THE PLASTIDIC 3-PHOSPHOGLYCERATE-]ACETYL-COA PATHWAY IN BARLEY LEAVESAND ITS INVOLVEMENT IN THE SYNTHESIS OF AMINO-ACIDS, PLASTIDIC ISOPRENOIDS AND FATTY-ACIDS DURING CHLOROPLAST DEVELOPMENT

Earlier studies on the synthesis of C3-derived amino acids, plastidic isoprenoids and fatty acids from CO2 by isolated chloroplasts in the l ight indicate the presence of a complete, but low-capacity, chloroplas t (chlp) 3-phosphoglycerate --> acetyl-CoA pathway which is predominan tely active in immature (developing) chloroplasts (A. Heintze et al., 1990, Plant Physiol. 93, 1121-1127). In this paper, we demonstrate the activity of the enzymes involved i.e. chlp phosphoglycerate mutase, c hlp enolase, chlp pyruvate kinase and chlp pyruvate-dehydrogenase comp lex (PDC), in the stroma of purified barley (Hordeum sativum L.) chlor oplasts of different developmental stages. The chlp phosphoglycerate m utase was partially purified for the first time. The activities of the enzymes of this chlp pathway (except PDC) were about a magnitude lowe r than those of the cytosolic enzymes. The chlp PDC of barley was more active than that of spinach. The apparent K(m) values of the enzymes of this pathway were about 100 muM or lower except for the chlp phosph oglycerate mutase which had a K(m) of 1.6-1.8 mM for 3-phospho-D-glyce rate. Interestingly, no appreciable change in the activity of these en zymes was observed during maturation of the chloroplasts. In contrast, the activity of the reversible NADP+-glyceraldehyde 3-phosphate dehyd rogenase increased about five times (from 140 to 590 nkat per g leaf d ry weight). The following hypothesis is put forward to explain the reg ulation of carbon metabolism during chloroplast development: 3-phospho -D-glycerate is withdrawn from a common pool by the actions of 3-phosp hoglycerate kinase and NADP+-glyceraldehyde-3-phosphate dehydrogenase, the activity of which increases considerably during maturation of chl oroplasts. This leads to an insufficient supply of 3-phospho-glycerate for the chlp phosphoglycerate mutase, which has a low affinity for it s substrate.