Mutational analysis of the nitrogenase carbon monoxide protective protein CowN reveals that a conserved C-terminal glutamic acid residue is necessary for its activity

Abstract

Nitrogenase is the only enzyme that catalyzes the reduction of nitrogen gas into ammonia. Nitrogenase is tightly inhibited by the environmental gas carbon monoxide (CO). Many nitrogen fixing bacteria protect nitrogenase from CO inhibition using the protective protein CowN. This work demonstrates that a conserved glutamic acid residue near CowNs C-terminus is necessary for its function. Mutation of the glutamic acid residue abolishes both CowNs protection against CO inhibition and CowNs ability to bind to nitrogenase. In contrast, a conserved C-terminal cysteine residue is not important for CO protection. Overall, this work uncovers structural features in CowN that are required for its function and provides new insights into its nitrogenase binding and CO protection mechanism.