Emerging strategies in microbial haem capture

Gram-negative pathogenic bacteria have evolved novel strategies to obtain i ron from host haem-sequestering proteins. These include the production of s pecific outer membrane receptors that bind directly to host haem-sequesteri ng proteins, secreted haem-binding proteins (haemophores) that bind haem/ha emoglobin/haemopexin and deliver the complex to a bacterial cell surface re ceptor and bacterial proteases that degrade haem-sequestering proteins. Onc e removed from haem-sequestering proteins, haem may be transported via the bacterial outer membrane receptor into the cell. Recent studies have begun to define the steps by which haem is removed from bacterial haem proteins a nd transported into the cell. This review describes recent work on the disc overy and characterization of these systems. Reference is also made to the transport of haem in serum (via haemoglobin, haemoglobin/haptoglobin, haemo pexin, albumin and lipoproteins) and to mechanisms of iron removal from the haem itself (probably via a haem oxygenase pathway in which the protoporph yrin ring is degraded). Haem protein-receptor interactions are discussed in terms of the criteria that govern protein-protein interactions in general, and connections between haem transport and the emerging field of metal tra nsport via metallochaperones are outlined.