Aldehyde-based activation of C2α-lactylthiamin diphosphate decarboxylation on bacterial 1-deoxy-d-xylulose 5-phosphate synthase.

1-Deoxy-d-xylulose 5-phosphate synthase (DXPS) catalyzes the thiamin diphosphate (ThDP)-dependent formation of DXP from pyruvate (donor substrate) and d-glyceraldehyde 3-phosphate (d-GAP, acceptor substrate) in bacterial central metabolism. DXPS uses a ligand-gated mechanism in which binding of a small molecule "trigger" activates the first enzyme-bound intermediate, C2α-lactylThDP (LThDP), to form the reactive carbanion via LThDP decarboxylation. d-GAP is the natural acceptor substrate for DXPS and also serves a role as a trigger to induce LThDP decarboxylation in the gated step. Additionally, we have shown that O2 and d-glyceraldehyde (d-GA) can induce LThDP decarboxylation. We hypothesize this ligand-gated mechanism poises DXPS to sense and respond to cellular cues in metabolic remodeling during bacterial adaptation. Here we sought to characterize features of small molecule inducers of LThDP decarboxylation. Using a combination of CD, NMR and biochemical methods, we demonstrate that the α-hydroxy aldehyde moiety of d-GAP is sufficient to induce LThDP decarboxylation en route to DXP formation. A variety of aliphatic aldehydes also induce LThDP decarboxylation. The study highlights the capacity of DXPS to respond to different molecular cues, lending support to potential multifunctionality of DXPS and its metabolic regulation by this mechanism.

19F NMR Reveals the Dynamics of Substrate Binding and Lid Closure for Iodotyrosine Deiodinase as a Complement to Steady-State Kinetics and Crystallography.

Active site lids are common features of enzymes and typically undergo conformational changes upon substrate binding to promote catalysis. Iodotyrosine deiodinase is no exception and contains a lid segment in all of its homologues from human to bacteria. The solution-state dynamics of the lid have now been characterized using 19F NMR spectroscopy with a CF3-labeled enzyme and CF3O-labeled ligands. From two-dimensional 19F-19F NMR exchange spectroscopy, interconversion rates between the free and bound states of a CF3O-substituted tyrosine (45 ± 10 s-1) and the protein label (40 ± 3 s-1) are very similar and suggest a correlation between ligand binding and conformational reorganization of the lid. Both occur at rates that are ∼100-fold faster than turnover, and therefore these steps do not limit catalysis. A simple CF3O-labeled phenol also binds to the active site and induces a conformational change in the lid segment that was not previously detectable by crystallography. Exchange rates of the ligand (130 ± 20 s-1) and protein (98 ± 8 s-1) in this example are faster than those above but remain self-consistent to affirm a correlation between ordering of the lid and binding of the ligand. Both ligands also protect the protein from limited proteolysis, as expected from their ability to stabilize a compact lid structure. However, the minimal turnover of simple phenol substrates indicates that such stabilization may be necessary but is not sufficient for efficient catalysis.

Disruption of an Active Site Network Leads to Activation of C2α-Lactylthiamin Diphosphate on the Antibacterial Target 1-Deoxy-d-xylulose-5-phosphate Synthase.

The bacterial metabolic enzyme 1-deoxy-d-xylulose-5-phosphate synthase (DXPS) catalyzes the thiamin diphosphate (ThDP)-dependent formation of DXP from pyruvate and d-glyceraldehyde-3-phosphate (d-GAP). DXP is an essential bacteria-specific metabolite that feeds into the biosynthesis of isoprenoids, pyridoxal phosphate (PLP), and ThDP. DXPS catalyzes the activation of pyruvate to give the C2α-lactylThDP (LThDP) adduct that is long-lived on DXPS in a closed state in the absence of the cosubstrate. Binding of d-GAP shifts the DXPS-LThDP complex to an open state which coincides with LThDP decarboxylation. This gated mechanism distinguishes DXPS in ThDP enzymology. How LThDP persists on DXPS in the absence of cosubstrate, while other pyruvate decarboxylases readily activate LThDP for decarboxylation, is a long-standing question in the field. We propose that an active site network functions to prevent LThDP activation on DXPS until the cosubstrate binds. Binding of d-GAP coincides with a conformational shift and disrupts the network causing changes in the active site that promote LThDP activation. Here, we show that the substitution of putative network residues, as well as nearby residues believed to contribute to network charge distribution, predictably affects LThDP reactivity. Substitutions predicted to disrupt the network have the effect to activate LThDP for decarboxylation, resulting in CO2 and acetate production. In contrast, a substitution predicted to strengthen the network fails to activate LThDP and has the effect to shift DXPS toward the closed state. Network-disrupting substitutions near the carboxylate of LThDP also have a pronounced effect to shift DXPS to an open state. These results offer initial insights to explain the long-lived LThDP intermediate and its activation through disruption of an active site network, which is unique to DXPS. These findings have important implications for DXPS function in bacteria and its development as an antibacterial target.

Metal Ion-Induced Large Fragment Deactivation: A Different Strategy for Site-Selectivity in a Complex Molecule.

Complex natural product functionalizations generally involve the use of highly engineered reagents, catalysts, or enzymes to react exclusively at a desired site through lowering of a select transition state energy. In this communication, we report a new, complementary strategy in which all transition states representing undesirable sites in a complex ionophore substrate are simultaneously energetically increased through the chelation of a metal ion to the large fragment we wish to neutralize. In the case of an electrophilic, radical based fluorination reaction, charge repulsion (electric field effects), induced steric effects, and electron withdrawal provide the necessary deactivation and proof of principle to afford a highly desirable natural product derivative. We envisage that many other electrophilic or charge based synthetic methods may be amenable to this approach as well.

Stability Islands and the Folding Cooperativity of a Seven-Repeat Array from Topoisomerase V.

Cooperativity is a central feature of protein folding, but the thermodynamic and structural origins of cooperativity remain poorly understood. To quantify cooperativity, we measured guanidine-induced unfolding transitions of single helix-hairpin-helix (HhH)2 repeats and tandem pairs from a seven-repeat segment of Methanopyrus kandleri Topoisomerase V (Topo V) to determine intrinsic repeat stability and interfacial free energies between repeats. Most single-repeat constructs are folded and stable; moreover, several pairs have unfolding midpoints that exceed midpoints of the single repeats they comprise, demonstrating favorable coupling between repeats. Analyzing unfolding transitions with a modified Ising model, we find a broad range of intrinsic and interfacial free energies. Surprisingly, the G repeat, which lacks density in the crystal structure of Topo V without DNA, is the most stable repeat in the array. Using nuclear magnetic resonance spectroscopy, we demonstrate that the isolated G repeat adopts a canonical (HhH)2 fold and forms an ordered interface with the F-repeat but not with the H repeat. Using parameters from our paired Ising fit, we built a partition function for the seven-repeat array. The multistate unfolding transition predicted from this partition function is in excellent agreement with the experimental unfolding transition, providing strong justification for the nearest-neighbor model. The seven-repeat partition function predicts a native state in which three independent segments ("stability islands") of interacting repeats are separated by two unstable interfaces. We confirm this segmented architecture by measuring the unfolding transition of an equimolar mixture of these three separate polypeptides. This segmented structural organization may facilitate wrapping around DNA.

Structural features of the Notch ankyrin domain-Deltex WWE2 domain heterodimer determined by NMR spectroscopy and functional implications.

The Notch signaling pathway, an important cell fate determination pathway, is modulated by the ubiquitin ligase Deltex. Here, we investigate the structural basis for Deltex-Notch interaction. We used nuclear magnetic resonance (NMR) spectroscopy to assign the backbone of the Drosophila Deltex WWE2 domain and mapped the binding site of the Notch ankyrin (ANK) domain to the N-terminal WWEA motif. Using cultured Drosophila S2R+ cells, we find that point substitutions within the ANK-binding surface of Deltex disrupt Deltex-mediated enhancement of Notch transcriptional activation and disrupt ANK binding in cells and in vitro. Likewise, ANK substitutions that disrupt Notch-Deltex heterodimer formation in vitro block disrupt Deltex-mediated stimulation of Notch transcription activation and diminish interaction with full-length Deltex in cells. Surprisingly, the Deltex-Notch intracellular domain (NICD) interaction is not disrupted by deletion of the Deltex WWE2 domain, suggesting a secondary Notch-Deltex interaction. These results show the importance of the WWEA:ANK interaction in enhancing Notch signaling.

Many dissimilar NusG protein domains switch between α-helix and ß-sheet folds.

Folded proteins are assumed to be built upon fixed scaffolds of secondary structure, α-helices and ß-sheets. Experimentally determined structures of >58,000 non-redundant proteins support this assumption, though it has recently been challenged by ~100 fold-switching proteins. Though ostensibly rare, these proteins raise the question of how many uncharacterized proteins have shapeshifting-rather than fixed-secondary structures. Here, we use a comparative sequence-based approach to predict fold switching in the universally conserved NusG transcription factor family, one member of which has a 50-residue regulatory subunit experimentally shown to switch between α-helical and ß-sheet folds. Our approach predicts that 24% of sequences in this family undergo similar α-helix ⇌ ß-sheet transitions. While these predictions cannot be reproduced by other state-of-the-art computational methods, they are confirmed by circular dichroism and nuclear magnetic resonance spectroscopy for 10 out of 10 sequence-diverse variants. This work suggests that fold switching may be a pervasive mechanism of transcriptional regulation in all kingdoms of life.

Site-Specific Detection and Characterization of Ubiquitin Carbamylation.

The physiological consequences of varying in vivo CO2 levels point to a general mechanism for CO2 to influence cellular homeostasis beyond regulating pH. Aside from a few instances where CO2 has been observed to cause post-translational protein modification, by forming long-lived carbamates, little is known about how transitory and ubiquitous carbamylation events could induce a physiological response. Ubiquitin is a versatile protein involved in a multitude of cellular signaling pathways as polymeric chains of various lengths formed through one of the seven lysines or N-terminal amine. Unique polyubiquitin (polyUb) compositions present recognition signals for specific ubiquitin-receptors which enables this one protein to be involved in many different cellular processes. Advances in proteomic methods have allowed the capture and identification of protein carbamates in vivo, and Ub was found carbamylated at lysines K48 and K33. This was shown to negatively regulate ubiquitin-mediated signaling by inhibiting polyUb chain formation. Here, we expand upon these observations by characterizing the carbamylation susceptibility for all Ub amines simultaneously. Using NMR methods which directly probe 15N resonances, we determined carbamylation rates under various environmental conditions and related them to the intrinsic pKas. Our results show that the relatively low pKas for half of the Ub amines are correlated with enhanced susceptibility to carbamylation under physiological conditions. Two of these carbamylated amines, not observed by chemical capture, appear to be physiologically relevant post-translational modifications. These findings point to a mechanism for varying the levels of CO2 due to intracellular localization, cellular stresses, and metabolism to affect certain polyUb-mediated signaling pathways.

The ASCC2 CUE domain in the ALKBH3-ASCC DNA repair complex recognizes adjacent ubiquitins in K63-linked polyubiquitin.

Alkylation of DNA and RNA is a potentially toxic lesion that can result in mutations and even cell death. In response to alkylation damage, K63-linked polyubiquitin chains are assembled that localize the Alpha-ketoglutarate-dependent dioxygenase alkB homolog 3-Activating Signal Cointegrator 1 Complex Subunit (ASCC) repair complex to damage sites in the nucleus. The protein ASCC2, a subunit of the ASCC complex, selectively binds K63-linked polyubiquitin chains via its coupling of ubiquitin conjugation to ER degradation (CUE) domain. The basis for polyubiquitin-binding specificity was unclear, because CUE domains in other proteins typically bind a single ubiquitin and do not discriminate among different polyubiquitin linkage types. We report here that the ASCC2 CUE domain selectively binds K63-linked diubiquitin by contacting both the distal and proximal ubiquitin. The ASCC2 CUE domain binds the distal ubiquitin in a manner similar to that reported for other CUE domains bound to a single ubiquitin, whereas the contacts with the proximal ubiquitin are unique to ASCC2. Residues in the N-terminal portion of the ASCC2 α1 helix contribute to the binding interaction with the proximal ubiquitin of K63-linked diubiquitin. Mutation of residues within the N-terminal portion of the ASCC2 α1 helix decreases ASCC2 recruitment in response to DNA alkylation, supporting the functional significance of these interactions during the alkylation damage response. Our study reveals the versatility of CUE domains in ubiquitin recognition.

Protein Domain Specific Covalent Inhibition of Human DNA Polymerase ß.

DNA polymerase ß (Pol ß) is a frequently overexpressed and/or mutated bifunctional repair enzyme. Pol ß possesses polymerase and lyase active sites, that are employed in two steps of base excision repair. Pol ß is an attractive therapeutic target for which there is a need for inhibitors. Two mechanistically inspired covalent inhibitors (1, IC50 =21.0 µM; 9, IC50 =18.7 µM) that modify lysine residues in different Pol ß active sites are characterized. Despite modifying lysine residues in different active sites, 1 and 9 inactivate the polymerase and lyase activities of Pol ß. Fluorescence anisotropy experiments indicate that they do so by preventing DNA binding. Inhibitors 1 and 9 provide the basis for a general approach to preparing domain selective inhibitors of bifunctional polymerases. Such molecules could prove to be useful tools for studying the role of wild type and mutant forms of Pol ß and other polymerases in DNA repair.

Selective Inhibition of DNA Polymerase ß by a Covalent Inhibitor.

DNA polymerase ß (Pol ß) plays a vital role in DNA repair and has been closely linked to cancer. Selective inhibitors of this enzyme are lacking. Inspired by DNA lesions produced by antitumor agents that inactivate Pol ß, we have undertaken the development of covalent small-molecule inhibitors of this enzyme. Using a two-stage process involving chemically synthesized libraries, we identified a potent irreversible inhibitor (14) of Pol ß (KI = 1.8 ± 0.45 µM, kinact = (7.0 ± 1.0) × 10-3 s-1). Inhibitor 14 selectively inactivates Pol ß over other DNA polymerases. LC-MS/MS analysis of trypsin digests of Pol ß treated with 14 identified two lysines within the polymerase binding site that are covalently modified, one of which was previously determined to play a role in DNA binding. Fluorescence anisotropy experiments show that pretreatment of Pol ß with 14 prevents DNA binding. Experiments using a pro-inhibitor (pro-14) in wild type mouse embryonic fibroblasts (MEFs) indicate that the inhibitor (5 µM) is itself not cytotoxic but works synergistically with the DNA alkylating agent, methylmethanesulfonate (MMS), to kill cells. Moreover, experiments in Pol ß null MEFs indicate that pro-14 is selective for the target enzyme. Finally, pro-14 also works synergistically with MMS and bleomycin to kill HeLa cells. The results suggest that pro-14 is a potentially useful tool in studies of the role of Pol ß in disease.

Conserved allosteric ensembles in disordered proteins using TROSY/anti-TROSY R2-filtered spectroscopy.

Defining the role of intrinsic disorder in proteins in the myriad of biological processes with which it is involved represents a significant goal in modern biophysics. Toward this end, NMR is uniquely suited for molecular studies of dynamic and disordered regions, but studying these regions in concert with their more structured domains and binding partners presents spectroscopic challenges. Here, we investigate the interactions between the structured and disordered regions of the human glucocorticoid receptor (GR). To do this, we developed an NMR strategy that relies on a novel relaxation filter for the simultaneous study of structured and unstructured regions. Using this approach, we conducted a comparative analysis of three translational isoforms of GR containing a folded DNA-binding domain (DBD) and two disordered regions that flank the DBD, one of which varies in size in the different isoforms. Notably, we were able to assign resonances that had previously been inaccessible because of the spectral complexity of the translational isoforms, which in turn allowed us to 1) identify a region of the structured DBD that undergoes significant changes in the local chemical environment in the presence of the disordered region and 2) determine differences in the conformational ensembles of the disordered regions of the translational isoforms. Furthermore, an ensemble-based thermodynamic analysis of the isoforms reveals conserved patterns of stability within the N-terminal domain of GR that persist despite low sequence conservation. These studies provide an avenue for further investigations of the mechanistic underpinnings of the functional relevance of the translational isoforms of GR while also providing a general NMR strategy for studying systems containing both structured and disordered regions.

Backbone Hydrogen Bond Energies in Membrane Proteins Are Insensitive to Large Changes in Local Water Concentration.

A hallmark feature of biological lipid bilayer structure is a depth-dependent polarity gradient largely resulting from the change in water concentration over the angstrom length scale. This gradient is particularly steep as it crosses the membrane interfacial regions where the water concentration drops at least a million-fold along the direction of the bilayer normal. Although local water content is often assumed to be a major determinant of membrane protein stability, the effect of the water-induced polarity gradient upon backbone hydrogen bond strength has not been systematically investigated. We addressed this question by measuring the free energy change for a number of backbone hydrogen bonds in the transmembrane protein OmpW. These values were obtained at 33 backbone amides from hydrogen/deuterium fractionation factors by nuclear magnetic resonance spectroscopy. We surprisingly found that OmpW backbone hydrogen bond energies do not vary over a wide range of water concentrations that are characteristic of the solvation environment in the bilayer interfacial region. We validated the interpretation of our results by determining the hydrodynamic and solvation properties of our OmpW-micelle complex using analytical ultracentrifugation and molecular dynamics simulations. The magnitudes of the backbone hydrogen bond free energy changes in our study are comparable to those observed in water-soluble proteins, the H-segment of the leader peptidase helix used in the von Heijne and White biological scale experiments, and several interfacial peptides. Our results agree with those reported for the transmembrane α-helical portion of the amyloid precursor protein after the latter values were adjusted for kinetic isotope effects. Overall, our work suggests that backbone hydrogen bonds provide modest thermodynamic stability to membrane protein structures and that many amides are unaffected by dehydration within the bilayer.

Macromolecular crowding induces compaction and DNA binding in the disordered N-terminal domain of hUNG2.

Many human DNA repair proteins have disordered domains at their N- or C-termini with poorly defined biological functions. We recently reported that the partially structured N-terminal domain (NTD) of human uracil DNA glycosylase 2 (hUNG2), functions to enhance DNA translocation in crowded environments and also targets the enzyme to single-stranded/double-stranded DNA junctions. To understand the structural basis for these effects we now report high-resolution heteronuclear NMR studies of the isolated NTD in the presence and absence of an inert macromolecular crowding agent (PEG8K). Compared to dilute buffer, we find that crowding reduces the degrees of freedom for the structural ensemble, increases the order of a PCNA binding motif and dramatically promotes binding of the NTD for DNA through a conformational selection mechanism. These findings shed new light on the function of this disordered domain in the context of the crowded nuclear environment.

A Second Backbone: The Contribution of a Buried Asparagine Ladder to the Global and Local Stability of a Leucine-Rich Repeat Protein.

Parallel ß-sheet-containing repeat proteins often display a structural motif in which conserved asparagines form a continuous ladder buried within the hydrophobic core. In such "asparagine ladders", the asparagine side-chain amides form a repetitive pattern of hydrogen bonds with neighboring main-chain NH and CO groups. Although asparagine ladders have been thought to be important for stability, there is little experimental evidence to support such speculation. Here we test the contribution of a minimal asparagine ladder from the leucine-rich repeat protein pp32 to stability and investigate lattice rigidity and hydrogen bond character using solution nuclear magnetic resonance (NMR) spectroscopy. Point substitutions of the two ladder asparagines of pp32 are strongly destabilizing and decrease the cooperativity of unfolding. The chemical shifts of the ladder side-chain HZ protons are shifted significantly downfield in the NMR spectrum and have low temperature coefficients, indicative of strong hydrogen bonding. In contrast, the HE protons are shifted upfield and have temperature coefficients close to zero, suggesting an asymmetry in hydrogen bond strength along the ladder. Ladder NH2 groups have weak 1H-15N cross-peak intensities; 1H-15N nuclear Overhauser effect and 15N CPMG experiments show this to be the result of high rigidity. Hydrogen exchange measurements demonstrate that the ladder NH2 groups exchange very slowly, with rates approaching the global exchange limit. Overall, these results show that the asparagine side chains are held in a very rigid, nondynamic structure, making a significant contribution to the overall stability. In this regard, buried asparagine ladders can be considered "second backbones" within the cores of their elongated ß-sheet host proteins.

Nitroxyl (HNO) targets phospholamban cysteines 41 and 46 to enhance cardiac function.

Nitroxyl (HNO) positively modulates myocardial function by accelerating Ca2+ reuptake into the sarcoplasmic reticulum (SR). HNO-induced enhancement of myocardial Ca2+ cycling and function is due to the modification of cysteines in the transmembrane domain of phospholamban (PLN), which results in activation of SR Ca2+-ATPase (SERCA2a) by functionally uncoupling PLN from SERCA2a. However, which cysteines are modified by HNO, and whether HNO induces reversible disulfides or single cysteine sulfinamides (RS(O)NH2) that are less easily reversed by reductants, remain to be determined. Using an 15N-edited NMR method for sulfinamide detection, we first demonstrate that Cys46 and Cys41 are the main targets of HNO reactivity with PLN. Supporting this conclusion, mutation of PLN cysteines 46 and 41 to alanine reduces the HNO-induced enhancement of SERCA2a activity. Treatment of WT-PLN with HNO leads to sulfinamide formation when the HNO donor is in excess, whereas disulfide formation is expected to dominate when the HNO/thiol stoichiometry approaches a 1:1 ratio that is more similar to that anticipated in vivo under normal, physiological conditions. Thus, 15N-edited NMR spectroscopy detects redox changes on thiols that are unique to HNO, greatly advancing the ability to detect HNO footprints in biological systems, while further differentiating HNO-induced post-translational modifications from those imparted by other reactive nitrogen or oxygen species. The present study confirms the potential of HNO as a signaling molecule in the cardiovascular system.

Increasing the length of poly-pyrimidine bulges broadens RNA conformational ensembles with minimal impact on stacking energetics.

Helical elements separated by bulges frequently undergo transitions between unstacked and coaxially stacked conformations during the folding and function of noncoding RNAs. Here, we examine the dynamic properties of poly-pyrimidine bulges of varying length (n = 1-4, 7) across a range of Mg2+ concentrations using HIV-1 TAR RNA as a model system and solution NMR spectroscopy. In the absence of Mg2+, helices linked by bulges with n ≥ 3 residues adopt predominantly unstacked conformations (stacked population <15%), whereas one-bulge and two-bulge motifs adopt predominantly stacked conformations (stacked population >74%). In the presence of 3 mM Mg2+, the helices predominantly coaxially stack (stacked population >84%), regardless of bulge length, and the midpoint for the Mg2+-dependent stacking transition is within threefold regardless of bulge length. In the absence of Mg2+, the difference between free energy of interhelical coaxial stacking across the bulge variants is estimated to be ∼2.9 kcal/mol, based on an NMR chemical shift mapping with stacking being more energetically disfavored for the longer bulges. This difference decreases to ∼0.4 kcal/mol in the presence of Mg2+ NMR RDCs and resonance intensity data show increased dynamics in the stacked state with increasing bulge length in the presence of Mg2+ We propose that Mg2+ helps to neutralize the growing electrostatic repulsion in the stacked state with increasing bulge length thereby increasing the number of coaxial conformations that are sampled. Energetically compensated interhelical stacking dynamics may help to maximize the conformational adaptability of RNA and allow a wide range of conformations to be optimally stabilized by proteins and ligands.

Oxidative decarboxylation of pyruvate by 1-deoxy-d-xyulose 5-phosphate synthase, a central metabolic enzyme in bacteria.

The underexploited antibacterial target 1-deoxy-d-xyluose 5-phosphate (DXP) synthase catalyzes the thiamin diphosphate (ThDP)-dependent formation of DXP from pyruvate and d-glyceraldehyde 3-phosphate (d-GAP). DXP is an essential intermediate in the biosynthesis of ThDP, pyridoxal phosphate, and isoprenoids in many pathogenic bacteria. DXP synthase catalyzes a distinct mechanism in ThDP decarboxylative enzymology in which the first enzyme-bound pre-decarboxylation intermediate, C2α-lactyl-ThDP (LThDP), is stabilized by DXP synthase in the absence of d-GAP, and d-GAP then induces efficient LThDP decarboxylation. Despite the observed LThDP accumulation and lack of evidence for C2α-carbanion formation in the absence of d-GAP, CO2 is released at appreciable levels under these conditions. Here, seeking to resolve these conflicting observations, we show that DXP synthase catalyzes the oxidative decarboxylation of pyruvate under conditions in which LThDP accumulates. O2-dependent LThDP decarboxylation led to one-electron transfer from the C2α-carbanion/enamine to O2, with intermediate ThDP-enamine radical formation, followed by peracetic acid formation en route to acetate. Thus, LThDP formation and decarboxylation and DXP formation were studied under anaerobic conditions. Our results support a model in which O2-dependent LThDP decarboxylation and peracetic acid formation occur in the absence of d-GAP, decreasing the levels of pyruvate and O2 in solution. The relative pyruvate and O2 concentrations then dictate the extent of LThDP accumulation, and its buildup can be observed when [pyruvate] > [O2]. The finding that O2 acts as a structurally distinct trigger of LThDP decarboxylation supports the hypothesis that a mechanism involving small molecule-dependent LThDP decarboxylation equips DXP synthase for diverse, yet uncharacterized cellular functions.

Histidine-Lysine Axial Ligand Switching in a Hemoglobin: A Role for Heme Propionates.

The hemoglobin of Synechococcus sp. PCC 7002, GlbN, is a monomeric group I truncated protein (TrHb1) that coordinates the heme iron with two histidine ligands at neutral pH. One of these is the distal histidine (His46), a residue that can be displaced by dioxygen and other small molecules. Here, we show with mutagenesis, electronic absorption spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy that at high pH and exclusively in the ferrous state, Lys42 competes with His46 for the iron coordination site. When b heme is originally present, the population of the lysine-bound species remains too small for detailed characterization; however, the population can be increased significantly by using dimethyl-esterified heme. Electronic absorption and NMR spectroscopies showed that the reversible ligand switching process occurs with an apparent pKa of 9.3 and a Lys-ligated population of ∼60% at the basic pH limit in the modified holoprotein. The switching rate, which is slow on the chemical shift time scale, was estimated to be 20-30 s-1 by NMR exchange spectroscopy. Lys42-His46 competition and attendant conformational rearrangement appeared to be related to weakened bis-histidine ligation and enhanced backbone dynamics in the ferrous protein. The pH- and redox-dependent ligand exchange process observed in GlbN illustrates the structural plasticity allowed by the TrHb1 fold and demonstrates the importance of electrostatic interactions at the heme periphery for achieving axial ligand selection. An analogy is drawn to the alkaline transition of cytochrome c, in which Lys-Met competition is detected at alkaline pH, but, in contrast to GlbN, in the ferric state only.