The evolution of oxygen transport hemoglobins occurred on at least two independent
occasions. The earliest event led to myoglobin and red blood cell hemoglobin in animals. In
plants, oxygen transport “leghemoglobins” evolved much more recently. In both events,
pentacoordinate heme sites capable of inert oxygen transfer evolved from hexacoordinate
hemoglobins that have unrelated functions. High sequence homology between
hexacoordinate and pentacoordinate hemoglobins in plants has poised them for potential …