Crystal structure of the IIB(Sor) domain of the sorbose permease from Klebsiella pneumoniae solved to 1.75A resolution

The phosphoenolpyruvate transferase system (PTS) is the major pathway by which bacteria import hexose sugars across the plasma membrane. The PTS transfers a phosphoryl group sequentially via several components from the glycolytic intermediate phosphoenolpyruvate (PEP) to the translocated sugar. It is comprised of the two general proteins enzyme I and HPr, and a sugar-specific enzyme II complex. Sugar translocation is through the membrane domain of the enzyme II complex. The enzyme II complex can belong to one of six families based upon sequence similarity, with the sorbose transporter from Klebsiella pneumoniae a member of the mannose family.The structure of the IIB(Sor) domain was solved to 1.75A resolution by molecular replacement. It has a central core of seven parallel beta-strands surrounded by a total of six alpha-helices. Three helices cover the front face, one the back face with the remaining two capping the central beta-sheet at the top and bottom. The catalytic His15 residue is situated on the surface-exposed loop between strand 1 and helix 1. In addition to the features previously observed in the homologous IIB(Lev) domain from Bacillus subtilis we see new features in the IIB(Sor) structure. First, the catalytic His15 side-chain is fixed in a specific conformation by forming a short hydrogen bond with Asp10, which in turn makes a salt-bridge with Arg8. Second, as observed in other phosphoproteins, an arginine residue (Arg12) is well poised to stabilize a phosphoryl group on His15. Third, we see an Asp/His pair reminiscent of that observed in the IIA(Man) domain from Escherichia coli. Finally, docking of IIA(Man) to IIB(Sor) shows that Arg12 in its current conformation is well positioned to assist the subsequent transfer of the phosphoryl group onto the sugar in line with previous mutagenesis studies.