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Symmetry plays a central role in conventional and topological phases of matter, making the ability to optically drive symmetry changes a critical step in developing future technologies that rely on such control. Topological materials, like topological semimetals, are particularly sensitive to a breaking or restoring of time-reversal and crystalline symmetries, which affect both bulk and surface electronic states. While previous studies have focused on controlling symmetry via coupling to the crystal lattice, we demonstrate here an all-electronic mechanism based on photocurrent generation. Using second harmonic generation spectroscopy as a sensitive probe of symmetry changes, we observe an ultrafast breaking of time-reversal and spatial symmetries following femtosecond optical excitation in the prototypical type-I Weyl semimetal TaAs. Our results show that optically driven photocurrents can be tailored to explicitly break electronic symmetry in a generic fashion, opening up the possibility of driving phase transitions between symmetry-protected states on ultrafast timescales.
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The antiferromagnetic (AFM) compound MnBi_{2}Te_{4} is suggested to be the first realization of an AFM topological insulator. We report on inelastic neutron scattering studies of the magnetic interactions in MnBi_{2}Te_{4} that possess ferromagnetic triangular layers with AFM interlayer coupling. The spin waves display a large spin gap and pairwise exchange interactions within the triangular layer are long ranged and frustrated by large next-nearest neighbor AFM exchange. The degree of frustration suggests proximity to a variety of magnetic phases, potentially including skyrmion phases, which could be accessed in chemically tuned compounds or upon the application of symmetry-breaking fields.
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'Sudden' quantum quench and prethermalization have become a cross-cutting theme for discovering emergent states of matter1-4. Yet this remains challenging in electron matter5-9, especially superconductors10-14. The grand question of what is hidden underneath superconductivity (SC) 15 appears universal, but poorly understood. Here we reveal a long-lived gapless quantum phase of prethermalized quasiparticles (QPs) after a single-cycle terahertz (THz) quench of a Nb3Sn SC gap. Its conductivity spectra is characterized by a sharp coherent peak and a vanishing scattering rate that decreases almost linearly towards zero frequency, which is most pronounced around the full depletion of the condensate and absent for a high-frequency pump. Above a critical pump threshold, such a QP phase with coherent transport and memory persists as an unusual prethermalization plateau, without relaxation to normal and SC thermal states for an order of magnitude longer than the QP recombination and thermalization times. Switching to this metastable 'quantum QP fluid' signals non-thermal quench of coupled SC and charge-density-wave (CDW)-like orders and hints quantum control beneath the SC.
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Teoria Quântica , Condutividade Elétrica , Modelos Químicos , SupercondutividadeRESUMO
BACKGROUND: Endoprosthetic replacement is a valuable treatment option following intercalary resection of bone tumours in the diaphysis. OBJECTIVES: To identify indication, operative technique, implants currently available, literature results and alternative procedures for the alloplastic reconstruction of segmental bone defects. MATERIALS AND METHODS: This review article summarizes the authors' own experiences and relevant clinical studies focussing on this topic. RESULTS: According to the literature, 10-year-survival rates of intercalary endoprostheses range between 64 and 80%. Yet, comparisons between different publications are difficult due to the limited number of cases, different implants, follow-up periods and the heterogeneous patient populations. Biological alternatives for reconstruction are autologous bone transplantation, distraction osteogenesis and bone transport, allogenic bone transplantation, and the induced membrane technique. Innovative tissue engineering approaches are still limited to preclinical testing. CONCLUSIONS: Short- to mid-term results for segmental endoprostheses following intercalary resections are satisfactory and may be regarded as superior to those of biological reconstructions due to the immediate full weight-bearing capability. However, they are mainly applied for elderly patients and in palliative situations because of potential long-term complications.
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Neoplasias Ósseas , Procedimentos de Cirurgia Plástica , Idoso , Neoplasias Ósseas/cirurgia , Transplante Ósseo , Humanos , Salvamento de Membro , Procedimentos de Cirurgia Plástica/instrumentação , Reimplante , Estudos Retrospectivos , Resultado do TratamentoRESUMO
OBJECTIVE: To correlate osteochondral repair assessed by validated macroscopic scoring systems with established semiquantitative histological analyses in an ovine model and to test the hypothesis that important macroscopic individual categories correlate with their corresponding histological counterparts. METHODS: In the weight-bearing portion of medial femoral condyles (n = 38) of 19 female adult Merino sheep (age 2-4 years; weight 70 ± 20 kg) full-thickness chondral defects were created (size 4 × 8 mm; International Cartilage Repair Society (ICRS) grade 3C) and treated with Pridie drilling. After sacrifice, 1520 blinded macroscopic observations from three observers at 2-3 time points including five different macroscopic scoring systems demonstrating all grades of cartilage repair where correlated with corresponding categories from 418 blinded histological sections. RESULTS: Categories "defect fill" and "total points" of different macroscopic scoring systems correlated well with their histological counterparts from the Wakitani and Sellers scores (all P ≤ 0.001). "Integration" was assessed in both histological scoring systems and in the macroscopic ICRS, Oswestry and Jung scores. Here, a significant relationship always existed (0.020 ≤ P ≤ 0.049), except for Wakitani and Oswestry (P = 0.054). No relationship was observed for the "surface" between histology and macroscopy (all P > 0.05). CONCLUSIONS: Major individual morphological categories "defect fill" and "integration", and "total points" of macroscopic scoring systems correlate with their corresponding categories in elementary and complex histological scoring systems. Thus, macroscopy allows to precisely predict key histological aspects of articular cartilage repair, underlining the specific value of macroscopic scoring for examining cartilage repair.
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Cartilagem Articular/patologia , Articulação do Joelho/patologia , Regeneração , Animais , Cartilagem Articular/lesões , Cartilagem Articular/cirurgia , Feminino , Ovinos , Suporte de CargaRESUMO
BACKGROUND: Osteoid osteoma is the third most common benign bone tumor and typically induces pain that is worse at night. OBJECTIVE: To identify the epidemiological, pathogenetic, histological and radiological characteristics of osteoid osteoma and to present the broad variety of treatment options. MATERIAL AND METHODS: This review article summarizes relevant clinical studies and meta-analyses on this topic. RESULTS: Osteoid osteoma is characterized by a central nidus smaller than 1.5 cm in diameter with surrounding bone sclerosis. In the majority of cases, the tumor occurs in the long bones of the lower extremities and is predominantly manifested in patients aged between 5 and 25 years. Pain is mediated by prostaglandins, which stimulate afferent peripheral nerve fibers. Besides plain radiographs, thin-section computed tomography represents the gold standard of diagnostics but should be complemented by magnetic resonance or nuclear medicine imaging modalities. The conservative treatment consists of long-term therapy with non-steroidal anti-inflammatory drugs. Minimally invasive radiofrequency ablation of the nidus is the current operative treatment of choice. CONCLUSION: Success rates of radiofrequency ablation and other minimally invasive procedures are high while treatment costs and length of hospital stay are low. Thus, open surgical curettage is reserved for rare indications and en bloc excision of the nidus should only be performed in cases of recurrent lesions.
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Neoplasias Ósseas/diagnóstico , Neoplasias Ósseas/cirurgia , Osteoma Osteoide/diagnóstico , Osteoma Osteoide/cirurgia , Adolescente , Adulto , Neoplasias Ósseas/epidemiologia , Neoplasias Ósseas/patologia , Osso e Ossos/patologia , Osso e Ossos/cirurgia , Criança , Pré-Escolar , Estudos Transversais , Diagnóstico Diferencial , Humanos , Osteoma Osteoide/epidemiologia , Osteoma Osteoide/patologia , Dor/etiologia , Prostaglandinas/metabolismo , Tomografia Computadorizada por Raios X , Adulto JovemRESUMO
OBJECTIVE: To test the hypothesis that changes in the subchondral bone induced by parathyroid hormone (PTH [1-34]) reciprocally affect the integrity of the articular cartilage within a naïve osteochondral unit in vivo. DESIGN: Daily subcutaneous injections of 10 µg PTH [1-34]/kg were given to adult rabbits for 6 weeks, controls received saline. Blood samples were continuously collected to monitor renal function. The subchondral bone plate and subarticular spongiosa of the femoral heads were separately assessed by micro-computed tomography. Articular cartilage was evaluated by macroscopic and histological osteoarthritis scoring, polarized light microscopy, and immunohistochemical determination of type-I, type-II, type-X collagen contents, PTH [1-34] receptor and caspase-3 expression. Absolute and relative extents of hyaline and calcified articular cartilage layers were measured histomorphometrically. The correlation between PTH-induced changes in subchondral bone and articular cartilage was determined. RESULTS: PTH [1-34] enhanced volume, mineral density, and trabecular thickness within the subarticular spongiosa, and increased thickness of the calcified cartilage layer (all P < 0.05). Moreover, PTH [1-34] led to cartilage surface irregularities and reduced matrix staining (both P < 0.03). These early osteoarthritic changes correlated with and were ascribed to the increased thickness of the calcified cartilage layer (P = 0.026) and enhanced mineral density of the subarticular spongiosa (P = 0.001). CONCLUSIONS: Modifications of the subarticular spongiosa by PTH [1-34] cause broadening of the calcified cartilage layer, resulting in osteoarthritic cartilage degeneration. These findings identify a mechanism by which PTH-induced alterations of the normal subchondral bone microarchitecture may provoke early osteoarthritis.
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Cartilagem Articular/patologia , Cabeça do Fêmur/efeitos dos fármacos , Osteoartrite/induzido quimicamente , Hormônio Paratireóideo/efeitos adversos , Animais , Biópsia por Agulha , Cartilagem Articular/diagnóstico por imagem , Cartilagem Articular/efeitos dos fármacos , Modelos Animais de Doenças , Relação Dose-Resposta a Droga , Esquema de Medicação , Cabeça do Fêmur/diagnóstico por imagem , Cabeça do Fêmur/patologia , Imuno-Histoquímica , Injeções Subcutâneas , Osteoartrite/diagnóstico por imagem , Osteoartrite/patologia , Hormônio Paratireóideo/administração & dosagem , Coelhos , Distribuição Aleatória , Valores de Referência , Sensibilidade e Especificidade , Microtomografia por Raio-X/métodosRESUMO
BACKGROUND: Changes of patellar position (height, tilt, and shift) and arthritis of the patellofemoral joint might potentially influence outcome after unicompartmental knee replacement. OBJECTIVES: The purpose of this work is to evaluate the influence of the aforementioned parameters on postoperative outcome. METHODS: Literature analysis via PubMed. RESULTS: A total of 12 relevant studies (three about Patellar height, two about patellar tilt and shift, seven about patellofemoral osteoarthritis) could be identified. Regarding Patellar height, two out of three studies demonstrated a postoperative decrease. With regard to patellar tilt and shift, only one study identified postoperative lateralization of the patella to be a predictor for poor outcome. The radiological appearance of arthritis of the patellofemoral joint does not significantly influence postoperative knee function except for cases where only the lateral patellar facet is affected. Anterior knee pain has no influence on clinical outcome. CONCLUSION: Literature data do not allow for a precise statement about the possible influence of patellar position on the outcome after unicompartmental knee replacement. With proper patient selection, good results can be achieved despite patellofemoral osteoarthritis.
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Artroplastia do Joelho/efeitos adversos , Prótese do Joelho/efeitos adversos , Osteoartrite do Joelho/etiologia , Patela/cirurgia , Articulação Patelofemoral/cirurgia , Ajuste de Prótese/métodos , Artroplastia do Joelho/instrumentação , Humanos , Osteoartrite do Joelho/prevenção & controle , Osteoartrite do Joelho/cirurgia , Prognóstico , Desenho de Prótese , Resultado do TratamentoRESUMO
Magnetic kagome metals are a promising platform to develop unique quantum transport and optical phenomena caused by the interplay between topological electronic bands, strong correlations, and magnetic order. This interplay may result in exotic quasiparticles that describe the coupled electronic and spin excitations on the frustrated kagome lattice. Here, we observe novel elementary magnetic excitations within the ferromagnetic Mn kagome layers in TbMn6Sn6 using inelastic neutron scattering. We observe sharp, collective acoustic magnons and identify flat-band magnons that are localized to a hexagonal plaquette due to the special geometry of the kagome layer. Surprisingly, we observe another type of elementary magnetic excitation; a chiral magnetic quasiparticle that is also localized on a hexagonal plaquette. The short lifetime of localized flat-band and chiral quasiparticles suggest that they are hybrid excitations that decay into electronic states.
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OBJECTIVE: The 1-34 amino acid segment of the parathyroid hormone (PTH [1-34]) mediates anabolic effects in chondrocytes and osteocytes. The aim of this study was to investigate whether systemic application of PTH [1-34] improves the repair of non-osteoarthritic, focal osteochondral defects in vivo. DESIGN: Standardized cylindrical osteochondral defects were bilaterally created in the femoral trochlea of rabbits (n = 8). Daily subcutaneous injections of 10 µg PTH [1-34]/kg were given to the treatment group (n = 4) for 6 weeks, controls (n = 4) received saline. Articular cartilage repair was evaluated by macroscopic, biochemical, histological and immunohistochemical analyses. Reconstitution of the subchondral bone was assessed by micro-computed tomography. Effects of PTH [1-34] on synovial membrane, apoptosis, and expression of the PTH receptor (PTH1R) were determined. RESULTS: Systemic PTH [1-34] increased PTH1R expression on both, chondrocytes and osteocytes within the repair tissue. PTH [1-34] ameliorated the macro- and microscopic aspect of the cartilaginous repair tissue. It also enhanced the thickness of the subchondral bone plate and the microarchitecture of the subarticular spongiosa within the defects. No significant correlations were established between these coexistent processes. Apoptotic levels, synovial membrane, biochemical composition of the repair tissue, and type-I/II collagen immunoreactivity remained unaffected. CONCLUSIONS: PTH [1-34] emerges as a promising agent in the treatment of focal osteochondral defects as its systemic administration simultaneously stimulates articular cartilage and subchondral bone repair. Importantly, both time-dependent mechanisms of repair did not correlate significantly at this early time point and need to be followed over prolonged observation periods.
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Cartilagem Articular/efeitos dos fármacos , Fêmur/lesões , Hormônio Paratireóideo/uso terapêutico , Animais , Apoptose/efeitos dos fármacos , Regeneração Óssea/efeitos dos fármacos , Regeneração Óssea/fisiologia , Cálcio/metabolismo , Cartilagem Articular/lesões , Cartilagem Articular/metabolismo , Cartilagem Articular/patologia , Condrócitos/metabolismo , Condrócitos/patologia , Colágeno/metabolismo , Avaliação Pré-Clínica de Medicamentos/métodos , Feminino , Fêmur/diagnóstico por imagem , Fêmur/metabolismo , Fêmur/fisiopatologia , Injeções Intramusculares , Osteócitos/metabolismo , Osteócitos/patologia , Hormônio Paratireóideo/administração & dosagem , Coelhos , Receptor Tipo 1 de Hormônio Paratireóideo/metabolismo , Membrana Sinovial/efeitos dos fármacos , Membrana Sinovial/patologia , Cicatrização/efeitos dos fármacos , Cicatrização/fisiologia , Microtomografia por Raio-X/métodosRESUMO
Alterations of the subchondral bone are pathological features associated with spontaneous osteochondral repair following an acute injury and with articular cartilage repair procedures. The aim of this review is to discuss their incidence, extent and relevance, focusing on recent knowledge gained from both translational models and clinical studies of articular cartilage repair. Efforts to unravel the complexity of subchondral bone alterations have identified (1) the upward migration of the subchondral bone plate, (2) the formation of intralesional osteophytes, (3) the appearance of subchondral bone cysts, and (4) the impairment of the osseous microarchitecture as potential problems. Their incidence and extent varies among the different small and large animal models of cartilage repair, operative principles, and over time. When placed in the context of recent clinical investigations, these deteriorations of the subchondral bone likely are an additional, previously underestimated, factor that influences the long-term outcome of cartilage repair strategies. Understanding the role of the subchondral bone in both experimental and clinical articular cartilage repair thus holds great promise of being translated into further improved cell- or biomaterial-based techniques to preserve and restore the entire osteochondral unit.
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Osso e Ossos/patologia , Cartilagem Articular/patologia , Pesquisa Translacional Biomédica , Cicatrização , Animais , Osso e Ossos/diagnóstico por imagem , Modelos Animais de Doenças , Humanos , Articulações/patologia , RadiografiaRESUMO
OBJECTIVE: Upward migration of the subchondral bone plate is associated with osteochondral repair. The aim of this study was to quantitatively monitor the sequence of subchondral bone plate advancement in a lapine model of spontaneous osteochondral repair over a 1-year period and to correlate these findings with articular cartilage repair. DESIGN: Standardized cylindrical osteochondral defects were created in the rabbit trochlear groove. Subchondral bone reconstitution patterns were identified at five time points. Migration of the subchondral bone plate and areas occupied by osseous repair tissue were determined by histomorphometrical analysis. Tidemark formation and overall cartilage repair were correlated with the histomorphometrical parameters of the subchondral bone. RESULTS: The subchondral bone reconstitution pattern was cylindrical at 3 weeks, infundibuliform at 6 weeks, plane at 4 and 6 months, and hypertrophic after 1 year. At this late time point, the osteochondral junction advanced 0.19 [95% confidence intervals (CI) 0.10-0.30] mm above its original level. Overall articular cartilage repair was significantly improved by 4 and 6 months but degraded after 1 year. Subchondral bone plate migration correlated with tidemark formation (r = 0.47; P < 0.0001), but not with the overall score of the repair cartilage (r = 0.11; P > 0.44). CONCLUSIONS: The subchondral bone plate is reconstituted in a distinct chronological order. The lack of correlation suggests that articular cartilage repair and subchondral bone reconstitution proceed at a different pace and that the advancement of the subchondral bone plate is not responsible for the diminished articular cartilage repair in this model.
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Regeneração Óssea/fisiologia , Cartilagem Articular/fisiologia , Consolidação da Fratura , Lâmina de Crescimento/patologia , Animais , Remodelação Óssea , Cartilagem Articular/lesões , Cartilagem Articular/patologia , Movimento Celular/fisiologia , Modelos Animais de Doenças , Lâmina de Crescimento/fisiopatologia , Coelhos , Fatores de TempoRESUMO
OBJECTIVE: To develop a new macroscopic scoring system which allows for an overall judgment of experimental articular cartilage repair and compare it with four existing scoring systems and high-field magnetic resonance imaging (MRI). METHODS: A new macroscopic scoring system was developed to assess the repair of cartilage defects. Cartilage repair was graded by three observers with different experience in cartilage research at 2-3 time points and compared with the protocol A of the international cartilage repair society (ICRS) cartilage repair assessment score, the Oswestry arthroscopy score, and macroscopic grading systems designed by Jung and O'Driscoll. Parameters were correlated with the two-dimensional (2D) magnetic resonance observation of cartilage repair tissue (MOCART) score based on a 9.4 T MRI as an external reference standard. RESULTS: All macroscopic scores exhibited high intra- and interobserver reliability and high internal correlation. The newly developed macroscopic scoring system had the highest intraobserver [0.866 ≤ intraclass correlation (ICC) ≤ 0.895] and the highest interobserver reliability (ICC = 0.905) for "total points". Here, Cronbach's alpha indicated good homogeneity and functioning of the items (mean = 0.782). "Total points" of the 2D MOCART score correlated with all macroscopic scores (all P < 0.0001). The newly developed macroscopic scoring system yielded the highest correlation for the MRI parameter "defect fill" (rho = 0.765; all P < 0.0001). CONCLUSIONS: "Total points" and "defect fill", two clinically relevant indicators of cartilage repair, can be reliably and directly assessed by macroscopic evaluation, using either system. These data support the use of macroscopic assessment to precisely judge cartilage repair in preclinical large animal models.
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Cartilagem Articular/patologia , Imageamento por Ressonância Magnética/métodos , Joelho de Quadrúpedes/patologia , Animais , Cartilagem Articular/cirurgia , Modelos Animais de Doenças , Feminino , Reprodutibilidade dos Testes , Ovinos , Joelho de Quadrúpedes/cirurgia , Resultado do TratamentoRESUMO
Electrons navigate more easily in a background of ordered magnetic moments than around randomly oriented ones. This fundamental quantum mechanical principle is due to their Bloch wave nature and also underlies ballistic electronic motion in a perfect crystal. As a result, a paramagnetic metal that develops ferromagnetic order often experiences a sharp drop in the resistivity. Despite the universality of this phenomenon, a direct observation of the impact of ferromagnetic order on the electronic quasiparticles in a magnetic metal is still lacking. Here we demonstrate that quasiparticles experience a significant enhancement of their lifetime in the ferromagnetic state of the low-density magnetic semimetal EuCd2As2, but this occurs only in selected bands and specific energy ranges. This is a direct consequence of the magnetically induced band splitting and the multi-orbital nature of the material. Our detailed study allows to disentangle different electronic scattering mechanisms due to non-magnetic disorder and magnon exchange. Such high momentum and energy dependence quasiparticle lifetime enhancement can lead to spin selective transport and potential spintronic applications.
RESUMO
Knowledge of magnetic symmetry is vital for exploiting nontrivial surface states of magnetic topological materials. EuIn2As2 is an excellent example, as it is predicted to have collinear antiferromagnetic order where the magnetic moment direction determines either a topological-crystalline-insulator phase supporting axion electrodynamics or a higher-order-topological-insulator phase with chiral hinge states. Here, we use neutron diffraction, symmetry analysis, and density functional theory results to demonstrate that EuIn2As2 actually exhibits low-symmetry helical antiferromagnetic order which makes it a stoichiometric magnetic topological-crystalline axion insulator protected by the combination of a 180∘ rotation and time-reversal symmetries: [Formula: see text]. Surfaces protected by [Formula: see text] are expected to have an exotic gapless Dirac cone which is unpinned to specific crystal momenta. All other surfaces have gapped Dirac cones and exhibit half-integer quantum anomalous Hall conductivity. We predict that the direction of a modest applied magnetic field of µ0H ≈ 1 to 2 T can tune between gapless and gapped surface states.
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The N and C termini of peptides presented by major histocompatibility complex (MHC) class I molecules are held within the peptide binding groove by a network of hydrogen bonds to conserved MHC residues. However, the published structure of the human allele HLA-B*3501 complexed with the nef octa-peptide VPLRPMTY, revealed non-standard positioning for both peptide termini. To investigate whether these deviations are indeed related to the length of the nef-peptide, we have determined the structure of HLA-B*3501 presenting a nona-peptide to 2.5 A resolution. A comparison of HLA-B*3501/peptide complexes with structures of other HLA molecules exhibits allele-specific properties of HLA-B*3501, as well as peptide-induced structural changes. Independent of the length of the bound peptide, HLA-B*3501 positions the peptide C terminus significantly closer to the alpha1-helix and nearer to the A pocket than observed for other HLA class I/peptide complexes. This reorientation is accompanied by a shift within the N-terminal part of the alpha2-helix towards the middle of the binding groove. Due to the short distance between the N and C termini, the nona-peptide is compressed and forced to zig-zag vertically within the binding groove. Its conformation rather resembles that of a deca-peptide than of other nona-peptides bound to class I molecules. Superposition of both HLA-B*3501/peptide complexes additionally reveals a significant, peptide-dependent deviation between the N-terminal parts of the alpha1-helices which might be due to different positioning of the peptide N termini. Taken together, these data illustrate the strong interdependence between the HLA class I molecule and the bound peptide.
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Antígenos Nucleares do Vírus Epstein-Barr/química , Antígeno HLA-B35/química , Peptídeos/química , Conformação Proteica , Alelos , Sequência de Aminoácidos , Sítios de Ligação , Antígenos Nucleares do Vírus Epstein-Barr/imunologia , Antígeno HLA-B35/imunologia , Humanos , Dados de Sequência Molecular , Peptídeos/imunologia , Relação Estrutura-AtividadeRESUMO
The 71 amino acid residue omega protein encoded by the Streptococcus pyogenes non-conjugative plasmid pSM19035 is a transcriptional repressor that regulates expression of genes for copy number control and stable maintenance of plasmids. The crystal structure of omega protein has been determined by multiple isomorphous replacement, including anomalous scattering and refined to an R-factor of 21.1 % (R(free)=23.2 %) at 1.5 A resolution. Two monomers related by a non-crystallographic 2-fold axis form a homodimer that occupies the asymmetric unit. Each polypeptide chain is folded into two alpha-helices and one beta-strand forming an antiparallel beta-ribbon in the homodimer. The N-terminal regions (1-23 and 1-22 in subunits I and II, respectively) are not defined in the electron density due to proteolysis of the N-terminal 20 amino acid residues during crystallisation and partial disorder. The omega protein belongs to the structural superfamily of MetJ/Arc repressors featuring a ribbon-helix-helix DNA-binding motif with the beta-ribbon located in and recognizing the major groove of operator DNA; according to a modelled omega protein-DNA complex, residues Arg31 and Arg31' on the beta-ribbon are in positions to interact with a nucleobase, especially guanine.
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Proteínas de Ligação a DNA/química , Plasmídeos/genética , Proteínas Repressoras/química , Streptococcus pyogenes/química , Streptococcus pyogenes/genética , Sequência de Aminoácidos , Proteínas de Bactérias/química , Proteínas de Bactérias/genética , Proteínas de Bactérias/metabolismo , Sequência de Bases , Dicroísmo Circular , Cristalografia por Raios X , DNA/química , DNA/genética , DNA/metabolismo , Replicação do DNA , Proteínas de Ligação a DNA/genética , Proteínas de Ligação a DNA/metabolismo , Dimerização , Dosagem de Genes , Ligação de Hidrogênio , Modelos Moleculares , Dados de Sequência Molecular , Conformação de Ácido Nucleico , Estrutura Quaternária de Proteína , Estrutura Secundária de Proteína , Proteínas Repressoras/genética , Proteínas Repressoras/metabolismo , Alinhamento de Sequência , Solventes , Eletricidade EstáticaRESUMO
The ribonuclease T1 variant 9/5 with a guanine recognition segment, altered from the wild-type amino acid sequence 41-KYNNYE-46 to 41-EFRNWQ-46, has been cocrystallised with the specific inhibitor 2'-GMP. The crystal structure has been refined to a crystallographic R factor of 0.198 at 2.3 A resolution. Despite a size reduction of the binding pocket, pushing the inhibitor outside by 1 A, 2'-GMP is fixed to the primary recognition site due to increased aromatic stacking interactions. The phosphate group of 2'-GMP is located about 4.2 A apart from its position in wild-type ribonuclease T1-2'-GMP complexes, allowing a Ca(2+), coordinating this phosphate group, to enter the binding pocket. The crystallographic data can be aligned with the kinetic characterisation of the variant, showing a reduction of both, guanine affinity and turnover rate. The presence of Ca(2+) was shown to inhibit variant 9/5 and wild-type enzyme to nearly the same extent.
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Escherichia coli/enzimologia , Variação Genética , Guanina/metabolismo , Ribonuclease T1/química , Ribonuclease T1/metabolismo , Sequência de Aminoácidos , Substituição de Aminoácidos , Sítios de Ligação , Cálcio/metabolismo , Cristalização , Cristalografia por Raios X , Variação Genética/genética , Guanosina Monofosfato/química , Guanosina Monofosfato/metabolismo , Ligação de Hidrogênio , Hidrólise , Cinética , Modelos Moleculares , Dados de Sequência Molecular , Fosfatos/metabolismo , Conformação Proteica , RNA/metabolismo , Ribonuclease T1/antagonistas & inibidores , Ribonuclease T1/genética , Especificidade por Substrato , Água/metabolismoRESUMO
The X-ray crystal structure analysis of inducer-free Tet repressor, TetR, at 2.4 A resolution identifies one of two openings of the tunnel-like binding site as the entrance for the inducer tetracycline-Mg2+, [Mg Tc]+. Recognition and binding of the inducer unleashes conformational changes leading to the induced state of TetR. In the first step, the C-terminal turn of alpha-helix 6 unwinds, thereby altering the orientation of alpha-helix 4. This different orientation of alpha-helix 4 is stabilized by a series of hydrogen bonds mediated through a chain of eight water molecules. The alpha-helix 4 connects the DNA-binding domain (alpha-helices 1 to 3) to the rigid TetR core, and thus regulates gene expression through its respective orientations.