Photorespiration

Abstract

Photorespiration results in the light-dependent uptake of O2 and release of CO2 Oxygenationof Ribulose-1,5-bisphosphate, an unavoidable process, leads to the production of glycolate-2-P. The recycling of glycolate-2-P into glycerate-3-P via the photorespiratory cycle (C2 cycle) requires a large machinery, consisting of more than 15 enzymes and translocators, distributed over three different organelles, i.e. the chloroplast, peroxisome and mitochondrion. Complex compartmentation is an essential trait of photorespiration. The most fascinating reaction in the C2 cycle occurs in the mitochondria when glycine molecules formed in the peroxisomes are broken down by a complex of proteins (H-, P-, T- and L-proteins) which by concerting their activities, catalyze the oxidative decarboxylation (glycine decarboxylase) and deamination of glycine with the formation of CO2, NH3 and the concomitant reduction of NAD+ to NADH. The remaining carbon of glycine is transferred to tetrahydropteroyl polyglutamate (H4PteGlun or folate) to form CH2H4PteGlun. The H-protein plays a pivotal role in the complete sequence of reactions since its prosthetic group (lipoic acid) interacts successively with the three other components of the complex and undergoes a cycle of reductive methylamination, methylamine transfer and electron transfer. The availability of folate to glycine decarboxylase and its recycling through serine hydroxymethyltransferase (SHMT) reaction is a critical step for glycine oxidation during photorespiration. Numerous shuttles exist to support transamination, ammonia refixation and the supply or export of reductants generated or consumed (via malate-oxaloacetate shuttles) in the photorespiratory pathway. A porin-like channel which is anion selective, represents the major permeability pathway of the peroxisomal membrane. It is tempting to parallel the accumulation of Rubisco in the stroma of the chloroplast with the accumulation of glycine decarboxylase in the matrix of mitochondria because both enzymes reach millimolar concentrations.