Figure 1.

The conventional and branched cross-bridge cycles. (A) A simplified four-state version of the conventional model of the cross-bridge cycle (6). In this model, P_i release is coupled to rotation of myosin’s lever arm and the rebinding of P_i to the AM.ADP state reverses this step. (B) Conventional (green) and branched (red) kinetic pathways where the rebinding of P_i to the AM.ADP state causes myosin to dissociate from actin in a postpowerstroke state and complete the release of P_i and ADP and the rebinding to ATP off actin. In this model, the powerstroke is not reversed with the rebinding of P_i to the AM.ADP state. From Walcott et al. (42), we can specify all parameters of the green pathway. There are then four free kinetic parameters, boxed and shown in bold for the red pathway.