Regulation of pyruvate dehydrogenase complex activity in plant cells.

The pyruvate dehydrogenase complex (PDC) is subjected to multiple interacting levels of control in plant cells. The first level is subcellular compartmentation. Plant cells are unique in having two distinct, spatially separated forms of the PDC; mitochondrial (mtPDC) and plastidial (plPDC). The mtPDC is the site of carbon entry into the tricarboxylic acid cycle, while the plPDC provides acetyl-CoA and NADH for de novo fatty acid biosynthesis. The second level of regulation of PDC activity is the control of gene expression. The genes encoding the subunits of the mt- and plPDCs are expressed following developmental programs, and are additionally subject to physiological and environmental cues. Thirdly, both the mt- and plPDCs are sensitive to product inhibition, and, potentially, to metabolite effectors. Finally, the two different forms of the complex are regulated by distinct organelle-specific mechanisms. Activity of the mtPDC is regulated by reversible phosphorylation catalyzed by intrinsic kinase and phosphatase components. An additional level of sensitivity is provided by metabolite control of the kinase activity. The plPDC is not regulated by reversible phosphorylation. Instead, activity is controlled to a large extent by the physical environment that exists in the plastid stroma.