The Regulation of Acetyl Coenzyme A Synthesis in Chloroplasts

Abstract The enzymatic activities of the pyruvate dehydrogenase complex (PDC) and acetyl-CoA synthetase (ACS) have been compared in extracts of plastids isolated from spinach leaves and from both green and etiolated pea seedlings. A ll plastid preparations were shown to be capable of synthesizing acetyl-CoA, not only via acetyl-CoA synthetase, but also via the pyruvate dehydroge­ nase complex, though, with different activities. Both pathways are apparently under metabolic control. Thus, the substrate levels in photosynthetically active spinach chloroplasts appear to favor acetyl-CoA synthesis via ACS (apparent Km for acetate of 0.1 mм) , because calculated stromal pyruvate levels (0.1 m M) appear to limit its formation via the PDC (apparent Km for pyruvate of 0.2-0.3 nм) . In spinach chloroplasts, therefore, the PDC pathway seems to be predominantly involved in providing precursors for branched-chain amino acid biosynthesis (vali­ne, leucine and isoleucine). Acetyl-CoA, synthesized via ACS, may additionally function as an inhibitor of the chloroplast PD C , because, as in mitochondria, relatively low concentrations of the end products NADH and acetyl-CoA strongly inhibit the PD C in chloroplast extracts. On the other hand, comparatively high concentrations of MgATP, a cofactor for ACS, inhibited the PDC complex. The pH optimum of about 8 and the high Mg-requirement distinguishes both enzymes from mitochondrial PDC and reflects an accomodation to stromal conditions in photosynthetically active chloroplasts.