Although with a distinct heme active site, both myoglobin (Mb) and cytochrome c oxidase (CcO) were found to function as a nitrite reductase (NIR) under hypoxic conditions. On the other hand, Mb was rationally designed to mimic native CcO by introduction of two distal histidines, i.e., L29H/F43H mutant, where His29, His43 and native His64 formed a metal-binding site. To probe the role of distal histidines in regulating the NIR activity of Mb, we herein designed a single mutant of L29H Mb that contains two distal histidines and solved its X-ray crystal structure, then we made both structural and...
