COOPERATIVE CHARGE FLUCTUATIONS BY MIGRATING PROTONS IN GLOBULAR-PROTEINS

A review of the hydrogen bonded network on the protein surface shows t he presence of a charged complex system with parallel and competitive interactions, including ionizable side-chains, migrating protons, boun d water and nearby backbone peptides. This system displays cooperative effects of dynamical nature, reviewed for lysozyme as a case. By incr easing the water coverage of the protein powder, the bound water clust er exhibits a percolative transition, detectable by the onset of large water-assisted displacements of migrating protons, with a parallel em ergence of protein mobility and biological function. By lowering the t emperature, migrating protons exhibit a glassy dielectric relaxation i n the low frequency range, pointing to a frustration by competing inte ractions similar to that observed in spin glasses and fragile glass fo rming liquids. The observation of these dissipative processes implies the occurrence of spontaneous charge fluctuations. A simplified model of the protein surface, where conformational and ionizable side-chain fluctuations are averaged out, is used to discuss the statistical phys ics of these cooperative effects. Some biological implications of this dynamical cooperativity for enzymatic activity are briefly suggested at the end. (C) 1998 Elsevier Science Ltd. All rights reserved.