1.7 A X-ray structure of the periplasmic ribose receptor from Escherichia coli

The X-ray structure of the periplasmic ribose receptor (binding protein) of Escherichia coli (RBP) was solved at 3 A resolution by the method of multiple isomorphous replacement. Alternating cycles of refitting and refinement have resulted in a model structure with an R-factor of 18.7% for 27,526 reflections from 7.5 to 1.7 A resolution (96% of the data). The model contains 2228 non-hydrogen atoms, including all 271 residues of the amino acid sequence, 220 solvent atoms and beta-D-ribose. The protein consists of two highly similar structural domains, each of which is composed of a core of parallel beta-sheet flanked on both sides by alpha-helices. The two domains are related to each other by an almost perfect 2-fold axis of rotation, with the C termini of the beta-strands of each sheet pointing toward the center of the molecule. Three short stretches of amino acid chain (from symmetrically related portions of the protein) link these two domains, and presumably act as a hinge to allow relative movement of the domains in functionally important conformational changes. Two water molecules are also an intrinsic part of the hinge, allowing crucial flexibility in the structure. The ligand beta-D-ribose (in the pyranose form) is bound between the domains, held by interactions with side-chains of the interior loops. The binding site is precisely tailored, with a combination of hydrogen bonding, hydrophobic and steric effects giving rise to tight binding (0.1 microM for ribose) and high specificity. Four out of seven binding-site residues are charged (2 each of aspartate and arginine) and contribute two hydrogen bonds each. The remaining hydrogen bonds are contributed by asparagine and glutamine residues. Three phenylalanine residues supply the hydrophobic component, packing against both faces of the sugar molecule. The arrangement of these hydrogen bonding and hydrophobic residues results in an enclosed binding site with the exact shape of the allowed sugar molecules; in the process of binding, the ligand loses all of its surface-accessible area. The sites of two mutations that affect the rate of folding of the ribose receptor are shown to be located near small cavities in the wild-type protein. The cavities thus allow the incorporation of the larger residues in the mutant proteins. Since these alterations would seriously affect the ability of the protein to build the first portion of the hydrophobic core in the first domain, it is proposed that this process is the rate-limiting step in folding of the ribose receptor.