Positioning and Reactivity in SyrB2

SyrB2 is a non-heme Fe(II) halogenase in the syringomycin biosynthetic pathway that chlorinates a methyl group of L-Thr. Halogenation at typically unreactive alkanes is important as a source for mimics useful in drug design and to enhance understanding of how antibiotics are synthesized naturally. Importantly, substrate delivery to the SyrB2 active site is dictated by whether or not the substrate can bind to a long, prosthetic phosphopantetheine tether. In that sense, the catalytic mechanism is no longer defined simply by the energetics of the substrate and the catalyst but also by the mechanical properties of phosphopantetheine. In order to elucidate these structure-function relationships at a quantum mechanical level, we have extended previous work that only focused on a free substrate to now look at how placement of phosphopantetheine and substrate with respect to the catalytic center alters reaction energetics. We elucidate energy profiles that, when used in concert with biochemical observations, can help us to determine where the substrate is most likely to be when it is functionalized in the enzyme. Future work will focus on developing large-scale QM/MM studies of the full enzyme complex.