Role of plant glyoxylate reductases during stress: a hypothesis

Abstract

Molecular modelling suggests that a group of proteins in plants known as the beta-hydroxyacid dehydrogenases, or the hydroxyisobutyrate dehydrogenase superfamily, includes enzymes that reduce succinic semialdehyde and glyoxylate to gamma-hydroxybutyrate and glycolate respectively. Recent biochemical and expression studies reveal that NADPH-dependent cytosolic (termed GLYR1) and plastidial (termed GLYR2) isoforms of succinic semialdehyde/glyoxylate reductase exist in Arabidopsis. Succinic semialdehyde and glyoxylate are typically generated in leaves via two distinct metabolic pathways, gamma-aminobutyrate and glycolate respectively. In the present review, it is proposed that the GLYRs function in the detoxification of both aldehydes during stress and contribute to redox balance. Outstanding questions are highlighted in a scheme for the subcellular organization of the detoxification mechanism in Arabidopsis.

Figure 1. Molecular models of HIBADH family members

(A) Crystal structure of TSAR (tartronic semialdehyde reductase) and its substrate analogue tartaric acid (TA) (PDB entry: 1VPD). (B) Predicted structure of GLYR1 generated by the fold-recognition server PHYRE (http://www.sbg.bio.ic.ac.uk/phyre/) using TSAR as a template. (C and D) Overlapping positions and identities of the predicted active-site residues for TSAR and GLYR1 respectively. Red, blue and green indicate negative, positive and hydrophobic regions of the residues.

Figure 2. The GABA shunt and its relationship with other metabolic pathways

The GABA shunt metabolites and enzymes are indicated as blue and orange balloons respectively, and the TCA cycle is shown as black and pink balloons. Glutamate, the precursor to GABA, is supplied predominantly via the glutamine synthetase/glutamate synthase cycle or the TCA cycle. The polyamine putrescine has been proposed as an alternative GABA precursor, although in planta contribution in mature leaves is likely to be minor. There are two branching routes for GABA catabolism via SSA: one to succinate and the TCA cycle and the other to GHB via SSA reductase (a yellow balloon, shown as GLYR in Figure 3), although the products of GHB catabolism in plants are currently unknown.

Figure 3. Proposed role for GLYRs during stress

The subcellular scheme relates recent and hypothetical information about GLYRs and NADKs to pyridine nucleotides and specific intermediates of the GABA and glycolate pathways. There is no consideration of stoichiometry in this scheme. Enzymes of the GABA and glycolate pathways, as well as photorespiratory nitrogen metabolism, are shown as orange, pink and brown balloons respectively. Enzymes associated with aldehyde detoxification and NADPH formation are shown as blue and turquoise balloons respectively. Components whose levels are probably elevated in response to stress are shown as yellow boxes, and potential diffusion pathways are shown as dotted lines. For further discussion and references, please see the text. FR, ferredoxin reductase; GGAT, glutamate:glyoxylate aminotransferase; GK, glycerate kinase; GO, glycolate oxidase; GOGAT, glutamate synthase; GS, glutamine synthetase; HPR, hydroxypyruvate reductase; PGP, phosphoglycolate phosphatase; SGAT, serine:glyoxylate aminotransferase; SHMT, serine hydroxymethyltransferase.