We demonstrate the high level expression of integral membrane proteins (IMPs) in a cell-free coupled transcription/translation system using a modified Escherichia coli S30 extract preparation and an optimized protocol. The expression of the E. coli small multidrug transporters EmrE and SugE containing four transmembrane segments (TMS), the multidrug transporter TehA with 10 putative TMS, and the cysteine transporter YfiK with six putative TMS, were analysed. All IMPs were produced at high levels yielding up to 2.7 mg of protein per mL of reaction volume. Whilst the vast majority of the synthesized IMPs were precipitated in the reaction mixture, the expression of a fluorescent EmrE-sgGFP fusion construct showed evidence that a small part of the synthesized protein 'remained soluble and this amount could be significantly increased by the addition of E. coli lipids into the cell-free reaction. Alternatively, the majority of the precipitated IMPs could be solubilized in detergent micelles, and modifications to the solubilization procedures yielded proteins that were almost pure. The folding induced 1 by formation of the proposed a-helical secondary structures of the IMPs after solubilization in various micelles was monitored by CD spectroscopy. Furthermore, the reconstitution of EmrE, SugE and TehA into proteoliposomes was demonstrated by freeze-fracture electron microscopy, and the function of EmrE was additionally analysed by the specific transport of ethidium. The cell-free expression technique allowed efficient amino acid specific labeling of the IMPs with 15 N isotopes, and the recording of solution NMR spectra of the solubilized EmrE, SugE and YfiK proteins further indicated a correctly folded conformation of the proteins.

Recently proposed self-consistent 3J coupling analysis (Schmidt, J. M.; Blümel, M.; Löhr, F.; Rüterjans, H. J. Biomol. NMR 1999, 14, 1-12) has been carried out to calibrate Karplus parameters constituting the empirical dependence of 3J coupling constants on the chi1 dihedral angle in amino acid side chains. The procedure involves simultaneous least-squares optimization of six sets of three Karplus coefficients related to all six 3J coupling types accessible in 15N,13C-labeled proteins. A simple concept of fundamental and incremental component couplings is proposed to account for substituent effects, eventually yielding amino acid topology-specific Karplus parameters. The method is exemplified with recombinant Desulfovibrio vulgaris flavodoxin (147 amino acids, 16 kDa) with reference to a total of 749 experimental 3JHalpha,Hbeta, 3JN',Hbeta, 3JC',Hbeta, 3JHalpha,Cgamma, 3JN',Cgamma, and 3JC',Cgamma coupling constants. Unlike other parametrizations, the present method does not make reference to X-ray coordinates, so that the Karplus coefficients obtained are not influenced by differences between solution and crystal states. Cross validation using X-ray torsion angles demonstrates the improvement relative to previous parametrizations. The Karplus coefficients derived are applicable to other proteins, too. Parameter refinement also yields a series of chi1 torsion angles, providing valuable constraints for protein structure determination, as well as optional parameters of local angular mobility in the contexts of Gaussian random fluctuation or a three-site jump model. The procedure permits automatic stereospecific assignments of Hbeta and Cgamma chemical shifts. The majority of the flavodoxin side-chain conformations agrees with high-resolution X-ray structures of the protein. Marked deviations between NMR and X-ray datasets are attributed to different rotameric states due to crystal-packing effects and to conformational equilibria between multiple chi1 rotamers.

A wide range of organophosphorus nerve agents, including Soman, Sarin, and Tabun is efficiently hydrolyzed by the phosphotriesterase enzyme diisopropyl fluorophosphatase (DFPase) from Loligo vulgaris. To date, the lack of available inhibitors of DFPase has limited studies on its mechanism. The de novo design, synthesis, and characterization of substrate analogues acting as competitive inhibitors of DFPase are reported. The 1.73 A crystal structure of O,O-dicyclopentylphosphoroamidate (DcPPA) bound to DFPase shows a direct coordination of the phosphoryl oxygen by the catalytic calcium ion. The binding mode of this substrate analogue suggests a crucial role for electrostatics in the orientation of the ligand in the active site. This interpretation is further supported by the crystal structures of double mutants D229N/N120D and D229N/N175D, designed to reorient the electrostatic environment around the catalytic calcium. The structures show no differences in their calcium coordinating environment, although they are enzymatically inactive. Additional double mutants E21Q/N120D and E21Q/N175D are also inactive. On the basis of these crystal structures and kinetic and mutagenesis data as well as isotope labeling we propose a new mechanism for DFPase activity. Calcium coordinating residue D229, in concert with direct substrate activation by the metal ion, renders the phosphorus atom of the substrate susceptible for attack of water, through generation of a phosphoenzyme intermediate. Our proposed mechanism may be applicable to the structurally related enzyme paraoxonase (PON), a component of high-density lipoprotein (HDL).