Mechanisms of interaction between photorespiration and respiration in photosynthetic cells

Abstract

Photorespiratory oxidation of glycine is a main activity for leaf mitochondria of C3 plants in the light. Glycine oxidation liberates ammonia and CO2 and increases the reduction level of both NAD(H) and NADP(H) pools in mitochondria. Reoxidation of photorespiratory NADH by the mitochondrial electron transport chain can lead to production of ATP which is exported to the cytosol. However, glycine oxidation also induces non-coupled pathways of electron transport including the alternative oxidase why the yield of ATP can vary. By the effects of ammonia, elevated NADH and ATP, glycine oxidation decreases the activation state of the pyruvate dehydrogenase complex and limits the activity resulting in restriction in the entry of glycolytic substrates to the tricarboxylic (TCA) cycle. An additional consequence of high reduction levels of mitochondrial NAD(H) and NADP(H) pools is suppression of isocitrate oxidation. The regulation of both NAD-ICDH and NADP-ICDH by the reduction level in mitochondria represents a mechanism to operate a partial TCA cycle in the light. Oxaloacetate formed from glycolysis is converted in mitochondria to citrate, which is transported to the cytosol. The reaction of cytosolic NADP-ICDH supplies 2-oxoglutarate for photorespiratory ammonia refixation and maintains an appreciable reduction level of the cytosolic NADP in the light. Taken together photorespiratory and respiratory carbon fluxes in the light form a highly flexible system to distribute the demands of energy (ATP) and reducing equivalents (NADH, NADPH) between different compartments.