Catalytic action of Mn-superoxide dismutase in scavenging superoxide radical anion by double hydrogen abstraction from dihydrolipoic acid: A theoretical study

The mechanism of scavenging superoxide radical anion (O-2(center dot-)) by dihydrolipoic acid (diLA) in absence and presence of the enzyme Manganese-superoxide dismutase (Mn-SOD) has been investigated using density functional theory. Mn-SOD was modelled by a complex of a manganese cation (Mn2+) bonded to three similar molecules having a histidine ring each and a water molecule. It has been shown that the scavenging mechanism involves double hydrogen abstraction by O-2(center dot-) from different pairs of neighboring sites of diLA. It has been found that diLA alone cannot scavenge superoxide radical anions efficiently as the barrier energies involved in the reactions are very high. However, in presence of Mn-SOD, owing to its catalytic action, the corresponding reactions become barrierless due to which superoxide radical anions would be scavenged highly efficiently. H2O2 formed from superoxide radical anion due to double hydrogen abstraction from diLA is scavenged by diLA alone barrierlessly without involving Mn-SOD or any other catalyst.