Substrate Flow from Photosynthetic Carbon Metabolism to Chloroplast Isoprenoid Synthesis in Spinach Evidence for a Plastidic Phosphoglycerate Mutase

Abstract

Abstract The carbon flow from 3-phosphoglycerate to pyruvate and acetyl-CoA within the chloroplast as well as the pathway for the formation of β-carotene, plastoquinone-9 etc. as plastidic isoprenoids from photosynthetically fixed CO2 hitherto remained unclear because the presence of the phos- phoglycerate mutase in chloroplasts had not unequivocally been proven. To clarify this question, the incorporation of 14CO2, [2-14C]acetate, [2-14C]pyruvate, [2-14C]glycine, [3-14C]serine and [2-14C]mevalonate into β-carotene, plastoquinone-9, sterols and fatty acids was studied using spinach and barley protoplasts and barley seedlings. In protoplasts as well as in seedlings under conditions of CO2 fixation, the largest portion of the acetyl-CoA derived compounds formed was β-carotene and plastoquinone-9 rather than fatty acids and sterols. High rates of fatty acid synthesis were obtained by supplying acetate as well. Mevalonate was incorporated into sterols but not into β-carotene and plastoquinone-9. Direct evidence was obtained for the hitherto questioned plastidic phosphoglycerate mutase. Low activities were found in spinach chloroplasts which were substantiated by the criteria of latency method. Substrate flow from 3-phosphoglycerate to pyruvate via 2-phosphoglycerate and phosphoenolpyruvate was shown by applying [1-14C]- glycerate and following its incorporation. From this the following conclusions were drawn: (i) The plastidic IPP synthesizing system is strongly separated from the cytosolic-ER one. (ii) As the formation of β-carotene and plastoquinone-9 is favoured under conditions of photosynthetic CO2 fixation, a direct carbon flow from 3-phosphoglycerate to isoprenoids via acetyl-CoA by a low- capacity pathway, which shows high affinities for the substrates, is suggested.