction follows a sequential ordered path where NADPH binds, followed by the substrate. Binding of NADPH induces a conformational change (Enzyme•NADPH → Enzyme*•NADPH) that involves hinge-like movement of a surface loop (residues 213–217) so as to cover a portion of the NADPH in a manner similar to that of a safety belt. The alcohol product is formed via a transfer of the pro-R hydride of NADPH to the re face of the substrate's carbonyl carbon. Following release of the alcohol product, another conformational change occurs (E*•NADP⁺ → E•NADP⁺) in order to release NADP⁺.^[8] Kinetic studies have shown that reorientation of this loop to permit release of NADP⁺ appears to represent the rate-limiting step in the direction of aldehyde reduction.^[9][10][11] As the rate of coenzyme release limits the catalytic rate, it can be seen that perturbation of interactions that stabilize coenzyme binding can have dramatic effects on the maximum velocity (Vmax).^[11]

The hydride that is transferred from NADP⁺ to glucose comes from C-4 of the nicotinamide ring at the base of the hydrophobic cavity. Thus, the position of this carbon defines the enzyme's active site. There exist three residues in the enzyme within a suitable distance of the C-4 that could be potential proton donors: Tyr-48, His-110 and Cys-298. Evolutionary, thermodynamic and molecular modeling evidence predicted Tyr-48 as the proton donor. This prediction was confirmed the results of mutagenesis studies.^[5][12][13] Thus, a [hydrogen-bonding] interaction between the phenolic hydroxyl group of Tyr-48 and the ammonium side chain of Lys-77 is thought to help to facilitate hydride transfer.^[5]

Role in diabetes

• AKR1B1
• Aldo-keto reductase

References