Methaemoglobin

Methemoglobin is a form of the heme group is in the Fe³⁺ state, not the Fe²⁺ of normal hemoglobin. Methemoglobin is unable to carry oxygen. It is chocolate-brown in color. The NADH-dependent enzyme methemoglobin reductase (AKA diaphorase I) is responsible for converting methemoglobin back to hemoglobin.