Volume 3, Issue 23 October 2006
Ironing Out Bacterial Infections
Kenneth Raymond works on iron-chelating siderophore molecules with graduate students Rebecca Abergel (left) and Trisha Hoette (center). (Courtesy the Raymond Lab)
From the moment bacteria invade a human cell, they launch a battle against the body to acquire iron. Like humans, bacteria need this mineral to make energy and replicate their genes. Though iron is the second-most abundant metal on Earth, it's in short supply inside cells, where it's bound to the storage proteins transferrin and ferritin.
Pathogenic bacteria have evolved an ingenious way to overcome this problem. By sending out small iron-chelating molecules known as siderophores, bacteria can literally wrest iron away from storage proteins. Many siderophores can bind to iron orders of magnitude more strongly than transferrin and ferritin.
UC Berkeley professor of chemistry Kenneth Raymond has been studying bacterial iron uptake for more than 30 years. It's part of his multifaceted work on coordination chemistry, which ranges from improving MRI contrast agents to devising radioactive waste cleanup methods. This September, the National Institute of Allergy and Infectious Diseases awarded Raymond a $998,325 grant to develop cocktails to treat radiation poisoning by trapping radioactive metal ions.
"I'm interested in how nature transports metals," Raymond says. "I'm interested in siderophores because they are amazingly selective at binding metal ions."
Hundreds of different siderophores have been isolated from dozens of bacterial species living in oceans, soils, and everywhere in between. The rogues' gallery includes some of nature's most deadly pathogens: those responsible for tuberculosis and cholera, salmonella and typhoid fever, bubonic plague and anthrax.
Luckily, humans aren't defenseless in the face of siderophore-bearing bacteria. About five years ago, Roland Strong of the Fred Hutchinson Cancer Research Center in Seattle found that a protein isolated from white blood cells formed ruby red crystals. The crystals' crimson color was a tipoff that they might contain iron. Strong asked Raymond to help determine whether the protein might be involved in thwarting bacterial iron acquisition. The collaboration opened up a fruitful new chapter in immunology.
"Siderocalin is essentially a vacuum cleaner for siderophores. It's the human immune system's response to interrupting bacterial infection," Raymond says. His laboratory is studying all three steps of the siderophore system—the structure of siderophores, the binding behavior of siderocalin, and how bacteria recover iron once it's bound to a siderophore—in hopes of finding ways to foil bacterial iron uptake.
E. coli bacteria use the siderophore enterobactin to steal iron from human proteins such as transferrin. The immune system protein siderocalin can intercept enterobactin, but can't recognize "stealth siderophores" such as aerobactin and salmochelin. (The Raymond Lab)
Raymond is studying why this immune system protein recognizes some siderophores but not others. "If you look at the siderocalin receptor site, it looks like a glove. What is the hand for which this glove was designed?" To probe which characteristics are most important for siderocalin recognition, the researchers are synthesizing natural siderophores with small, custom-made variations. They've found the protein is a generalist that can bind to a wide variety of siderophores a good thing if it's the body's main defense against these molecules.
But the deadliest bacterial strains aren't so easily deterred. To ensure an iron supply, they produce second- or even third-line siderophores. At first glance, these molecules don't seem very formidable; Raymond has found them to be far weaker iron chelators than their first-line cousins. However, "the weaker chelators have the wrong structure to fit into siderocalin. These second chelators are a way to evade the immune system," Raymond says. "This is what I call siderophore stealth."
To determine why siderocalin doesn't bind second-line siderophores, they studied the example of salmochelin. Produced by the bacterium responsible for typhoid fever, it's identical to enterobactin, except that it brandishes big glucose groups on two of its three iron-binding arms. "What this does is make the siderophore too water-soluble and bulky to bind to siderocalin. This glycosylation is another way for the organism to evade the human immune system," Raymond says.
One receptor that binds many types of siderophores helps bacteria recover iron with a novel shuttle mechanism. (from Stintzi et al. Proc. National Academy Science, 2000, 97, 10691-1096)
The third prong in Raymond's research program is determining how bacteria recover iron-siderophore complexes. Already he has discovered that some bacteria use just one general receptor to recognize many siderophores. In this system, the iron-siderophore complex binds to the bacterial receptor and passes the iron to an empty shuttle siderophore, which is what ultimately ferries the metal inside the bacterium. "Iron comes in with a date to the dance, but it never goes into the cell with that date; it always exchanges," Raymond says.
Raymond's discoveries about siderophores have opened up a whole new universe of antibiotic design. Already researchers are developing molecules meant to clog siderophore receptors and essentially starve infecting microbes of iron.
For a decade, Raymond was the only chemist working on siderophores. Today, there is an entire conference devoted to siderocalin alone, attended by dozens of immunologists, biochemists, and chemists. Raymond himself isn't sure where the field is headed. "There have been more twists and turns in this field than an Agatha Christie novel," Raymond says. "If you'd asked me ten years ago whether the human immune system is involved in intercepting iron transport, I'd have said there is no evidence of that. There have been many such surprises over the years I've been working on these compounds; I'm sure I'll see more."