Abstract
Thermoregulation (homeothermy) in animals involves a complex mechanism involving thermal receptors throughout the body and integration in the hypothalamus that controls shivering and non-shivering thermogenesis. The flowers of some ancient families of seed plants show a similar degree of physiological thermoregulation, but by a different mechanism. Here, we show that respiratory control in homeothermic spadices of skunk cabbage (Symplocarpus renifolius) is achieved by rate-determining biochemical reactions in which the overall thermodynamic activation energy exhibits a negative value. Moreover, NADPH production, catalyzed by mitochondrial isocitrate dehydrogenase in a chemically endothermic reaction, plays a role in the pre-equilibrium reaction. We propose that a law of chemical equilibrium known as Le Châtelier’s principle governs the homeothermic control in skunk cabbage.
Highlights
We propose here that the negative activation energy in homeothermic skunk cabbage could be produced via biochemical pre-equilibrium reactions comprising reversible reactions catalysed by cellular dehydrogenases and a rate-determining reaction catalysed by the mitochondrial terminal oxidases alternative oxidase (AOX) and c oxidase (COX) (Fig. 2)
Because citrate is one of the most abundant organic acids in thermoregulatory male tissues of Dracunculus vulgaris[20] and because our analysis with isolated mitochondria showed that NADP+-dependent isocitrate dehydrogenase (ICDH) is the major enzyme that catabolizes isocitrate, we focused on the pre-equilibrium reaction mediated by ICDH and type-II rotenone-insensitive internal
NADPH-NADPH dehydrogenase (NDA)/ICDH-mediated oxygen consumption did exhibit negative activation energy, the temperature at which Eo was zero (22.3 °C) was higher than it was in intact spadices (15.2 °C; Fig. 3c)
Summary
We propose here that the negative activation energy in homeothermic skunk cabbage could be produced via biochemical pre-equilibrium reactions comprising reversible reactions catalysed by cellular dehydrogenases and a rate-determining reaction catalysed by the mitochondrial terminal oxidases AOX and COX (Fig. 2). It is conceivable that Ea + Ea′′ < Ea′ , in which case the activation energy is negative and the rate will decrease as temperature increases.
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