Detailed state model of CaMKII activation and autophosphorylation

By combining biochemical experiments with computer modelling of biochemical reactions we elucidated some of the currently unresolved aspects of calcium-calmodulin-dependent protein kinase II (CaMKII) activation and autophosphorylation that might be relevant for its physiological function and provided a model that incorporates in detail the mechanism of CaMKII activation and autophosphorylation at T286 that is based on experimentally determined binding constants and phosphorylation rates. To this end, we developed a detailed state model of CaMKII activation and autophosphorylation based on the currently available literature, and constrained it with data from CaMKII autophosphorylation essays. Our model takes exact phosphorylation patterns of CaMKII holoenzymes into account, and is valid at physiologically relevant conditions where the concentrations of calcium and calmodulin are not saturating. Our results strongly suggest that even when bound to less than fully calcium-bound calmodulin, CaMKII is in the active state, and indicate that the autophosphorylation of T286 by an active non-phosphorylated CaMKII subunit is significantly faster than by an autophosphorylated CaMKII subunit. These results imply that CaMKII can be efficiently activated at significantly lower calcium concentrations than previously thought, which may explain how CaMKII gets activated at calcium concentrations existing at synapses in vivo. We also investigated the significance of CaMKII holoenzyme structure on CaMKII autophosphorylation and obtained estimates of previously unknown binding constants.