a cycle of phosphorylation and dephosphorylation catalyzed by two enzymes that normally function in two different metabolic pathways; the net effect is the hydrolysis of ATP and the generation of heat, e.g., the futile cycle from the unregulated action of 6-phosphofructokinase and fructose-1,6-bisphosphatase in muscle; such cycles may have an important role in heat production, in the fine-tuning of the regulation of certain pathways, and may be a factor in malignant hyperthermia. SYN: substrate cycle.