Theoretical analysis of the effects of two pH regulation patterns on the temperature sensitivities of biological systems in nonhomeothermic animals

Abstract

When protonation reactions comprise a part of biochemical reaction schemes in vivo, the temperature sensitivities of chemical equilibria and of enzymatic rates are modulated according to the variation of pH with T B (body temperature). Two patterns of pH regulation have been established, each pattern controlling the ionization states of different titratable groups as T B changes: dpH dT B ≅ 0 pH unit/° C for the blood of heterothermic mammals (constant charge for phosphate groups, e.g.), and dpH dT B ≅ −0.017 pH unit/° C for intracellular and blood compartments of vertebrate and invertebrate poikilotherms (constant charge for histidine imidazole and -SH groups). Calculations demonstrate the feasibility of the following. (i) A large effect of T B on a metabolic branch point is possible when the competition of two pathways for common substrate is governed by two titratable groups with large differences in Δ H i o values (ionization enthalpy), e.g., phosphate and alpha amino groups. (ii) The effect of temperature on the amount of free energy released from equilibrium chemical reactions (as might occur for reactions not controlled by rate-limiting enzymes) would depend strongly on the value of Δ H i 0, and on which of the two pH regulation patterns was present. (iii) If free energy released from ionization reactions were coupled to activation processes of enzyme catalysis, so as to effectively decrease free energy barriers, the temperature sensitivities of reaction rates would be smaller in pH environments in which pH varied inversely with T B than in environments of constant pH, regardless of Δ H i 0. (iv) Different mechanisms for the modulation of metabolism as a function of T B might have evolved for animals with different values of dpH dT B .