Hemophore Has A

Abstract

To satisfy their iron needs, several gram‐negative bacteria use a heme uptake system that relies on the secretion in the extracellular medium of heme‐binding proteins called HasA hemophores . These proteins scavenge heme from the host, either free or hemoprotein‐bound, and shuttle it to a specific outer membrane transporter, HasR, whereby it is internalized by a TonB‐dependent process and then used as an iron source. HasA hemophores form a family of highly conserved 19–20 kDa proteins without either sequential or structural similarities to other known proteins. The HasA hemophore from Serratia marcescens , the most deeply studied, is described here with a special focus on its heme binding site and its unusual pair of axial ligands, a histidine and a tyrosine. The spectroscopic properties and the structures of the heme loaded and unloaded hemophore, free or complexed to the HasR transporter are presented. The mechanisms of heme capture by the hemophore and heme transfer to HasR are discussed.