Regulation of Enzymes in C4 Photosynthesis* *Abbreviations used in the text: NADP-ME-type, NADP malic enzyme type; PCKtype, PEP carboxykinase type; NAD-ME-type, NAD malic enzyme type; PCR cycle, Photosynthetic Carbon Reduction Cycle; PEP, phosphoenolpyruvate.

Abstract

This chapter discusses what is known about the regulation of a metabolic variant of photosynthesis known as the C 4 pathway. Photosynthetic carbon metabolism is an inherently complex process. A cyclic sequence of reactions is essential, as the primary CO 2 acceptor must be continually regenerated from assimilated carbon, and there must be at least one branch point in the cycle from which the accumulated carbon is channeled off into end products. A further degree of complexity is introduced by the dependency of this system upon ATP and NADPH generated by light-dependent processes in chloroplast membranes. Most photosynthetic organisms, including the majority of higher plants, assimilate CO 2 directly into 3-phosphoglycerate (3-PGA) via the enzyme ribulose-1,5-P 2 carboxylase. Subsequent reactions reduce 3-PGA to products that are metabolized to reform ribulose-1,5-P 2 by the pathway known as the photosynthetic carbon reduction (PCR) cycle or the Calvin cycle. The C 4 pathway does not replace the PCR cycle but rather operates as a complex appendage to this cycle.