Multiple acyl-CoA thioesterases occupy distinct functional niches within the mitochondrial matrix

Acyl-CoA thioesterases (Acots) hydrolyze fatty acyl-CoA esters. Acots in the mitochondrial matrix are poised to mitigate β-oxidation overload that may contribute to lipotoxicity. Several Acots associate with mitochondria, but whether they all localize to the matrix, and are redundant or have different roles is unresolved. We compared mitochondrial Acots (Acot2, 7, 9, and 13) in terms of suborganellar localization, activity, expression and regulation, in mitochondria from multiple mouse tissues and from a new model of Acot2 depletion. Acot7, 9 and 13 localized to the matrix, joining Acot2 that was previously shown to localize there. Mitochondria from heart, skeletal muscle, brown adipose tissue and kidney robustly expressed Acot2, 9 and 13, though Acot9 was substantially higher in brown adipose tissue and kidney mitochondria, as was activity for C4:0-CoA, a unique substrate of Acot9. In all these tissues, Acot2 accounted for ~half of the thioesterase activity for C14-CoA and C16:0-CoA. In contrast, liver mitochondria from fed and fasted mice expressed little Acot activity, and this activity was confined to long-chain CoAs, and due mainly to Acot7 and Acot13 activity. Matrix Acots occupied different functional niches, based on substrate specificity (Acot9 vs. Acot2 and 13) and strong CoA inhibition (Acot7, 9, 13 but not Acot2). Interpreting these results in the context of β-oxidation, CoA inhibition would prevent Acot-mediated suppression of β-oxidation while providing for an Acot-mediated release valve when CoA is limiting. This release valve would operate across a wide range of acyl-CoA chain lengths. In contrast, CoA-insensitive Acot2 could provide a constitutive syphon for long-chain fatty acyl-CoAs. These results reveal how the family of matrix Acots can help to mitigate β-oxidation overload and prevent a CoA limitation.