[File:NADH cytochrome B5 reductase 1UMK.png|thumbnail|right|The structure of the enzyme that converts methemoglobin to hemoglobin]]
Methemoglobin (English:
methaemoglobin) (pronounced "met-hemoglobin") is a form of the
oxygen-carrying
metalloprotein hemoglobin, in which the
iron in the
heme group is in the Fe
3+ (
ferric) state, not the Fe
2+ (
ferrous) of normal hemoglobin. Methemoglobin cannot bind oxygen, unlike
oxyhemoglobin. It is bluish chocolate-brown in color. In human blood a trace amount of methemoglobin is normally produced spontaneously, but when present in excess the blood becomes abnormally dark bluish brown. The NADH-dependent enzyme
methemoglobin reductase (diaphorase I) is responsible for converting methemoglobin back to hemoglobin.