| Abstract: | We investigated the temperature dependence of several functions of octopine dehydrogenase, a monomeric enzyme extracted from the shell fish Pecten maximus L. We found that six enzymatic functions are temperature independent or change only negligibly with temperatue. These are the dissociation constants of three coenzyme complexes and the Michaelis Km values for NAD, NADH and one of the substrates (D-octopine). This is taken as an indication of a temperature-regulatory mechanism which enables the enzyme to maintain a constant level of NAD, NADH and D-octopine in binary and ternary complexes independent of fluctuations of the external temperature. This is discussed with reference to enzymes from other poikilotherms, which reportedly display similar biologically meaningful response to temperature. We also discuss the meaning of our data from a thermodynamic viewpoint. Considering that in a temperature-independent binding process only entropy changes contribute to the standard free-energy change, we speculate on possible molecular models which might account for our results. We also investigate the activation-energy parameters for the reaction catalyzed by octopine dehydrogenase, as obtained from the temperature dependence of V. It is found that octopine dehydrogenase, relative to other dehydrogenases, is provided with a rather low delta H not equal to, which enables the enzyme to change its turnover number by only a small factor in the temperature range 5--35 degrees C. |
