PubMed · 6762401
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T Arakawa, S N Timasheff. 1982. [On protein-structure stabilizing solvent].. https://pubmed.ncbi.nlm.nih.gov/6762401/
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We have used a simple binding model to consider how the thermodynamics of denaturant-protein interactions might influence the shape of protein stability curves (free energy change as a function of temperature), and how these effects translate into a temperature dependence of the apparent m-value (sensitivity of unfolding free energy to denaturant). We find that for an exothermic binding reaction with no binding heat capacity increment, increasing denaturant concentrations produces an apparent increase in curvature in the protein stability curve, giving rise to an increase in the heat capacity increment of unfolding. Similar increases are seen if the binding heat capacity increment is taken as positive. However, for a negative binding heat capacity increment, increasing denaturant concentrations decreases the curvature of the stability curve, giving rise to a decrease in the heat capacity of unfolding. At very high denaturant concentrations (above which the heat capacity of denaturation becomes negative) the stability curve becomes dimpled, showing two separate maxima rather than one. These three models result in very different temperature dependencies of apparent m-values. For urea-induced unfolding of the ankyrin-domain of the Drosophila Notch protein, we find a dependence of experimental m-values on temperature that is similar to that produced by a negative binding heat capacity increment. This temperature dependence is consistent with the observed decrease in heat capacity of unfolding as denaturant is added.
Thermal effects on seed germination are considered through the changes brought about by temperature in the germination capacity, in the germination rate, and in the distribution of the relative frequency of germination along the incubation times. A number of questions of general thermobiological interest are thus raised, entailing the need of an analysis of the temperature dependence of the seed germination rate. A treatment of these rates by the activation-energy approach cannot be general, for their Arrhenius plots are not always linear. Moreover, it is shown that any process displaying a temperature optimum (as happens in the germination of most seed species) cannot follow one of the fundamental tenets of the collision rate theory. The need of a theoretical treatment stressing the essential role of the partition of energy within the seed system has led to an anlysis using the absolute reaction rate theory. New experimental prospects for the physiology of seed germination are thus raised, concerning the meaning of the temperature cardinal points, the growth pattern of the embryo in germinating seeds, the dual effect of protein thermodenaturation, the effects of high hydrostatic pressures, and a whole pharmacological line of work. The cybernetic counterpart of the thermodynamic view of seed germination appears in the study of the distribution of the relative frequency of germination along the isothermal incubation time. In some species of seeds the thermal communication between the environment and the seed growth effector can be shown to proceed by molecular collisions at all germination isotherms. In the seeds of Dolichos biflorus this communication through random thermal noise prevails only at temperatures close to both extreme limits of germination. Both in this species and in Calotropis procera there is a temperature range (encompassing the optimum) within which a temperature signal is superimposed upon the gaussian noise. An interpretation is proposed according to which the temperature signal is transduced in a protein-conformation code.
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