Many microbial pathogens are known to possess heme acquisition systems that allow the organisms to utilize heme or heme-bound proteins as sources of iron. These systems have been demonstrated to procure iron from heme, hemoglobin, haptoglobin, or hemopexin and are identified as nonsiderophore heme iron acquisition systems. Heme oxygenase activity from mammalian cells was first demonstrated in the 1960s. This enzyme is capable of degrading heme to α-biliverdin with the liberation of free iron. Bacterial heme oxygenase activity was first found in the gram-positive pathogen Corynebacterium diphtheriae, the causative organism of the respiratory disease diphtheria. Structure-function relationships have been investigated by site-directed mutagenesis in the hopes of identifying crucial residues participating in the binding of heme and the enzymatic cleavage of the porphryin ring. Overall, human heme oxygenase HO-1 and bacterial heme oxygenase HmuO are mechanistically the same, and they show only slight structural variations when investigated by such methods as electron paramagnetic resonance, resonance Raman spectroscopy, and nuclear magnetic resonance studies.