Figure 1.

Simplified model of the regulatory phosphorylation of C₄ PPDK by its bifunctional RP in the mesophyll-chloroplast stroma. As a consequence of dark-induced increases in stromal [ADP], the RP-catalyzed kinase reaction is favored and the “target” enzyme is inactivated by introduction of a dianionic (2⁻) charge into the active-site domain. In the light, stromal ADP levels decrease, favoring the RP-catalyzed dephosphorylation reaction and subsequent restoration of PPDK activity. The strictly conserved, catalytically essential His-P residue at the P+2 position (e.g. His-458 in maize C₄ PPDK; Chastain et al., 2000; Wei et al., 2000) is also indicated. Modified from Figure 50 in Malkin and Niyogi (2000).