Figure 2. Flavohaemoglobin as a pre-eminent nitric oxide (NO)-detoxifying protein.

A typical (Gram-negative) envelope is shown that allows ready access of extracellular NO to intracellular Hmp. A small contribution to the cellular NO pool from intracellular sources is indicated. Hmp comprises a haem domain (red), an NAD(P)H-oxidizing domain (green) and an FAD domain (cyan). The redox centres are shown. The primary reaction catalysed is the conversion, by a dioxygenase mechanism, of O ₂ and NO to form nitrate. Minor reactions also reported are shown below in the blue box. Transcriptional regulators identified thus far are shown at the right (green box), and the numerous consequences of Hmp activity are indicated at the right (orange box).