6 - ENZYME REACTIONS IN BIOLOGICAL MEMBRANES

Abstract

This chapter discusses enzyme reactions in biological membranes. A large number of cellular enzymes are located in membranes. Indeed, it is likely that most particulate enzymes are in truth membrane-bound enzymes whose particulate nature reflects their association with the lipid matrix of biological membranes. Included in this category are enzymes located in plasma membranes, mitochondria, microsomes, and in other sub-cellular organelles. In fact, if one is willing to extend the definition to proteins that catalyze the physical translocation of substrates, the class of membrane enzymes can be extended to include proteins responsible for transmembrane transport, such as bacterial permeases. Membrane lipids can also participate in enzyme-catalyzed reactions by forming covalently linked intermediates in the reaction sequence. So far, this has been shown to occur in the synthesis of macromolecules in bacteria and in all cases examined, the carrier lipids are polyiso-prenoid compounds. In general, the solubilization of membrane proteins requires the disruption of the organized membrane structure. Nonpolar bonds (lipid–lipid, lipid–protein, and protein–protein) are believed to play a major role in stabilizing the structure of membranes and it is, therefore, not surprising that agents that disrupt such bonds are often effective in solubilizing membrane-bound enzymes.