Glyoxylate reductase, first isolated from spinach leaves, is an enzyme that catalyzes the reduction of glyoxylate to glycolate, using the cofactorNADH or NADPH. The systematic name of this enzyme class is glycolate:NAD+ oxidoreductase. Other names in common use include NADH-glyoxylate reductase, glyoxylic acid reductase, and NADH-dependent glyoxylate reductase.
Structure
The crystal structure of the glyoxylate reductase enzyme from the hyperthermophilic archeon Pyrococcus horiskoshii OT3 has been reported. The enzyme exists in the dimeric form. Each monomer has two domains: a substrate-binding domain where glyoxylate binds, and a nucleotide-binding domain where the NADH cofactor binds.
Mechanism
The enzyme catalyzes the transfer of a hydride from NADH to glyoxylate, causing a reduction of the substrate to glycolate and an oxidation of the cofactor to NAD+. Figure 2 shows the mechanism for this reaction. It is thought that the two residues Glu270 and His288 are important for the enzyme's catalytic function, while the residue Arg241 is thought to be important for substrate specificity.
Function
The glyoxylate reductase enzyme localizes to the cell cytoplasm in plants. It can use both NADPH and NADH as a cofactor, but prefers NADPH. The enzyme substrate, glyoxylate, is a metabolite in plant photorespiration, and is produced in the peroxisome. Glyoxylate is important in the plant cell as it can deactivate RUBISCO and inhibit its activation. Hence, glyoxylate levels are important in regulating photosynthesis. The enzyme is thought of as a glyoxylate-glycolate shuttle that helps in the disposal of excess reducing equivalents from photosynthesis. This is supported by the following findings: glycolate biosynthesis in the chloroplasts is highest at low CO2 concentrations, the enzyme is quite specific for the NADPH cofactor which is a final product of electron transfer in the chloroplasts during photosynthesis, and when isolated chloroplasts are exposed to light, they absorb glyoxylate and reduce it, but they do not absorb glycolate. Due to the link between glyoxylate levels and photosynthesis, an increase in glyoxylate levels indicates that the plant is under stress. As glyoxylate levels continue to increase, they can harm the plant by reacting with DNA, oxidizing membrane lipids, modifying proteins, and increasing the transcription of stress-related genes in the plant. This highlights the importance of glyoxylate reductase, as it helps keep plant cells healthy and detoxifies the cell by reducing glyoxylate levels. In the absence of the enzyme, the side-effects of increased glyoxylate activity can cause cellular and developmental problems in the plant. Glyoxylate reductase can be used as a tool for studying photorespiratory carbon metabolism in plant leaves. Such studies can be carried out using acetohydroxamate and aminooxyacetate, which have been found to inhibit glyoxylate reductase activity. These inhibitors are not fully specific, but provide fully reversible inhibition of the enzyme and so provide a flexible tool for metabolic studies in plants.
Glyoxylate is an important component of the glyoxylate cycle, a variant of the citric acid cycle, whereby acetyl-CoA is converted to succinate and then other carbohydrates in plants, bacteria, protists, and fungi. Studies have been conducted to trace the genes for the glyoxylate cycle enzymes to animals. The studies have shown that these genes are in fact present in animals, but the redistribution of the genes suggest that either that these genes encode other enzymes that take part in the glyoxylate cycle, but are not orthologous to the known enzymes in the cycle, or animals have developed a new function for these enzymes that have yet to be characterized.