The Effects of Increased Viscosity on the Function of Integral Membrane Proteins

For many years the idea that the activity of integral membrane proteins is regulated by the fluidity of the lipid matrix was popular and appeared to be quite rational. However, as information about the effect of viscosity on the function of different membrane proteins became available, the correlation between the two became increasingly unclear. The purpose of this article is to readdress this issue in light of our recent pressure and temperature studies. This chapter is divided into seven parts: (1) the effect of viscosity on enzyme activity; (2) the effect of viscosity on the local motions of proteins; (3) characterization of membrane viscosity; (4) demonstration of changes in protein-lipid contacts brought about by changes in viscosity; (5) an example of a protein in which the viscosity appears to stabilize a particular conformational state: (6) relations between membrane viscosity and protein function; and (7) conclusions. The effect of viscosity (η) on the rate (k) of a chemical reaction was first given by Kramers (1940): . . . k=A/ηe−Ea/RT (1) . . . In this expression, viscosity will affect the rate of a reaction by limiting the rate of diffusion of reactants. Viscosity will thus modify the frequency factor (A) and should not affect the activation energy. This expression has been applied to aqueous soluble enzymes (for example, Gavish, 1979; Gavish & Werber, 1979; Somogyi et al., 1984), and it appears that, in general, enzymes obey Kramers’s relation, although in some cases the exponent of η is less than one. Viscosity can affect enzymatic rates not only by limiting the diffusion of substrates but also by damping internal motions of the protein chains. It seems reasonable that a high enough viscosities, the protein would be damped sufficiently so that large activation energies will be required for the backbone motions that allow substrates and products to diffuse into and out of the active site. This viscosity-induced increase in activation energy was shown by studies of the reassociation of carbon monoxide and dioxygen to the heme site of myoglobin after flash photodissociation (Austin et al., 1975; Beece et al., 1980).