Crystal structure of enoyl-CoA hydratase from Thermus thermophilus HB8

Fatty-acid degradation is an oxidative process that involves four enzymatic steps and is referred to as the beta-oxidation pathway. During this process, long-chain acyl-CoAs are broken down into acetyl-CoA, which enters the mitochondrial tricarboxylic acid (TCA) cycle, resulting in the production of energy in the form of ATP. Enoyl-CoA hydratase (ECH) catalyzes the second step of the beta-oxidation pathway by the syn addition of water to the double bond between C2 and C3 of a 2-trans-enoyl-CoA, resulting in the formation of a 3-hydroxyacyl CoA. Here, the crystal structure of ECH from Thermus thermophilus HB8 (TtECH) is reported at 2.85 angstrom resolution. TtECH forms a hexamer as a dimer of trimers, and wide clefts are uniquely formed between the two trimers. Although the overall structure of TtECH is similar to that of a hexameric ECH from Rattus norvegicus (RnECH), there is a significant shift in the positions of the helices and loops around the active-site region, which includes the replacement of a longer alpha(3) helix with a shorter alpha-helix and 3(10)-helix in RnECH. Additionally, one of the catalytic residues of RnECH, Glu144 (numbering based on the RnECH enzyme), is replaced by a glycine in TtECH, while the other catalytic residue Glu164, as well as Ala98 and Gly141 that stabilize the enolate intermediate, is conserved. Their putative ligand-binding sites and active-site residue compositions are dissimilar.