A tertiary pathway for the oxidation of ethanol is carried out by catalase, a peroxisomal enzyme that also catalyzes the removal of hydrogen peroxide (H2O2). Although catalase has a much smaller role in alcohol oxidation than ADH or CYP2E1, it is important in cerebral function as inhibiting catalase has been found to decrease the rate of oxidation of ethanol to acetaldehyde by the brain (3).
In conditions with high hydrogen peroxide concentration, catalase works to convert two molecules of H2O2 into two molecules of water. The first H2O2 molecule enters the active site where it oxidizes the haem iron to produce an oxyferryl group with a π-cationic porphyrin radical and a molecule of water. A second molecule of hydrogen peroxide then enters the active site and is oxidized to molecular oxygen and water. In low concentrations of H2O2, however, an alternative pathway will occur due to the strong oxidant nature of the oxyferryl group formed after the first molecule of H2O2 is oxidized. The hydroxyl radicals will exit the channel to the active site and oxidize ethanol molecules to acetaldehyde molecules outside of the active site (10).
Figure 10: Mechanism of catalase. Under high hydrogen peroxide conditions, catalase follows the mechanism on the left to oxidize two peroxide molecules. Under low concentrations of hydrogen peroxide, catalase follows the radical mechanism on the right and produces two hydroxyl radicals.
Figure 11: The hydroxyl radicals produced in the radical catalase mechanism are channeled out of the active site via this mechanism. The radicals then react with ethanol, which is held in proximity to the active site. If ethanol is not present, NADPH is oxidized.
The active site of catalase is a haem moiety with His-75 playing a major role in the catalytic activity; it uses a charge relay (residues Tyr358, Arg354, His218 and Asp348) in order to carry out reactions without disrupting peroxide binding. When the oxyferryl group is formed, the charge relay system is initiated through the continuous movement of the coordination bond electrons from one bond to the next. The resonance attributed to the movement of the coordination bond electrons allows an electron cloud to form around the iron center in which electrons can be donated to the catalytic reactions (10).
Figure 12: The active site of catalase contains a heme (yellow) with catalytic His75 on the distal side (purple), and several residues, Tyr358, Arg354, His218 and Asp348, contributing to a charge relay system on the proximal side, shown in orange (pdb file: 1qqw).
November 10, 2016 at 4:36 pm
Figure 12 looks amazing! It’s well laid out too. It might add some clarity to label to oxyferryl group within the figure, though. Also, you sort of lost me in the third paragraph when you mentioned the charge relay being in the “forward” or “reverse” directions, but other than that this is a great page.
November 10, 2016 at 5:28 pm
I think it would be beneficial to add what orange, yellow, and purple color coding represents in Figure 14.
November 11, 2016 at 1:04 am
You show the active site residues of catalase and describe the importance of the “charge relay” between the residues to carry out the reaction. Would it be possible to show a figure with these interactions of charges? And how this relates to peroxide binding?