No abstract

No abstract

Normally, bimolecular reactions are analyzed in terms of the Smoluchowski theory. However, when one attempts to generalize this analysis to cases where diffusion proceeds in two other than in three dimensions, one soon encounters severe conceptual difficulties. Although kinetic studies of membrane enzymes are generally difficult because the usual kinetic formalism refers to nonaggregated homogenous solutions, a major goal of our research is to define the molecular mechanism(s) by which alterations in membrane-bound substrate contents affect the enzyme activity in the same membrane. For that purpose, a simplified random-walk model was adopted in the present work. The enzyme reaction in the two-dimensional membrane could be calculated theoretically by applying the classical analysis of heat equation. As a result, the theoretical rate equation well accounting experimental findings was derived on the model of the liver microsomal NADHcytochrome b5 reductase reaction. Furthermore, it was found that the modification of the simple rigid-sphere collision theory by including a term called the steric factor was not necessary in this derived equation.The fluid-mosaic membrane model regards biological membranes as two-dimensional protein solutions in a fluid lipid bilayer phase (17). The key idea is that the lipid components of the membrane constitute a fluid environment in which there are no barriers to lateral movement. The interaction of membrane-bound enzymes presents an interesting problem in kinetics since the usual assumptions concerning enzyme-substrate interaction in free solution may not be suitable.The endoplasmic reticulum of liver has many functions that are mediated by membrane-bound enzymes. In the case of cytochrome b5, a well known integral protein of the endoplasmic reticulum (21), a relatively detailed picture of how this integral protein is bound to the microsomal membrane has been established (6). Cytochrome b5 in microsomes is known to be reduced by NADH-cytochrome b5 reductase (EC 1.6.2.2). NADH-cytochrome b5 reductase and cytochrome b5 are intimately associated with the microsomal membrane (14, 15) and their kinetic interaction affords a unique opportunity for kinetic studies. This NADH-dependent reduction reaction has received much attention and shows very complex kinetics, exhibiting at least a biphasic time course (12).As suggested by Scheel et al. (16), the classical Michaelis-Menten theory, which is applicable to water-soluble (or solubilized) enzymes and substrates, may not be applicable to membrane-bound enzymes, particularly if the substrate is also in the membrane. This is due in part to geometric restrictions when both the enzyme and the substrate are confined to

No abstract

No abstract