Abstract
Plant nitrite reductase (NiR) and sulfite reductase (SiR) have common features in structure and function. Both enzymes are generally distinguished in terms of preferences for nitrite and sulfite as substrates. The Cyanidioschyzon merolae genome encodes two SiR homologs, termed CmSiRA and CmSiRB, but no NiR homolog. As most known SiR have a low NiR activity and CmSiRB is mapped between the genes for nitrate transporter and nitrate reductase, CmSiRB could be a nitrite-reducing enzyme. We characterized catalytic activities of recombinant CmSiRB. The catalytic-center activity of nitrite reduction of CmSiRB was relatively high, and the affinity for nitrite was low. The catalytic-center activity of sulfite reduction was extremely low, and the affinity for sulfite was very high. This result demonstrates that CmSiRB is a special SiR having reduced activity of sulfite reduction and enhanced activity of nitrite reduction.
