Oxygen binding and nitric oxide dioxygenase activity of cytoglobin are altered to different extents by cysteine modification

Cytoglobin (Cygb), like other members of the globin family, is a nitric oxide (NO) dioxygenase, metabolizing NO in an oxygen (O(2))‐dependent manner. We examined the effect of modification of cysteine sulfhydryl groups of Cygb on its O(2) binding and NO dioxygenase activity. The two cysteine sulfhydryls of Cygb were modified to form either an intramolecular disulfide bond (Cygb_SS), thioether bonds to N‐ethylmaleimide (NEM; Cygb_SC), or were maintained as free SH groups (Cygb_SH). It was observed that the NO dioxygenase activity of Cygb only slightly changed (~ 25%) while the P(50) of O(2) binding to Cygb changed over four‐fold with these modifications. Our results suggest that it is possible to separately regulate one Cygb function (such as O(2) binding) without largely affecting the other Cygb functions (such as its NO dioxygenase activity).