Figure 1. Scheme of the initial steps of the phototransduction cascade, starting from photon absorption by rhodopsin (R, state 0) and proceeding to the formation of active PDE (the TαGTP × PDE* complex, state 7). Encircled numbers designate the intermediate states of the components of the cascade. t0 to t6 are time constants of corresponding transitions. At the transition from state 4 to 5, R* is released from transducin and returns to state 1 initiating a new loop of T activation. There also is a parallel path
of activation via the cycle driven by rhodopsin partially inactivated by phosphorylation (Rp). It is suggested that phosphorylation slows the transition from state 1 to state 2 (t1) by a factor ap greater than 1; all following reactions are thought to be unaffected.