Acetyl coenzyme A biosynthesis in the chloroplast : What is the physiological precursor?

Alternative Carbon Sources for Isoprene Emission

Isoprene and other plastidial isoprenoids are produced primarily from recently assimilated photosynthates via the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. However, when environmental conditions limit photosynthesis, a fraction of carbon for MEP pathway can come from extrachloroplastic sources. The flow of extrachloroplastic carbon depends on the species and on leaf developmental and environmental conditions. The exchange of common phosphorylated intermediates between the MEP pathway and other metabolic pathways can occur via plastidic phosphate translocators. C1 and C2 carbon intermediates can contribute to chloroplastic metabolism, including photosynthesis and isoprenoid synthesis. Integration of these metabolic processes provide an example of metabolic flexibility, and results in the synthesis of primary metabolites for plant growth and secondary metabolites for plant defense, allowing effective use of environmental resources under multiple stresses.

Acetyl-CoA-carboxylase activity in normally developing wheat leaves

In order to investigate the role of acetyl CoA carboxylase (ACC) in the regulation of fatty-acid biosynthesis in chloroplasts, the activities and relative amounts of the enzyme have been measured in the tissue of wheat (Triticum aestivum L.) leaves undergoing development and cellular differentiation. The total activity in the first leaves of 5- to 7-d-old plants was similar but decreased to less than half in 9-d-old plants. The activity of ACC in the cells of the first leaf of 7-d-old plants doubled when cell age increased from 24 to 48 h, remained relatively constant for a further 24 h and then declined. The amount of ACC in cells increased 15-fold during the first 36 h of cell enlargement. Cells more than 36 h old contained about two-thirds the maximum amount of ACC found in younger cells. The most rapid phase of fatty-acyl accumulation in lipids was in cells aged between 60 and 84 h. Tenfold changes in the activity of ACC were observed when the assay conditions with respect to ATP, ADP, Mg(2+) and pH were changed to correspond to the physiological conditions in chloroplasts during light/dark transitions. This observation and the magnitude of the changes in the optimum activity and amount of ACC in leaf cells undergoing development are consistent with a role for ACC in the regulation of the flow of carbon from acetyl CoA to fatty acids in chloroplasts.

Carnitine acetyltransferase in pea cotyledon mitochondria

Purified pea cotyledon mitochondria did not oxidise acetyl-CoA in the presence of carnitine. However, acetylcarnitine was oxidised. It was concluded that acetylcarnitine passed through the mitochondrial membrane barrier but acetyl-CoA did not. Only a sensitive radioactive assay detected carnitine acetyltransferase in intact mitochondrion or intact mitoplast preparations. When the mitochondria or mitoplasts were burst, acetyl-CoA substrate was available to the matrix carnitine acetyltransferase and a high activity of the enzyme was measured. The inner mitochondrial membrane is there-fore the membrane barrier to acetyl-CoA but acetylcarnitine is suggested to be transported through this membrane via an integral carnitine: acylcarnitine translocator. Evidence is presented to indicate that when the cotyledons from 48-h-grown peas are oxidising pyruvate, acetylcarnitine formed in the mitochondrial matrix by the action of matrix carnitine acetyltransferase may be transported to extra-mitochondrial sites via the membrane translocator.

Carnitine acyltransferases in chloroplasts of Pisum sativum L

Carnitine-acetyltransferase (EC 2.3.1.7) and carnitine-palmitoyltransferase (EC 2.3.1.21) activities were shown to be present in chloroplasts of green pea leaves and possibly to occur in leaf mitochondrial and peroxisomal fractions. A role for the enzymes in the transfer of acyl groups across membranes is suggested.

Oleate metabolism in microsomes from developing leaves ofPisum sativum L

Microsomes from young leaves of pea,Pisum sativum L., metabolized oleate principally by the reactions mediated by oleoyl-CoA synthetase, oleoyl-CoA thioesterase, oleoyl-CoA: phosphatidylcholine acyltransferase and oleoyl phosphatidylcholine desaturase. Hydrogen peroxide specifically inhibited oleate desaturation and the evidence presented argues for a specific inhibition of the terminal enzyme of the desaturase system, i.e. oleoyl phosphatidylcholine desaturase. Catalase, ascorbic acid, or ascorbate peroxidase, in conjunction with ascorbic acid, stimulated oleate desaturation, possibly by the removal of hydrogen peroxide. Lysophosphatidylcholine was found to be the preferred acceptor for acyl transfer from oleoyl-CoA, which indicates that the transfer of oleoyl moieties was catalyzed predominantly by oleoyl-CoA:lysophosphatidylcholine acyltransferase. Acyl exchange between oleoyl-CoA and phosphatidylcholine, with a possible involvement of phospholipases, was also detected but at much lower rates than acyl transfer. When intact or broken chloroplasts were added to microsomes, which had been preincubated with oleoyl-CoA, some stimulation of the reactions catalyzed by oleoyl-CoA:phosphatidylcholine acyltransferase and oleoyl phosphatidylcholine desaturase was observed. However, only minor amounts of microsomal linoleoyl phosphatidylcholine were converted to galactolipids containing linolenoyl moieties.

The synthesis of short- and long-chain acylarnitine by etio-chloroplasts of greening barley leaves

Etio-chloroplasts of barley, purified on sucrose density gradients were shown to possess carnitine long-chain acyltransferase (carnitine palmitoyltransferase, EC 2.3.1.21) activity and carnitine short-chain acyltransferase (carnitine acetyltransferase EC 2.3.1.7) activity. These enzymes may play a role in the transport of acyl groups as acylcarnitines through the membrane barrier of barley etio-chloroplasts and also 'or alternatively' may spare CoA by transferring short- and long-chain acyl groups from short-and long-chain acyl CoA to carnitine.