ECH is used in β-oxidation to add a hydroxyl group and a
proton to the unsaturated
β-carbon on a fatty-acyl CoA. ECH 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 ECH 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. This configuration requires that the active site for ECH is extremely rigid, 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 (shown in the picture) 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. ECH is mechanistically similar to
fumarase. ==References==