As indicated by the ΔGO°’, hydrolysis of a CoA thioester is highly exothermic. Most plants and other organisms contain high activities of CoA thioesterase as well as non-specific hydrolases that act on CoA esters in vitro. Until we started our research, no plant CoA thioesterase had been purified to homogeneity or studied at the genetic level. In other organisms, biochemical and molecular approaches have provided evidence for several unrelated families of CoA thioesterases. Questions about the relevance of CoA thioesterase activities are complicated by the difficulty of defining exactly how the enzymes fit into metabolism. For example, fatty acyl-CoA thioesterases are found in both peroxisomes and mitochondria of animal cells – the two sites of b-oxidation. What might be their role and what mechanisms prevent them from disrupting b-oxidation by hydrolyzing the CoA intermediates? For the mammalian and yeast enzymes, it is proposed that branched-chain and other fatty acid structures that are recalcitrant to β-oxidation must be hydrolyzed to prevent sequestration of the CoA pool. Consistent with this suggestion, these enzymes are strongly inhibited by free CoA (IC50 10-15 µM) suggesting that they will only be activated when CoA becomes depleted. As we have shown, plant homologues do not fit this paradigm.
The coordinated analysis of the families of CoA thioesterases will be especially important given the conundrum of their metabolic significance in all organisms. Furthermore, fatty acid breakdown is only one of many proposed functions of b-oxidation in plants. Some reaction sequences such as those leading to the production of jasmonoyl-CoA and indole-3-acetyl-CoA, presumably require the action of an appropriately specific CoA thioesterase to release the free acid form of the hormone.