Chapter 15 Structure, Function, and Post-translational Regulation of C4 Pyruvate Orthophosphate Dikinase

Abstract

Pyruvate orthophosphate dikinase is a cardinal enzyme of the C4 pathway. Its role in C4 photosynthesis is to catalyze the regeneration of PEP, the primary carboxylation substrate from pyruvate, Pi, and ATP in the chloroplast stroma of leaf-mesophyll cells. It is the most abundant of C4 enzymes, comprising up to 10% of the soluble protein of C4 leaves, and thus may exert a limitation on the rate of CO2 assimilation into the C4-cycle. Studies dating back to the 1970s documented its biochemical properties as related to its role in C4 photosynthetic process. Later studies originating in the early 1980s discovered how the enzyme is regulated in a light/dark manner by reversible phosphorylation of an active-site threonine. A bifunctional protein kinase/protein phosphatase with unprecedented properties, the PPDK Regulatory Protein (RP), was identified as the enzyme catalyzing this reversible phosphorylation event. However, the gene encoding this unusual enzyme had eluded cloning for some two decades until modern cloning methods allowed its recent isolation from maize. Although the enzyme properties of C4-PPDK are well understood, the molecular basis of its post-translational light/dark regulation by RP is poorly understood. Because of the significance of PPDK regulation to the C4-photosynthetic process, this chapter addresses the current state-of-knowledge on how C4-PPDK is post-translationally regulated by its companion regulatory enzyme, RP. This includes proposed models that describe how phosphorylation of PPDK by RP leads to complete inactivation of enzyme activity and the mechanism regulating the direction of RP's opposing PPDK-dephosphorylation and PPDK-phosphorylation activities. Also reviewed are the recent bioinformatic analyses of the RP polypeptide primary structure. These revealed that vascular plant RP represents a fundamentally new and novel kind of protein kinase with evolutionary origins in PPDK-containing anaerobic bacteria.