Structural and biochemical study of Bacillus subtilis HmoB in complex with heme

Most bacteria have developed a hemoprotein degradation system to acquire iron from their hosts. Bacillus subtilis HmoB, a heme monooxygenase, is involved in the degradation of heme and subsequent release of iron. HmoB contains a C-terminal ABM domain, which is similar in sequence and structure to other heme monooxygenases. Heme degradation assay showed that highly conserved residues (N70, W128, and H138) near the heme-binding site were critical for activity of HmoB. However, HmoB was shown to be different from other bacterial heme oxygenases due to its longer N-terminal region and formation of a biological monomer instead of a dimer. The degradation product of B. subtilis HmoB was identified as staphylobilin from mass spectrometric analysis of the product and release of formaldehyde during degradation reaction.