NapABC

NapABC is a bacterial multienzyme complex that forms part of the periplasmic nitrate reductase system involved in anaerobic respiration. It participates in dissimilatory nitrate reduction, enabling some bacteria to use nitrate as terminal electron acceptor when oxygen is scarce.

In NapABC, NapA is a molybdenum-containing catalytic subunit that reduces nitrate to nitrite in the periplasm; NapB is a small c-type cytochrome that shuttles electrons; NapC is an inner membrane–associated cytochrome that transfers electrons from the quinol pool to NapB and NapA.

The napABC genes encode the components; transcription is regulated by environmental conditions such as oxygen and nitrate levels, often via two-component regulatory systems or FNR-like regulators; accessory factors such as NapD may assist NapA maturation, while in some bacteria additional Nap proteins participate in electron transfer.

NapABC is widespread among Gram-negative bacteria, including Paracoccus denitrificans, Rhodobacter sphaeroides, and Shewanella species, and is found in organisms that perform nitrate respiration under anaerobic or microaerophilic conditions. The complex forms part of a larger nitrate-reducing network that can interact with other electron transport branches; electron flow generally proceeds from intracellular donors through NapC to NapB and NapA, with NapA catalyzing nitrate reduction in the periplasm.

Its study informs understanding of microbial energy metabolism, environmental nitrate cycling, and potential applications in bioremediation and bioenergy.