Localization of Glycerate Kinase and Some Enzymes for Sucrose Synthesis in C 3 and C 4 Plants

Abstract

The localization of some key enzymes leading to sucrose synthesis in photosynthetic tissue of C(3) and C(4) species was investigated. These included UDP-glucose (UDPG) pyrophosphorylase, sucrose phosphate synthetase, and glycerate kinase. Whether glycerate kinase is localized exclusively in the chloroplast or partly outside the chloroplast could influence the fate of carbon flow to sucrose through the glycolate pathway.In the C(3) species wheat, intact chloroplasts were isolated from protoplasts. Following separation of the chloroplasts by differential centrifugation and by sucrose density gradient centrifugation, UDPG pyrophosphorylase, sucrose phosphate synthetase, and sucrose synthetase were found outside the chloroplast while glycerate kinase was localized in the chloroplast.In the C(4) species (maize, NADP-malic enzyme-type; Panicum miliaceum, NAD-malic enzyme type and Brachiaria erucaeformis, phosphoenolpyruvate carboxykinase type) the distribution of UDPG pyrophosphorylase and glycerate kinase between mesophyll and bundle sheath cells and their intracellular localization in mesophyll protoplasts was determined. Substantial activity of UDPG pyrophosphorylase was found in both mesophyll and bundle sheath cells while glycerate kinase was localized only in mesophyll cells. From C(4) mesophyll protoplasts, UDPG pyrophosphorylase was found to be cytoplasmic while glycerate kinase appears exclusively localized in the chloroplasts as determined by differential centrifugation and sucrose density gradients.It seems that in both C(3) and C(4) plants, the terminal steps of sucrose synthesis occur exclusively in the cytoplasm while carbon flow in the glycolate pathway to sucrose must occur through glycerate kinase in the chloroplasts. The localization of glycerate kinase in mesophyll cells of C(4) plants may have further implications for intercellular flow of carbon during metabolism in the glycolate pathway in these species.