15 December 2009

Natural linking – though not of fragments

We’ve previously discussed the appeal and challenges of fragment linking. A new paper in Science describes how a naturally occurring antibiotic makes use of a linking strategy, albeit using rather Brobdingnagian fragments.

Simocyclinone D8 (SD8) is a dumbbell shaped molecule isolated several years ago from that ultimate micro-pharma, Streptomyces. Although SD8 blocks the action of bacterial DNA gyrase, which is also the target of fluoroquinolones such as ciprofloxacin and aminocoumarins such as novobiocin, it is mechanistically distinct from these older antibiotics. To understand why, Anthony Maxwell and colleagues at the John Innes Centre in Norwich, UK, solved the co-crystal structure of SD8 bound to GyrA. The structure reveals that the protein forms a dimer of dimers, with four molecules of SD8 bound to the four subunits of GyrA. Weirdly, each molecule of SD8 cross-links two separate subunits of GyrA, although mass-spectrometry, analytical ultracentrifugation, and modeling suggest that a single molecule of SD8 could also bind to a single GyrA subunit. The crystal structure shows that SD8 binds near – but not at – the fluoroquinolone binding site, blocking the DNA-binding portion of GyrA.

What caught my eye is the fact that both halves of the molecule are active by themselves, albeit with a loss in potency (see figure). The linker is over one nanometer long and doesn’t appear to make significant interactions with the protein; it would be fun to know how something like this evolved.

Antibiotics gleefully seem to ignore the Rule of 5, but it wouldn’t hurt to get a smaller, less complicated analog. Replacing either of the two ends with smaller fragments may be a productive approach, as would optimizing the individual “fragments” themselves.

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