Enoyl CoA Isomerase - Enzyme Structure

Enzyme Structure

All classes of enoyl-CoA isomerases belong to a family of enzymes, the hydratase/isomerase or crotonase superfamily, and when examined with x-ray crystallography, exhibit a common structural feature of the family, the N-terminal core with a spiral fold composed of four turns, each turn consisting of two beta-sheets and one alpha-helix.

In enoyl-CoA isomerase, the two beta-sheets are part of the catalytic site, since the NH groups of residues following the beta-sheets attach to the carbonyl oxygen of the acyl-CoA intermediate. The formation of this oxyanion hole stabilizes the transition state of the enzyme-catalyzed reaction.

Moreover, a glutamate residue located next to body cavities filled with water molecules and lined with hydrophobic or apolar side chains has also been identified as a part of the catalytic site. In its deprotonated form, the glutamate can act as a base and remove a proton from the acyl-CoA intermediate. The body cavities aid in rearranging the glutamate side chain to retain the proton and later deliver it back to the acyl-CoA, on a different carbon position.

The NH-containing residues have been identified as Ala70 and Leu126 and the glutamate as Glu158 in peroxisomal enzymes in a yeast species, Saccharomyces cerevisiae. Their relative locations on the enzyme can be compared in Figure 2.

The enzymes of the hydratase/isomerase or crotonase superfamily are typically trimeric disks dimerized into hexamers. The wide range of their substrate-enzyme specificity derives from the variations in the distances between the trimeric disks and their orientation. However, the human mitochondrial enoyl-CoA isomerase is a trimer and orients the fatty acid tail in a completely different direction from that seen in the hexamers. The trimeric disk of peroxisomal enzymes in Saccharomyces cerevisiae is displayed in Figure 3.