Insights into mitochondrial fatty acid synthesis from the structure of heterotetrameric 3-ketoacyl-ACP reductase/3R-hydroxyacyl-CoA dehydrogenase

Mitochondrial fatty acid synthesis (mtFAS) is essential for respiratory growth in yeast and mammalian embryonic survival. The human 3-ketoacyl-acyl carrier protein (ACP) reductase (KAR) of mtFAS is a heterotetrameric α 2 β 2 -assembly composed of 17β-hydroxysteroid dehydrogenase type-8 (HSD17B8, α-subunit) and carbonyl reductase type-4 (CBR4, β-subunit). Here we provide a structural explanation for the stability of the heterotetramer from the crystal structure with NAD + and NADP + bound to the HSD17B8 and CBR4 subunits, respectively, and show that the catalytic activity of the NADPH- and ACP-dependent CBR4 subunit is crucial for a functional Hs KAR. Therefore, mtFAS is NADPH- and ACP dependent, employing the 3R-hydroxyacyl-ACP intermediate. HSD17B8 assists in the formation of the competent Hs KAR assembly. The intrinsic NAD + - and CoA-dependent activity of the HSD17B8 subunit on the 3R-hydroxyacyl-CoA intermediates may indicate a role for this subunit in routing 3R-hydroxyacyl-CoA esters, potentially arising from the metabolism of unsaturated fatty acids, into the mitochondrial β-oxidation pathway. 3-Ketoacyl-ACP reductase is required for mitochondrial fatty acid synthesis. Venkatesan et al. present structures of this enzyme, show that the β-subunit is involved in fatty acid synthesis and propose a role for the α-subunit in routing unsaturated fatty acids into β-oxidation.