A myoglobin evolved from indoleamine 2,3-dioxygenase, a tryptophan-degrading enzyme.

The distribution, isolation, spectral and oxygen-binding properties, stability of ferrous state (autoxidation), amino acid sequence and gene structure of indoleamine 2,3-dioxygenase (IDO)-like myoglobins are summarized, and their evolution is discussed. Although it has long been thought that all hemoglobins and myoglobins have evolved from a common ancestral gene encoding a 14-16 kDa polypeptide, the discovery of IDO-like myoglobin from several gastropod molluscs clearly indicates that there was an alternative pathway for myoglobin evolution.