This is the N-terminal tail of the respiratory nitrate reductase alpha chain. The nitrate reductase complex is a dimer of heterotrimers each consisting of an alpha, beta and gamma chain. The N-terminal tail of the alpha chain interacts with the beta chain and contributes to the stability of the heterotrimer [1].

1: Insights into the respiratory electron transfer pathway from the structure of nitrate reductase A. Bertero MG, Rothery RA, Palak M, Hou C, Lim D, Blasco F, Weiner JH, Strynadka NC; Nat Struct Biol. 2003;10:681-687. PMID:12910261