Direct Detection of the Terminal Hydride Intermediate in [FeFe] Hydrogenase by NMR Spectroscopy.

Hydride state intermediates are known to occur in various hydrogen conversion enzymes, including the highly efficient [FeFe] hydrogenases. The intermediate state involving a terminal iron-bound hydride has been recognized as crucial for the catalytic mechanism, but its occurrence has up to now eluded unequivocal proof under (near) physiological conditions. Here we show that the terminal hydride in the [FeFe] hydrogenase from Chlamydomonas reinhardtii can be directly detected using solution 1H NMR spectroscopy at room temperature, opening new avenues for detailed in situ investigations under catalytic conditions.

1H NMR Spectroscopy of [FeFe] Hydrogenase: Insight into the Electronic Structure of the Active Site.

The [FeFe] hydrogenase HydA1 from Chlamydomonas reinhardtii has been studied using 1H NMR spectroscopy identifying the paramagnetically shifted 1H resonances associated with both the [4Fe-4S]H and the [2Fe]H subclusters of the active site "H-cluster". The signal pattern of the unmaturated HydA1 containing only [4Fe-4S]H is reminiscent of bacterial-type ferredoxins. The spectra of maturated HydA1, with a complete H-cluster in the active Hox and the CO-inhibited Hox-CO state, reveal additional upfield and downfield shifted 1H resonances originating from the four methylene protons of the azadithiolate ligand in the [2Fe]H subsite. The two axial protons are affected by positive spin density, while the two equatorial protons experience negative spin density. These protons can be used as important probes sensing the effects of ligand-binding to the catalytic site of the H-cluster.

Structural Insight into the Complex of Ferredoxin and [FeFe] Hydrogenase from Chlamydomonas reinhardtii.

The transfer of photosynthetic electrons by the ferredoxin PetF to the [FeFe] hydrogenase HydA1 in the microalga Chlamydomonas reinhardtii is a key step in hydrogen production. Electron delivery requires a specific interaction between PetF and HydA1. However, because of the transient nature of the electron-transfer complex, a crystal structure remains elusive. Therefore, we performed protein-protein docking based on new experimental data from a solution NMR spectroscopy investigation of native and gallium-substituted PetF. This provides valuable information about residues crucial for complex formation and electron transfer. The derived complex model might help to pinpoint residue substitution targets for improved hydrogen production.

Nitrite dismutase reaction mechanism: kinetic and spectroscopic investigation of the interaction between nitrophorin and nitrite.

Nitrite is an important metabolite in the physiological pathways of NO and other nitrogen oxides in both enzymatic and nonenzymatic reactions. The ferric heme b protein nitrophorin 4 (NP4) is capable of catalyzing nitrite disproportionation at neutral pH, producing NO. Here we attempt to resolve its disproportionation mechanism. Isothermal titration calorimetry of a gallium(III) derivative of NP4 demonstrates that the heme iron coordinates the first substrate nitrite. Contrary to previous low-temperature EPR measurements, which assigned the NP4-nitrite complex electronic configuration solely to a low-spin (S = 1/2) species, electronic absorption and resonance Raman spectroscopy presented here demonstrate that the NP4-NO2(-) cofactor exists in a high-spin/low-spin equilibrium of 7:3 which is in fast exchange in solution. Spin-state interchange is taken as evidence for dynamic NO2(-) coordination, with the high-spin configuration (S = 5/2) representing the reactive species. Subsequent kinetic measurements reveal that the dismutation reaction proceeds in two discrete steps and identify an {FeNO}(7) intermediate species. The first reaction step, generating the {FeNO}(7) intermediate, represents an oxygen atom transfer from the iron bound nitrite to a second nitrite molecule in the protein pocket. In the second step this intermediate reduces a third nitrite substrate yielding two NO molecules. A nearby aspartic acid residue side-chain transiently stores protons required for the reaction, which is crucial for NPs' function as nitrite dismutase.

Hybrid [FeFe]-hydrogenases with modified active sites show remarkable residual enzymatic activity.

[FeFe]-hydrogenases are to date the only enzymes for which it has been demonstrated that the native inorganic binuclear cofactor of the active site Fe2(adt)(CO)3(CN)2 (adt = azadithiolate = [S-CH2-NH-CH2-S](2-)) can be synthesized on the laboratory bench and subsequently inserted into the unmaturated enzyme to yield fully functional holo-enzyme (Berggren, G. et al. (2013) Nature 499, 66-70; Esselborn, J. et al. (2013) Nat. Chem. Biol. 9, 607-610). In the current study, we exploit this procedure to introduce non-native cofactors into the enzyme. Mimics of the binuclear subcluster with a modified bridging dithiolate ligand (thiodithiolate, N-methylazadithiolate, dimethyl-azadithiolate) and three variants containing only one CN(-) ligand were inserted into the active site of the enzyme. We investigated the activity of these variants for hydrogen oxidation as well as proton reduction and their structural accommodation within the active site was analyzed using Fourier transform infrared spectroscopy. Interestingly, the monocyanide variant with the azadithiolate bridge showed ∼50% of the native enzyme activity. This would suggest that the CN(-) ligands are not essential for catalytic activity, but rather serve to anchor the binuclear subsite inside the protein pocket through hydrogen bonding. The inserted artificial cofactors with a propanedithiolate and an N-methylazadithiolate bridge as well as their monocyanide variants also showed residual activity. However, these activities were less than 1% of the native enzyme. Our findings indicate that even small changes in the dithiolate bridge of the binuclear subsite lead to a rather strong decrease of the catalytic activity. We conclude that both the Brønsted base function and the conformational flexibility of the native azadithiolate amine moiety are essential for the high catalytic activity of the native enzyme.

Superhelical DNA as a preferential binding target of 14-3-3γ protein.

The 14-3-3 protein family is a highly conserved and widely distributed group of proteins consisting of multiple isoforms in eukaryotes. Ubiquitously expressed, 14-3-3 proteins play key roles in DNA replication, cell cycle regulation, and apoptosis. The function of 14-3-3 proteins is mediated by interaction with a large number of other proteins and with DNA. It has been demonstrated that 14-3-3γ protein binds strongly to cruciform structures and is crucial for initiating replication. In this study, we analyzed DNA binding properties of the 14-3-3γ isoform to linear and supercoiled DNA. We demonstrate that 14-3-3γ protein binds strongly to long DNA targets, as evidenced by electrophoretic mobility shift assay on agarose gels. Binding of 14-3-3γ to DNA target results in the appearance of blurry, retarded DNA bands. Competition experiments with linear and supercoiled DNA on magnetic beads show very strong preference for supercoiled DNA. We also show by confocal microscopy that 14-3-3 protein in the HCT-116 cell line is co-localized with DNA cruciforms. This implies a role for the 14-3-3γ protein in its binding to local DNA structures which are stabilized by DNA supercoiling.

Atomic structure of the KEOPS complex: an ancient protein kinase-containing molecular machine.

Kae1 is a universally conserved ATPase and part of the essential gene set in bacteria. In archaea and eukaryotes, Kae1 is embedded within the protein kinase-containing KEOPS complex. Mutation of KEOPS subunits in yeast leads to striking telomere and transcription defects, but the exact biochemical function of KEOPS is not known. As a first step to elucidating its function, we solved the atomic structure of archaea-derived KEOPS complexes involving Kae1, Bud32, Pcc1, and Cgi121 subunits. Our studies suggest that Kae1 is regulated at two levels by the primordial protein kinase Bud32, which is itself regulated by Cgi121. Moreover, Pcc1 appears to function as a dimerization module, perhaps suggesting that KEOPS may be a processive molecular machine. Lastly, as Bud32 lacks the conventional substrate-recognition infrastructure of eukaryotic protein kinases including an activation segment, Bud32 may provide a glimpse of the evolutionary history of the protein kinase family.

Assignment-free solution NMR method reveals CesT as an unswapped homodimer.

The X-ray structure of the homodimeric chaperone CesT is the only structure among the type three secretion system (TTSS) chaperones that shows a domain swap. This swap has potential importance for the mechanism of effector translocation through a TTSS. Here we present two nuclear magnetic resonance strategies exploiting pre-existing structural models and residual dipolar couplings (RDCs), which reveal the unswapped 35.4-kDa dimer to be present in solution. Particularly efficient is the discrimination of a swapped and unswapped structural state performed simultaneously to automatic backbone assignment using only HN-RDCs and carbonyl backbone chemical shifts. This direct approach may prove to be generally useful to rapidly differentiate two structural models.

High-resolution structure determination of the CylR2 homodimer using paramagnetic relaxation enhancement and structure-based prediction of molecular alignment.

Structure determination of homooligomeric proteins by NMR spectroscopy is difficult due to the lack of chemical shift perturbation data, which is very effective in restricting the binding interface in heterooligomeric systems, and the difficulty of obtaining a sufficient number of intermonomer distance restraints. Here we solved the high-resolution solution structure of the 15.4 kDa homodimer CylR2, the regulator of cytolysin production from Enterococcus faecalis, which deviates by 1.1 angstroms from the previously determined X-ray structure. We studied the influence of different experimental information such as long-range distances derived from paramagnetic relaxation enhancement, residual dipolar couplings, symmetry restraints and intermonomer Nuclear Overhauser Effect restraints on the accuracy of the derived structure. In addition, we show that it is useful to combine experimental information with methods of ab initio docking when the available experimental data are not sufficient to obtain convergence to the correct homodimeric structure. In particular, intermonomer distances may not be required when residual dipolar couplings are compared to values predicted on the basis of the charge distribution and the shape of ab initio docking solutions.

Structure and DNA-binding properties of the cytolysin regulator CylR2 from Enterococcus faecalis.

Enterococcus faecalis is one of the major causes for hospital-acquired antibiotic-resistant infections. It produces an exotoxin, called cytolysin, which is lethal for a wide range of Gram-positive bacteria and is toxic to higher organisms. Recently, the regulation of the cytolysin operon was connected to autoinduction by a quorum-sensing mechanism involving the CylR1/CylR2 two-component regulatory system. We report here the crystal structure of CylR2 and its properties in solution as determined by heteronuclear NMR spectroscopy. The structure reveals a rigid dimer containing a helix-turn-helix DNA-binding motif as part of a five-helix bundle that is extended by an antiparallel beta-sheet. We show that CylR2 is a DNA-binding protein that binds specifically to a 22 bp fragment of the cytolysin promoter region. NMR chemical shift perturbation experiments identify surfaces involved in DNA binding and are in agreement with a model for the CylR2/DNA complex that attributes binding specificity to a complex network of CylR2/DNA interactions. Our results propose a mechanism where repression is achieved by CylR2 obstruction of the promoter preventing biosynthesis of the cytolysin operon transcript.