Heme is ubiquitous, abundant and necessary for energy metabolism. Most bacteria have a heme biosynthesis pathway, but nevertheless, since heme is a major source of iron (an essential metal), microbes take up exogenous heme to retrieve iron. To grab heme, microbes extract it from host hemoproteins. This is achieved by two non-exclusive distinct pathways. One pathway involves proteins secreted by bacteria (hemophores) that scavenge heme from host hemoproteins. The second pathway involves microbial cell surface receptors that catch hemoproteins circulating in the vicinity of the cell surface. Both pathways lead to heme docking to cell surface receptors. In Gram-negative bacteria, docked heme is transported through the outer membrane by an energy-dependent process. In Gram-positive bacteria, docked heme is transferred to membrane-anchored heme binding lipoproteins. In all thus far described systems, heme is actively transported through the plasma membrane by an ATP hydrolysis-powered ABC transporter. Heme is either degraded into biliverdin, CO and iron by heme oxygenases, or iron is retrieved from heme, keeping the tetrapyrrol ring intact by recently identified enzymes. As excess heme is toxic, heme uptake, efflux and degradation are usually highly regulated. In most cases, intracytoplasmic heme or iron released during heme degradation are cofactors along with transcriptional regulators. In several cases, heme uptake and efflux are regulated by extracellular heme.

Further reading: Iron Uptake and Homeostasis in Microorganisms

Labels: Exogenous heme, Haeme, Heme, Heme biosynthesis pathway, Heme degradation, Heme efflux, Heme uptake