CAM: Rhythms of Enzyme Capacity and Activity as Adaptive Mechanisms

Abstract

There are some explanations for the fact that circadian endogenous rhythmicity, which is a basic general property of the physiology of eukaryotes, appears as a rather disturbing phenomenon for the physiologist, both from conceptual and experimental points of view. a) The existence of circadian rhythms discloses the annoying situation that very little is known about the inner processes which, in eukaryotic organisms, confer an adaptive ability for identifying local time and measuring the progress of a season. Nevertheless, substantial advances have been achieved in recent years on understanding the chemical and genetic mechanisms of the biological clock. b) The ubiquity of circadian rhythms results in metabolic networks shifting continuously between periodic patterns (even in the absence of external periodicisms). Here iterative measurements over 24 h are necessary if one intends to obtain a realistic description of a complex physiological function, particularly when two or more pathways are likely to be involved. In CAM, the "double carboxylation" mechanism for malate synthesis, the source of phosphoenolpyruvate (PEP), the fate of pyruvate, and the variability of delta13 C are probably good examples of the mixing of different pathways in different ratios according to the time of day. Attempting to integrate the effects of endogenous rhythmicity obviously makes interpretation of data and elaboration of comprehensive models more difficult.