Leghemoglobin (Lb) (also Leghaemoglobin) is an oxygen binding hemeprotein of about 16 kDa in size. Hemeproteins have various functions including oxygen transport, catalysis and active membrane transport among others. It is able to bind oxygen due to the iron containing heme at its center. It occurs in the root nodules of leguminous plants where it facilitates the diffusion of oxygen to ultimately promote nitrogen fixation. It is produced by legumes in response to nitrogen-fixating bacterial production, rhizobia. Like hemoglobin, Lb is red in color. Bacteroids synthesize the heme, and sometimes mitochondria in situations when the bacteria are defective or when Lb is produced in uninfected cells of nodules. Lb must be in the ferrous, reduced state in order to carry oxygen. Healthy nodules contain about 80% ferrous Lb and 20% oxyferrous Lb. This indicates that a mechanism exists in the nodules that maintains the reduced form of Lb.