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 CO 2 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 14 CO 2 , [2- 14 C]acetate, [2- 14 C]pyruvate, [2- 14 C]glycine, [3- 14 C]serine and [2- 14 C]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 CO 2 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- 14 C]- 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 CO 2 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.