The plastidic 3-phosphoglycerate ? acetyl-CoA pathway in barley leaves and its involvement in the synthesis of amino acids, plastidic isoprenoids and fatty acids during chloroplast development

Abstract

Earlier studies on the synthesis of C3-derived amino acids, plastidic isoprenoids and fatty acids from CO2 by isolated chloroplasts in the light indicate the presence of a complete, but low-capacity, chloroplast (chlp) 3-phosphoglycerate → acetyl-CoA pathway which is predominantely 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, chlp enolase, chlp pyruvate kinase and chlp pyruvate-dehydrogenase complex (PDC), in the stroma of purified barley (Hordeum sativum L.) chloroplasts of different developmental stages. The chlp phosphoglycerate mutase was partially purified for the first time. The activities of the enzymes of this chlp pathway (except PDC) were about a magnitude lower 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 μM or lower except for the chlp phosphoglycerate mutase which had a K m of 1.6–1.8 mM for 3-phospho-d-glycerate. Interestingly, no appreciable change in the activity of these enzymes was observed during maturation of the chloroplasts. In contrast, the activity of the reversible NADP+-glyceraldehyde 3-phosphate dehydrogenase increased about five times (from 140 to 590 nkat per g leaf dry weight). The following hypothesis is put forward to explain the regulation of carbon metabolism during chloroplast development: 3-phospho-d-glycerate is withdrawn from a common pool by the actions of 3-phosphoglycerate kinase and NADP+-glyceraldehyde-3-phosphate dehydrogenase, the activity of which increases considerably during maturation of chloroplasts. This leads to an insufficient supply of 3-phospho-glycerate for the chlp phosphoglycerate mutase, which has a low affinity for its substrate.