Phosphoenolpyruvate carboxykinase in C-4 plants: Its role and regulation

Some of the recent findings which revise our view of the role and regulation of phosphoenolpyruvate carboxykinase (PEPCK) in C-4 plants are discussed. Evidence is presented that PEPCK is present at appreciable activities in the bundle-sheath of some NADP-malic enzyme-type C-4 plants, such as maize, but it was not detectable in NAD-malic enzyme-type C-4 plants. PEPCK is rapidly inactivated in crude extracts of leaves of the C-4 plant, Panicum maximum. This inactivation could be prevented by high concentrations of dithiothreitol or by the inclusion of ADP or ATP, suggesting the involvement of thiols at the active site. PEPCK is also subject to rapid proteolysis in crude extracts of a range of C-4 plants, resulting in cleavage to a smaller (62 kDa) form. This can be reduced by extraction at high pH and by the inclusion of SDS, but it means that intact PEPCK has never been purified from a C-4 plant. The molecular mass of PEPCK varies considerably in C-4 plants, unlike C-3 and CAM plants in which it is usually 74 kDa. PEPCK is phosphorylated during darkness (and reversed by light) in some C-4 plants with PEPCK of a larger molecular mass, such as Panicum maximum (71 kDa), but it was not phosphorylated in the PEPCK-type C-4 plant, Sporobolus pyramidalis (69 kDa). The known regulatory properties of PEPCK are discussed in relation to its role in C-4 photosynthesis, in particular its sensitivity to regulation by adenylates and by Mn2+.