A cytochrome c is the natural electron acceptor for nicotine oxidoreductase

Nicotine oxidoreductase (NicA2), a member of the flavin-containing amine oxidase family, is of medical relevance as it shows potential as a therapeutic to aid cessation of smoking due to its ability to oxidize nicotine into a non-psychoactive metabolite. However, the use of NicA2 in this capacity is stymied by its dismal O2-dependent activity. Unlike other enzymes in the amine oxidase family, NicA2 reacts very slowly with O2, severely limiting its nicotine-degrading activity. Instead of using O2 as an oxidant, we discovered that NicA2 donates electrons to a cytochrome c, which means that NicA2 is actually a dehydrogenase. This is surprising, as enzymes of the flavin-containing amine oxidase family were invariably thought to use O2 as an electron acceptor. Our findings establish new perspectives for engineering this potentially useful therapeutic and prompt a reconsideration of the term ‘oxidase’ in referring to members of the flavin-containing amine ‘oxidase’ family. Unlike other amine oxidase-family enzymes, nicotine oxidoreductase (NicA2) reacts very slowly with oxygen, prompting the search for and identification of a cytochrome c protein (CycN) that is responsible for accepting electrons from NicA2 in vivo.