Light-Activated Electron Transfer and Catalytic Mechanism of Carnitine Oxidation by Rieske-Type Oxygenase from Human Microbiota

Oxidation of quaternary ammonium substrate, carnitine by non-heme iron containing Acinetobacter baumannii (Ab) oxygenase CntA/reductase CntB is implicated in the onset of human cardiovascular disease. Herein, we develop a blue-light (365 nm) activation of NADH coupled to electron paramagnetic resonance (EPR) measurements to study electron transfer from the excited state of NADH to the oxidized, Rieske-type, [2Fe-2S](2+) cluster in the AbCntA oxygenase domain with and without the substrate, carnitine. Further electron transfer from one-electron reduced, Rieske-type [2Fe-2S](1+) center in AbCntA-WT to the mono-nuclear, non-heme iron center through the bridging glutamate E205 and subsequent catalysis occurs only in the presence of carnitine. The electron transfer process in the AbCntA-E205A mutant is severely affected, which likely accounts for the significant loss of catalytic activity in the AbCntA-E205A mutant. The NADH photo-activation coupled with EPR is broadly applicable to trap reactive intermediates at low temperature and creates a new method to characterize elusive intermediates in multiple redox-centre containing proteins.

Automated summary

The oxidation of quaternary ammonium substrate, carnitine, by non-heme iron-containing Acinetobacter baumannii (Ab) oxygenase CntA/reductase CntB is associated with cardiovascular disease. A study using blue-light activation of NADH coupled with EPR measurements revealed that electron transfer from NADH to the oxidized [2Fe-2S] cluster in AbCntA only occurs in the presence of carnitine. The AbCntA-E205A mutant showed severely affected electron transfer, leading to a significant loss of catalytic activity.