Biochimica et Biophysica Acta vol:995 issue:2 pages:151-9
A model of multiphasic systems, based on the assumption of zero-order partition of substrates and products into the membranes, is applied to reversible mono-substrate and bi-substrate reactions catalysed by membrane-bound enzymes. Apart from replacement of single-phase kinetic constants by apparent kinetic constants, the derived kinetic expressions are formally identical with those for corresponding single-phase systems. The model confers to the apparent kinetic constants an experimentally verifiable meaning. For full characterization of membrane-kinetic systems, experiments at various concentrations of enzyme-embedding phospholipid are required. Extrapolation to zero phospholipid concentration of each Km app then yields the corresponding true kinetic constant characteristic of the membrane-bound enzyme and also provides a technique for determination of the membrane-partition constants. The procedure implies that the phospholipid content should be assayed for full characterization of membrane-bound enzymes. If, for practical reasons, the assays have to be limited to a single enzyme concentration, correction of the apparent kinetic constants is still possible provided the phospholipid concentration and the partition constants of the reactants are known. The model has permitted prediction of a number of previous observations reflecting the multiphasic nature of the systems. The assumptions, underlying the model, and their implications are examined as well as some commonly used experimental designs for determination of the type of enzymic site.