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. We characterized catalytic activities of recombinant CmSiRB. The turnover number of nitrite reduction of CmSiRB was relatively high, and the affinity for nitrite was low. The turnover number 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. We obtained site-directed mutated CmSiRB, which has twice the sulfite reducing activity and half the nitrite reducing activity of the wild-type enzyme.
