Kinetics of protein modification reactions.

Abstract

or partially covalent, bond of the protein under study (Cohen, 1970), hence also the more general term 'covalent modification' (Brocklehurst, 1982). Protein modification studies have so far aimed at: (a) the determination of the kind and number of reactive groups of the protein molecule under study, (b) the development of site-specific protein modifying agents, and (c) the elucidation of enzyme protein catalytic function. None of these objectives may be fully realized without a thorough kinetic analysis of the chemical reactions of the protein modification process. In this article a review of the kinetic approaches so far developed in the study of protein modification reactions will be presented. Emphasis will mainly be placed on formalistic descriptions of actual or hypothetical cases of protein modification, but aspects of molecular kinetics (influence of temperature on the rate of chemical modification) will also be considered. Modification of enzyme protein is frequently accompanied by loss of enzyme catalytic activity (irreversible enzyme inhibition), and in such cases enzyme protein modification and inactivation are best studied simultaneously. Loss of enzyme activity may result if the protein molecule is modified in ways other than a straightforward derivatization of amino acid residues. Loss of protein tertiary and quaternary structure, modification of enzyme-bound cofactors, ligand binding and conformational isomerism of the protein are important from the kinetics point of view, in that these factors may determine the shape of the plot of concentration of modified residues versus reaction time. Enzyme activity loss, consequent to protein modification, may be complete or partial. In the latter case, inactivation may be the expression of a diminished V, or an increased K., of the enzyme, or both. It is because of considerations such as these that the kinetics of chemical modification of proteins cannot be considered entirely apart from the kinetics of such processes as heat denaturation of proteins, photoinactivation and radiationmediated inactivation of enzymes, and high-ionicstrength-mediated protein unfolding.