Iron is an essential component of many enzymes, and microbes synthesize and secrete siderophores-small molecules that bind avidly to free iron in soils and in oceans-for the purpose of sequestering as much of it as they can. Many of these siderophores contain 2,3-dihy-droxybenzoic acid (2,3-DHBA) as an iron-chelating moiety; not coincidentally, the mammalian innate immunity protein siderocalin defends against pathogens by enveloping 2,3-DHBA siderophores and thus blocking bacterial iron acquisition. The anthrax bacterium makes petrobactin, a siderophore that uses the less common 3,4-DHBA as its iron chelator, which enables it to evade the clutches of siderocalin.
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