Gaseous ligand binding to Porphyromonas gingivalis HmuY hemophore-like protein in complex with heme

Porphyromonas gingivalis is the main pathogenic player in the development of periodontitis. To acquire heme, being an essential source of iron and protoporphyrin IX, P. gingivalis utilizes TonB-dependent outer membrane heme receptor (HmuR) and heme-binding hemophore-like protein (HmuY) as the main system for heme uptake from host hemoproteins. In this work, we present an extensive spectroscopic characterization of the binding of exogenous gaseous ligands to the holo-form of the HmuY (HmuY-heme) to unravel the mechanistic basis of heme release. Our data are consistent with a scenario where heme release from HmuY-heme is a multistep process that requires the initial rupture of one of the two heme‑iron coordination bonds with endogenous histidines.