Comparison of alkyl hydroperoxide reductase (AhpR) and water-forming NADH oxidase from Lactococcus lactis ATCC 19435

We have successfully applied the sequence comparison-based approach to develop a peroxidase (gene AhpC) and a water-forming NADH oxidase from Lactococcus lactis (L. lactis). We found a considerably lower maximum specific activity of nox-1 (AhpF) (15 U/mg) from L. lactis compared to its nox-2 counterpart (95 U/mg). Both nox-1 and nox-2 are turnover-limited, as expected for enzymes with labile, redox-active thiols in the active site. In the absence of exogenously added thiols, both nox-1 and nox-1/peroxidase. are considerably more stable against overoxidation than nox-2: the total turnover number TTN is 82,000 for nox-1 and nox-1/peroxidase vs. 39,000 for nox-2. Addition of exogenous thiols, however, increases nox-2 stability by a factor of two, up to the level of nox-1. Kinetic and stability analysis does not reveal any clear advantage for oxygen scavenging via the nox-1 or the nox-2 routes in lactic acid bacteria. Expression levels in lactic acid bacteria upon exposure to oxidative stress rather than kinetic performance more likely account for the previously observed superiority of nox-2 effectiveness over nox-1.