Etude du système Hxu d'acquisition de l'hème de l'hémopexine de Haemophilus influenzae

Pathogens and commensals Gram-Negative bacteria have evolved different Systems to acquire iron and heme from hemoproteins. These involve specific receptors located in the outer membrane. Haemophilus influenzae is a Gram-negative bacterium that has an absolute requirement for exogenous heme, as it does not possess the enzymes necessary for the heme biosynthetic pathway except for the last one. It can in particular acquire heme from the serum glycoprotein, hemopexin, that has a very high affinity for heme, via the HxuCBA System. HxuC is a TonB dependent outer membrane receptor; it eau acquire free heme but needs HxuB and Hxu A to acquire heme from hemopexin. HxuB and HxuA are the two components of a TPS system (two partner secretion), The HxuCBA System, has been functionally reconstituted in E, coli where it allows the acquisition of heme from hemopexin. Tins activity is strictly dependent of the presence of H. Influenzae TonB complex in the bacterium. HxuA and a variant of HxuA protein, which is released in the medium, were purified and we showed that they can restore the acquisition of heme from hemopexin in a hxuA mutant of H. Influenzae and in a E. Coli. Strain that only expresses the HxuC receptor. We showed by various experiments that HxuA and hemopexin form a 1:1 stoechiometric complex. The affinity of HxuA for apo and holo-hemopexin was determined and is in the nanomolar range. When the complex between HxuA and holo-hemopexin is formed heme changes environment and seems to be released from hemopexin. It therefore allows an exogenous heme/hemophore receptor to utilize heme from hemopexin. In this work, we describe a new mechanism of heme acquisition from a hemoprotein.