Utilization of the free energy of the reversible binding of protein and modifying agent towards the rate-enhancement of protein covalent modification

Abstract

An analysis is presented of the catalytic factors responsible for the rate-enhancement that may be observed when a protein modification reaction is compared with a reaction of the same modifying agent with a model micromolecular compound exhibiting the same reactive group as the protein under study. It is seen that affinity-mediated rate-enhancement of protein modification is realized by the loss of activation entropy. On the assumption that attainment of maximal affinity-mediated rate-enhancement presents with an activation entropy of the protein modification reaction equal to zero, whereas the activation enthalpy of the reaction remains unchanged, it is shown that the value for maximal affinity-mediated rate-enhancement is equal to e-delta s++/R. Accordingly, protein modification reactions may be differentiated into (i) reactions the rate-enhancement of which (relative to the reaction of the same modifying agent with a model compound) is primarily entropy-controlled and (ii) reactions the rate-enhancement of which is primarily enthalpy-controlled. It is seen that modifying agents of low reactivity towards model compounds, but with a high, i.e. highly negative, activation entropy are better suited as prospective affinity-based protein-modifying agents, since the potential affinity-mediated rate-enhancement, and hence the selectivity, of these compounds is necessarily high. Kinetic and thermodynamic constants of the reaction of modifying agents with proteins, and with model compounds, and values of maximal affinity-mediated rate-enhancement, based on published data of the reaction of several modifying agents with model compounds, are presented and discussed.

Molecular Mechanisms of Subtype-Specific Inhibition of Neuronal T-Type Calcium Channels by Ascorbate

“…However, there are numerous biological examples of reversible covalent modification serving as a mechanism for the regulation of protein function (Rakitzis, 1990;Veal et al, 2007), including modulation of ion channel gating, as exemplified by the activation of TRPA1 channels by natural isothiocyanate-containing compounds (Hinman et al, 2006). Additionally, there are several reactions involving reversible covalent modifications of histidine residues in other systems (including several mediated by ascorbate), despite the fact that the reversibility is predicted to be energetically unfavorable (Farver et al, 1998;Njus et al, 2001).…”

Perspectives for biophysicochemical modifications of enzymes

“…Thermodynamic and kinetic analysis of protein modification and enzyme inactivation reactions and of regeneration by dilution of covalently modified proteins have been performed (34)(35)(36). Examples are known in which kinetic properties have been altered after modification of an enzyme and there are also cases in which new catalytic activities have been introduced [37,38].…”