Abstract
Thermal dependence of the enzymes SOD, CAT, and POD was investigated in leaves of Iris pumila plants inhabiting two contrasting light environments, a sun-exposed dune site and a woodland understory. At the same assay temperature, both the specific activity and the activation energy of SOD and CAT were higher in plants inhabiting vegetation shade than in those experiencing full sunlight. Conversely, the temperature optima for the two enzymes did not differ between alternative radiation environments. The specific activity of POD increased with temperature increase, and was always greater in plants growing under full sunlight than in those from vegetation shade. The activation energy of POD was higher than that of SOD or CAT, being lower in sun-than in shade-exposed plants.
Highlights
Temperature is one of the major abiotic factors in determining the rate of enzyme activity
The events taking place during the catalytic process are complex, but it has been emphasized that the key determinant of the speed of an enzymatic reaction is the velocity of rate-limiting conformational changes (Hochachka and Somero, 2002)
Since under conditions of reduced thermal energy, the activation energy barriers to conformational changes may slow the rate of an enzymatic reaction, it has been hypothesized that in cold habitats, the optimal adaptive strategy would be development of enzymes with the lowest possible energy barriers to catalytic conformational changes, whereas in warm habitats, enzymes with enhanced resistance to heat denaturation would be more selectively advantageous than others (Hochachka and Somero, 2002)
Summary
Temperature is one of the major abiotic factors in determining the rate of enzyme activity. The events taking place during the catalytic process are complex, but it has been emphasized that the key determinant of the speed of an enzymatic reaction is the velocity of rate-limiting conformational changes (Hochachka and Somero, 2002). There is strong experimental evidence for the presence of an inverse correlation between enzyme conformational stability and specific activity, mediated via molecular flexibility (Jaenicke, 1991; Daniel et al, 1996). As might be expected from this flexibility/stability/activity association, “thermophilic” enzymes (tolerant to high temperatures) have been shown to be more rigid and less active at room temperature than their mesophilic orthologs (Jaenicke, 1991; Daniel et al, 1996). There is growing evidence, that the way in which enzymes respond to temperature may depend upon their intrinsic biochemical properties (Daniel et al, 1996; Thomas and Scopes, 1998; Lu et al, 2008), and upon the environmental growth conditions to which the organisms were exposed (Hull et al, 1997; Peltzer et al, 2002)
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