Functional and structural characterisation of a viral cytochrome b5

<jats:p>Cytochrome <jats:italic>b</jats:italic>5 is a ubiquitous electron transport protein. The sequenced viral OtV‐2 genome, which infects <jats:italic>Ostreococcus tauri</jats:italic>, was predicted to encode a putative cytochrome <jats:italic>b</jats:italic>5 enzyme. Using purified OtV‐2 cytochrome <jats:italic>b</jats:italic>5 we confirm this protein has identical spectral properties to purified human cytochrome <jats:italic>b</jats:italic>5 and additionally that the viral enzyme can substitute for yeast cytochrome <jats:italic>b</jats:italic>5 in yeast cytochrome P450 51 mediated sterol 14α‐demethylation. The crystal structure of the OtV‐2 cytochrome <jats:italic>b</jats:italic>5 enzyme reveals a single domain, comprising four β sheets, four α helices and a haem moiety, which is similar to that found in larger eukaryotic cytochrome proteins. As a product of a horizontal gene transfer event involving a subdomain of the host fumarate reductase‐like protein, OtV‐2 cytochrome <jats:italic>b</jats:italic>5 appears to have diverged in function and is likely to have evolved an entirely new role for the virus during infection. Indeed, lacking a hydrophobic C‐terminal anchor, OtV‐2 encodes the first cytosolic cytochrome <jats:italic>b</jats:italic>5 characterised. The lack of requirement for membrane attachment (in contrast to all other microsomal cytochrome <jats:italic>b</jats:italic>5s) may be a reflection of the small size of the host cell, further emphasizes the unique nature of this virus gene product and draws attention to the potential importance of cytochrome <jats:italic>b</jats:italic>5 metabolic activity at the extremes of cellular scale.</jats:p>