Chapter 8
Pressure Dependence of Enzyme Catalysis Peter C. Michels and Douglas S. Clark
Downloaded by GEORGE MASON UNIV on March 17, 2016 | http://pubs.acs.org Publication Date: July 7, 1992 | doi: 10.1021/bk-1992-0498.ch008
Department of Chemical Engineering, University of California—Berkeley, Berkeley, CA 94720
Pressure effects on enzyme function are strongly influenced by envi ronmental factors such as temperature, pH, salt and substrate con centrations, and pressure itself. This article examines these effects in relation to volume changes associated with enzymatic reactions and enzyme denaturation. Depending on the experimental conditions, elevated pressure can enhance or inhibit enzyme activity and increase or decrease enzyme stability. Difficulties inherent in the design and interpretation of high pressure experiments are discussed, as are eco nomic considerations pertaining to high pressure enzyme technology.
Intuitively, pressure effects on physicochemical processes are simple to understand as a direct extension of Le Chatelier's principle: elevated pressures favor changes that reduce a system's overall volume. Thus, by comparing the total volumes of the reactants versus products, of the ground state versus activated state, or of the dis sociated versus bound complex, the effect of pressure on reaction equilibria or rates can be estimated. If the volume of the products is smaller than the volume of the reactants, the reaction equilibrium will shift to favor the products during a rise in pressure. Similarly, if the overall volume of the activated state is less than that of the ground state, the velocity of the reaction will increase with pressure. The effect of pressure on the equilibrium constant of a reaction can be derived from the equilibrium condition: Σν μ «0 (
(1)
4
i
where ν and μ are the stoichiometric coefficient and chemical potential of species f
t
i, respectively. Substituting the definition of activity into equation (1) yields Σν μ? = - R T l n n a p = - R T l n K έ
i
a
(2)
i
where μ? is the chemical potential of species i at the standard state, defined here as one mole of pure component i at the system temperature and pressure, aj is the
0097-6156/92/0498-0108$06.00/0 © 1992 American Chemical Society
Adams and Kelly; Biocatalysis at Extreme Temperatures ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
8.
Pressure Dependence of Enzyme Catalyst109
MICHELS AND CLARK
activity of species i, and K is the equilibrium constant defined in terms of activities. Equivalently, we can write a
Σν μ°
= -RT(ln/
