FORCES AND FACTORS THAT CONTRIBUTE TO THE STRUCTURAL STABILITY OF MEMBRANE-PROTEINS

While a considerable amount of literature deals with the structural en ergetics of water-soluble proteins, relatively little is known about t he forces that determine the stability of membrane proteins. Similarly , only a few membrane protein structures are known at atomic resolutio n, although new structures have recently been described. In this artic le, we review the current knowledge about the structural features of m embrane proteins. We then proceed to summarize the existing literature regarding the thermal stability of bacteriorhodopsin, cytochrome-e ox idase, the band 3 protein, Photosystem II and porins. We conclude that a fundamental difference between soluble and membrane proteins is the high thermal stability of intrabilayer secondary structure elements i n membrane proteins. This property manifests itself as incomplete unfo lding, and is reflected in the observed low enthalpies of denaturation of most membrane proteins. By contrast, the extramembranous parts of membrane proteins may behave much like soluble proteins. A brief gener al account of thermodynamics factors that contribute to the stability of water soluble and membrane proteins is presented.