W. Wriggers and K. Schulten
Beckman Institute and Department of Physics, University of Illinois Urbana Champaign, Urbana, IL 61801.
We have recently considered the pathways of water diffusion into actin's enzymatic site and suggested, based on computer simulations, that a "back door" diffusion pathway is used in the release of the inorganic phosphate (Pi) after ATP hydrolysis. To substantiate this hypothesis, we have modelled the dissociation of Pi through the proposed back door pathway. After cleaveage from ATP, the phosphate rotates around the Ca2+ cation, turning away from the negatively charged ADP. The unbinding of the phosphate has been studied further by measuring adhesion forces when pulling the substrate out of its binding pocket. The dissociation from the divalent cation appears to be the limiting factor in the release of Pi. The position of His73 indicates a control function of this residue in the dissociation process. In the F-actin model [Lorenz et al., JMB 234:826 (1993)], His73 is constrained by phalloidin which would sterically block the release channel. Accordingly, the proposed back door mechanism is consistent with a phalloidin-induced delay of phosphate release in F-actin [Danker and Hess, Biochim. Biophys. Acta, 1035:197 (1990)].