Enzyme promiscuity, metabolite damage, and metabolite damage control systems of the tricarboxylic acid cycle.

Promiscuous enzymes and spontaneous chemical reactions can convert normal cellular metabolites into noncanonical or damaged metabolites. These damaged metabolites can be a useless drain on metabolism and may be inhibitory and/or reactive, sometimes leading to toxicity. Thus, mechanisms to prevent metabolite damage from occurring (metabolite damage preemption) or to convert damaged metabolites back to physiological forms (metabolite repair) are essential for sustained operation of metabolic networks. Some iconic examples of metabolite damage and its repair or preemption are associated with the tricarboxylic acid (TCA) cycle, and other metabolite damage control systems are likely to exist here due to the inherent promiscuity of TCA cycle enzymes and reactivity of TCA cycle intermediates. Here, we review known metabolite damage reactions and metabolite damage control systems associated with the TCA cycle. This includes a previously unrecognized metabolite damage control system - an oxaloacetate (OAA) enol-keto tautomerase activity that is 'built-in' to the TCA cycle. This activity is required to remove the highly inhibitory enol form of OAA and is likely to be critical for TCA cycle operation. By cataloging these instances, we show that metabolite damage and its repair or preemption is a prevalent feature of the TCA cycle and suggest many more metabolite damage control systems are likely to exist.