Mechanism
Enoyl-CoA hydratase (ECH) is used in β-oxidation to add a hydroxyl group and a proton to the unsaturated β-carbon on a fatty-acyl CoA. The enzyme functions by providing two glutamate residues as catalytic acid and base. The two amino acids hold a water molecule in place allowing it to attack in a syn addition to an α-β unsaturated acyl-CoA at the β-carbon. The α-carbon then grabs another proton which completes the formation of the beta-hydroxy-acyl-CoA. It is also known from experimental data that no other sources of protons reside in the active site. This means that the proton which the α-carbon grabs is from the water that just attacked the β-carbon. What this implies is that the hydroxyl group and the proton from water are both added from the same side of the double bond, a syn addition. This allows the enzyme to make an S stereoisomer from 2-trans-enoyl-CoA and an R stereoisomer from the 2-cis-enoyl-CoA. This is made possible by the two glutamate residues which hold the water in position directly adjacent to the α-β unsaturated double bond as seen in figure 1. As one might guess this configuration requires that the active site for this enzyme is extremely rigid so as to hold the water in a very specific configuration with regard to the acyl-CoA. The data for a mechanism for this reaction is not conclusive as to whether this reaction is concerted or occurs in consecutive steps. If occurring in consecutive steps, the intermediate is identical to that which would be generated from an E1cb elimination reaction. Both mechanisms are shown below.
| Reaction Mechanisms | |
|---|---|
| Figure 2: Both Mechanisms | Figure 3: Concerted Mechanism |
The enzyme is mechanistically similar to fumarase.
It is classified as EC 4.2.1.17.
Read more about this topic: Enoyl-Co A Hydratase
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