Phosphoeno/pyruvate carboxylation

The initial step in the C4 pathway is the carboxylation of phosphoeno/pyruvate (PEP) in the cytoplasm of the mesophyll cells. The reaction is catalysed by PEP carboxylase (Eqn. 4). 

The earlier flux analyses based on positional enrichment patterns did not succeed in resolving the two C3-carboxylating enzymes phosphoeno/pyruvate carboxylase (PEPCx) and pyruvate carboxylase (PyrCx) because the carbon routes in both reactions to oxaloacetate are identical and because no differences... 

C4 plants green plants in which the primary product of CO2 fixation is not 3-phosphoglycerate (cf. C3 plants) but a C4 acid such as oxaloacetate, malate or aspartate. These plants possess two types of photi>-synthesizing cells. In mesophyll cells near the leaf surface, CO2 is fixed into C4-compounds. This prefixation of CO2 is due to the action of the cytosolic enzyme, phosphoeno/pyruvate carboxylase (EC 4.1.1.31), which carboxylates phosphoenolpyruvate to oxaloacetic acid (see Hatch-Slack-Kortschak cycle). The Calvin cycle (see) operates in the the vascular bundle cells of C4 plants, and CO2 for the Calvin cycle is derived from the decarboxylation C4 compounds rather than directly from the atmosphere. This Kranz anatomy , i.e. photosynthetically active bundle sheath cells with a photosynthetically active layer... 

Conversion of acetyl-CoA to triose phosphate (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate) requires that two of the reactions of the catabolic pathway be replaced by alternative reactions catalysed by different enzymes these are (i) the pyruvate dehydrogenase complex-catalysed oxidative decarboxylation of pyruvate to acetyl-CoA, which is replac by the pyruvate synthase (EC 1.2.7.1)-catalysed reductive carboxylation of acetyl-CoA (eq. 3 I, Fig.), in which the reductant is reduced ferredoxin (Fd,ed). and (ii) the pyruvate kinase (EC 2.7.1.40)-catalysed, ATP-generating conversion of phosphoeno/pyruvate to pyruvate, which is replaced by the pyruvate, orthophosphate dikinase (EC 2.7.9.1)-catalysed, ATP-dtiven conversion of pyruvate to phosphoeno/pyruvate (eq. 4 F, Fig.),... 

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