Antibacterial Resistance – Learning Bacterial Tricks
Mar23

Antibacterial Resistance – Learning Bacterial Tricks

Virulent bacteria are growing increasingly resilient against our best antibiotics. Each day seems to bring a new story: MRSA outbreaks, resistant salmonella, or tough-to-treat tuberculosis. Just last week, World Health Organization director-general Dr. Margaret Chan delivered an address in Copenhagen, where she cautioned: “We are losing our first-line antimicrobials . . . in terms of replacement antibiotics, the pipeline is virtually dry. The cupboard is nearly bare.” (Click here for The Haystack’s past coverage of the development of new antibacterials). Why have our drugs stopped working? Recent research from St. Jude’s (Science, 2012, 1110) attempted to answer that question. Using X-ray crystallography, a technique used to see structures at the atomic level, the researchers were able to capture a critical moment when a drug binds to DHPS, its bacterial enzyme target. The scientists could then predict how bacteria evolve to dodge further biocidal bullets. The antibacterial medicines caught in the act by the St. Jude’s researchers are the sulfa drugs (see right), former front-line treatments many doctors push to the bottom of treatment regimens, due to increasingly resistant bacterial strains. Researchers knew resistance had something to do with the drugs’ mechanism of action; sulfa drugs mimic the binding of PABA – para-aminobenzoic acid, a compound found in many sunscreens (Chemical Note: PABA occurs naturally as bacterial vitamin H1, and can also be found in yeast and plants. Chemists often borrow naturally-occurring compounds for industrial uses; two prominent examples are vanillin and Vitamin C). Disruption of this PABA binding shuts down bacterial DNA replication, stopping reproduction. Before now, however, no one had succeeded in growing crystals of the active site that actually showed the drugs interacting with the enzymatic intermediate. Let’s take one more step back: how does PABA attach itself? The enzyme we’re discussing, DHPS, catalyzes bond formation between PABA and intermediates known as pterins (see picture, left). Earlier researchers believed that this molecular hook-up operated by an SN2 mechanism, a reaction where the PABA kicks out a small piece of the pterin to form a new C-N bond. We chemists would say that SN2 means concerted bond formation, meaning that PABA would bind at the same time as the leaving group (OPPi), well, leaves. Turns out that picture’s not quite right: it’s more SN1-like, which means that the pterin first forms a positively-charged, enzyme-stabilized species! As you can imagine, this is no small feat, since the reaction works at physiological pH, in water, which could hydrate the intermediate (but doesn’t). Nope – instead, this charged molecule sits around waiting for a PABA – or a sulfa drug – to bind to it. When PABA binds, the complex exits...

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