Lipid-Dependent Membrane Enzymes. A Kinetic Model for Cooperative Activation in the Absence of Cooperativity in Lipid Binding

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Abstract
The dependence of integral membrane enzymes on lipid activators is analyzed in terms of multiple binding site kinetics. Rate equations for an enzyme with n independent and identical lipid binding sites are derived for the case that enzyme activity is proportional to the total amount of lipid bound, or that only fully substituted enzyme is active. A third equation applies to the case that lipids bind with infinite cooperativity to give fully substituted and active enzyme. None of the 3 models was entirely consistent with existing experimental data. The following kinetic model accommodates the degree of cooperativity observed in lipid activation experiments as well as the number of independent lipid-binding sites determined by ESR measurements. The membrane enzyme is assumed to have n non-interacting and identical lipid-binding sites. Only fully substituted enzyme (ELn) and the next most highly substituted forms such as ELn-1 and ELn-2 may possess enzyme activity. These assumptions lead to cooperativity in activation. Cooperativity reaches a maximum when enzyme activity starts to appear with about 80% of the fully lipid substitution. The increase in cooperativity is accompanied by a decrease in the lipid concentration required for half-maximal activation. Further kinetic aspects of a dynamic boundary lipid layer around integral membrane enzymes are discussed.