RESUMEN
The principle of continuity posits that some central features of primordial biocatalytic mechanisms should still be present in the genetically dependent pathway of protein synthesis, a crucial step in the emergence of life. Key bimolecular reactions of this process are catalyzed by DNA-dependent RNA polymerases, aminoacyl-tRNA synthetases, and ribosomes. Remarkably, none of these biocatalysts contribute chemically active groups to their respective reactions. Instead, structural and functional studies have demonstrated that nucleotidic α-phosphate and ß-d-ribosyl 2' OH and 3' OH groups can help their own catalysis, a process which, consequently, has been called "substrate-assisted". Furthermore, upon binding, the substrates significantly lower the entropy of activation, exclude water from these catalysts' active sites, and are readily positioned for a reaction. This binding mode has been described as an "entropy trap". The combination of this effect with substrate-assisted catalysis results in reactions that are stereochemically and mechanistically simpler than the ones found in most modern enzymes. This observation is consistent with the way in which primordial catalysts could have operated; it may also explain why, thanks to their complementary reactivities, ß-d-ribose and phosphate were naturally selected to be the central components of early coding polymers.
Asunto(s)
Biosíntesis de Proteínas , Aminoacil-ARNt Sintetasas/metabolismo , Aminoacil-ARNt Sintetasas/genética , Biocatálisis , Ribosomas/metabolismo , ARN Polimerasas Dirigidas por ADN/metabolismo , ARN Polimerasas Dirigidas por ADN/químicaRESUMEN
This study shows how the NiFeSe site of an anaerobically purified O2-resistant hydrogenase reacts with air to give a seleninate as the first product. Less oxidized states of the active site are readily reduced in the presence of X-rays. Reductive enzyme activation requires an efficient pathway for water escape.
Asunto(s)
Desulfovibrio/enzimología , Hidrogenasas/metabolismo , Oxígeno/química , Dominio Catalítico , Cristalografía por Rayos X , Hidrógeno/química , Hidrogenasas/química , Oxidación-Reducción , Selenio/química , Azufre/químicaRESUMEN
In hydrogenases and many other redox enzymes, the buried active site is connected to the solvent by a molecular channel whose structure may determine the enzyme's selectivity with respect to substrate and inhibitors. The role of these channels has been addressed using crystallography and molecular dynamics, but kinetic data are scarce. Using protein film voltammetry, we determined and then compared the rates of inhibition by CO and O2 in ten NiFe hydrogenase mutants and two FeFe hydrogenases. We found that the rate of inhibition by CO is a good proxy of the rate of diffusion of O2 toward the active site. Modifying amino acids whose side chains point inside the tunnel can slow this rate by orders of magnitude. We quantitatively define the relations between diffusion, the Michaelis constant for H2 and rates of inhibition, and we demonstrate that certain enzymes are slowly inactivated by O2 because access to the active site is slow.
Asunto(s)
Desulfovibrio/enzimología , Hidrogenasas/química , Oxígeno/química , Aminoácidos/química , Monóxido de Carbono/química , Dominio Catalítico , Cristalografía por Rayos X/métodos , Difusión , Electroquímica/métodos , Espectroscopía de Resonancia por Spin del Electrón , Hidrógeno/química , Cinética , Modelos Moleculares , Conformación Molecular , Simulación de Dinámica MolecularRESUMEN
Hydrogenases catalyze the conversion between 2H(+) + 2e(-) and H(2)(1). Most of these enzymes are inhibited by O(2), which represents a major drawback for their use in biotechnological applications. Improving hydrogenase O(2) tolerance is therefore a major contemporary challenge to allow the implementation of a sustainable hydrogen economy. We succeeded in improving O(2) tolerance, which we define here as the ability of the enzyme to resist for several minutes to O(2) exposure, by substituting with methionines small hydrophobic residues strongly conserved in the gas channel. Remarkably, the mutated enzymes remained active in the presence of an O(2) concentration close to that found in aerobic solutions in equilibrium with air, while the wild type enzyme is inhibited in a few seconds. Crystallographic and spectroscopic studies showed that the structure and the chemistry at the active site are not affected by the mutations. Kinetic studies demonstrated that the inactivation is slower and reactivation faster in these mutants. We propose that in addition to restricting O(2) diffusion to the active site of the enzyme, methionine may also interact with bound peroxide and provide an assisted escape route for H(2)O(2) toward the gas channel. These results show for the first time that it is possible to improve O(2)-tolerance of [NiFe] hydrogenases, making possible the development of biohydrogen production systems.
Asunto(s)
Hidrogenasas/metabolismo , Metionina/metabolismo , Oxígeno/metabolismo , Dominio Catalítico , Difusión , Gases/química , Gases/metabolismo , Hidrogenasas/química , Metionina/química , Oxígeno/químicaRESUMEN
hBChE [human BChE (butyrylcholinesterase)] naturally scavenges OPs (organophosphates). This bioscavenger is currently in Clinical Phase I for pretreatment of OP intoxication. Phosphylated ChEs (cholinesterases) can undergo a spontaneous time-dependent process called 'aging' during which the conjugate is dealkylated, leading to creation of an enzyme that cannot be reactivated. hBChE inhibited by phosphoramidates such as tabun displays a peculiar resistance to oxime-mediated reactivation. We investigated the basis of oxime resistance of phosphoramidyl-BChE conjugates by determining the kinetics of inhibition, reactivation (obidoxime {1,1'-(oxybis-methylene) bis[4-(hydroxyimino) methyl] pyridinium dichloride}, TMB-4 [1,3-trimethylene-bis(4-hydroxyiminomethylpyridinium) dibromide], HLö 7 {1-[[[4-(aminocarbonyl) pyridinio]methoxy]methyl]-2,4-bis-[(hydroxyimino)methyl] pyridinium dimethanesulfonate)}, HI-6 {1-[[[4-(aminocarbonyl) pyridinio] methoxy] methyl]-2-[(hydroxyimino)methyl]pyridinium dichloride monohydrate} and aging, and the crystal structures of hBChE inhibited by different N-monoalkyl and N,N-dialkyl tabun analogues. The refined structures of aged hBChE conjugates show that aging proceeds through O-dealkylation of the P(R) enantiomer of N,N-diethyl and N-propyl analogues, with subsequent formation of a salt bridge preventing reactivation, similarly to a previous observation made on tabun-ChE conjugates. Interestingly, the N-methyl analogue projects its amino group towards the choline-binding pocket, so that aging proceeds through deamination. This orientation results from a preference of hBChE's acyl-binding pocket for larger than 2-atoms linear substituents. The correlation between the inhibitory potency and the N-monoalkyl chain length is related to increasingly optimized interactions with the acyl-binding pocket as shown by the X-ray structures. These kinetics and X-ray data lead to a structure-activity relationship that highlights steric and electronic effects of the amino substituent of phosphoramidate. This study provides the structural basis to design new oximes capable of reactivating phosphoramidyl-hBChE conjugates after intoxication, notably when hBChE is used as pretreatment, or to design BChE-based catalytic bioscavengers.
Asunto(s)
Butirilcolinesterasa/metabolismo , Inhibidores de la Colinesterasa/farmacología , Reactivadores de la Colinesterasa/farmacología , Organofosfatos/farmacología , Oximas/farmacología , Dominio Catalítico , Inhibidores de la Colinesterasa/química , Reactivadores de la Colinesterasa/química , Humanos , Cinética , Modelos Moleculares , Estructura Molecular , Oximas/química , Conformación Proteica , Relación Estructura-Actividad , Factores de TiempoRESUMEN
The reaction of the new and structurally characterized covalent {Mn(CO)(3)(H(2)O)(2)}(+)-lysozyme adduct with NiS(4) and NiN(2)S(2) complexes generates binuclear Ni-Mn complexes; relevance to the reactivity of the protein-bound {Fe(CO)(CN)(2)} intermediate during maturation of [NiFe] hydrogenases is discussed.
Asunto(s)
Manganeso/química , Muramidasa/química , Compuestos Organometálicos/química , Cristalografía por Rayos X , Espectroscopía Infrarroja por Transformada de FourierRESUMEN
Innate immunity relies critically upon the ability of a few pattern recognition molecules to sense molecular markers on pathogens, but little is known about these interactions at the atomic level. Human L- and H-ficolins are soluble oligomeric defence proteins with lectin-like activity, assembled from collagen fibers prolonged by fibrinogen-like recognition domains. The X-ray structures of their trimeric recognition domains, alone and in complex with various ligands, have been solved to resolutions up to 1.95 and 1.7 A, respectively. Both domains have three-lobed structures with clefts separating the distal parts of the protomers. Ca(2+) ions are found at sites homologous to those described for tachylectin 5A (TL5A), an invertebrate lectin. Outer binding sites (S1) homologous to the GlcNAc-binding pocket of TL5A are present in the ficolins but show different structures and specificities. In L-ficolin, three additional binding sites (S2-S4) surround the cleft. Together, they define an unpredicted continuous recognition surface able to sense various acetylated and neutral carbohydrate markers in the context of extended polysaccharides such as 1,3-beta-D-glucan, as found on microbial or apoptotic surfaces.
Asunto(s)
Glicoproteínas/química , Lectinas/química , Acetilación , Acetiltransferasas/metabolismo , Secuencia de Aminoácidos , Sitios de Unión , Galactosa/metabolismo , Glicoproteínas/genética , Glicoproteínas/metabolismo , Humanos , Inmunidad Innata , Lectinas/genética , Lectinas/metabolismo , Ligandos , Modelos Moleculares , Datos de Secuencia Molecular , Regiones Promotoras Genéticas , Subunidades de Proteína/química , Subunidades de Proteína/metabolismo , Homología de Secuencia de Aminoácido , Especificidad por Sustrato , beta-Glucanos/metabolismo , FicolinasRESUMEN
Drosophila E75 is a member of the nuclear receptor superfamily. These eukaryotic transcription factors are involved in almost all physiological processes. They regulate transcription in response to binding of rigid hydrophobic hormone ligands. As it is the case for many nuclear receptors, the E75 hormone ligand was originally unknown. Recently, however, it was shown that the ligand binding domain (LBD) of E75 contains a tightly bound heme prosthetic group and is gas responsive. Here we have used site-directed mutagenesis along with UV-visible and electron paramagnetic resonance (EPR) spectroscopies to characterize and assign the heme iron axial ligands in E75. The F370Y mutation and addition of hemin to the growth medium during expression of the protein in Escherichia coli were necessary to produce good yields of heme-enriched E75 LBD. EPR studies revealed the presence of several species containing a strongly iron bound thiolate. The involvement of cysteines 396 and 468 in heme binding was subsequently shown by single and double mutations. Using a similar approach, we have also established that the sixth iron ligand of a well-defined coordination conformation, which accounts for approximately half of the total species, is histidine 574. The other iron coordination pairs are discussed. We conclude that E75 is a new example of a thiolate hemoprotein and that it may be involved in hormone synthesis regulation.
Asunto(s)
Proteínas de Unión al ADN/metabolismo , Proteínas de Drosophila/metabolismo , Drosophila melanogaster/metabolismo , Hemoproteínas/metabolismo , Receptores Citoplasmáticos y Nucleares/metabolismo , Azufre/química , Factores de Transcripción/metabolismo , Alanina/genética , Secuencia de Aminoácidos , Animales , Cisteína/genética , Proteínas de Unión al ADN/química , Proteínas de Unión al ADN/aislamiento & purificación , Proteínas de Drosophila/química , Proteínas de Drosophila/aislamiento & purificación , Espectroscopía de Resonancia por Spin del Electrón , Expresión Génica , Hemo/química , Hemoproteínas/química , Hemoproteínas/aislamiento & purificación , Histidina/genética , Humanos , Ligandos , Datos de Secuencia Molecular , Mutación/genética , Unión Proteica , Estructura Terciaria de Proteína , Receptores Citoplasmáticos y Nucleares/química , Receptores Citoplasmáticos y Nucleares/aislamiento & purificación , Alineación de Secuencia , Solubilidad , Espectrofotometría Ultravioleta , Factores de Transcripción/química , Factores de Transcripción/aislamiento & purificaciónRESUMEN
Human phosphate-binding protein (HPBP) was serendipitously discovered by crystallization and X-ray crystallography. HPBP belongs to a eukaryotic protein family named DING that is systematically absent from the genomic database. This apoprotein of 38 kDa copurifies with the HDL-associated apoprotein paraoxonase (PON1) and binds inorganic phosphate. HPBP is the first identified transporter capable of binding phosphate ions in human plasma. Thus, it may be regarded as a predictor of phosphate-related diseases such as atherosclerosis. In addition, HPBP may be a potential therapeutic protein for the treatment of such diseases. Here, the purification, detergent-exchange protocol and crystallization conditions that led to the discovery of HPBP are reported.
Asunto(s)
Proteínas de Transporte de Fosfato/química , Apoproteínas/química , Apoproteínas/aislamiento & purificación , Apoproteínas/metabolismo , Arildialquilfosfatasa/química , Arildialquilfosfatasa/aislamiento & purificación , Aterosclerosis/metabolismo , HDL-Colesterol/química , Cristalización , Cristalografía por Rayos X , Humanos , Proteínas de Transporte de Fosfato/aislamiento & purificación , Proteínas de Transporte de Fosfato/metabolismoRESUMEN
Iron regulatory proteins (IRPs) control the translation of proteins involved in iron uptake, storage and utilization by binding to specific noncoding sequences of the corresponding mRNAs known as iron-responsive elements (IREs). This strong interaction assures proper iron homeostasis in animal cells under iron shortage. Conversely, under iron-replete conditions, IRP1 binds a [4Fe-4S] cluster and functions as cytosolic aconitase. Regulation of the balance between the two IRP1 activities is complex, and it does not depend only on iron availability. Here, we report the crystal structure of human IRP1 in its aconitase form. Comparison with known structures of homologous enzymes reveals well-conserved folds and active site environments with significantly different surface shapes and charge distributions. The specific features of human IRP1 allow us to propose a tentative model of an IRP1-IRE complex that agrees with a range of previously obtained data.
Asunto(s)
Aconitato Hidratasa/química , Citosol/enzimología , Proteína 1 Reguladora de Hierro/química , Aconitato Hidratasa/metabolismo , Secuencia de Aminoácidos , Animales , Dominio Catalítico , Bovinos , Cristalización , Cristalografía por Rayos X , Dimerización , Humanos , Proteína 1 Reguladora de Hierro/metabolismo , Datos de Secuencia Molecular , Unión Proteica , Pliegue de Proteína , Estructura Terciaria de Proteína , Alineación de SecuenciaRESUMEN
Nuclear receptors form an important class of transcription regulators in metazoans. To learn more about the evolution of these proteins, we have initiated structural studies on nuclear receptor ligand-binding domains from various animals. Here we present the crystal structure of the ligand-binding domain (LBD) of the retinoid X receptor (RXR) from the mollusc Biomphalaria glabrata. The structure reveals a novel tetrameric association in which each monomer is complexed to the human RXR ligand 9-cis retinoic acid and to a human co-activator-derived peptide. The ligand and the co-activator peptide are bound in essentially the same manner as observed in previously reported human RXR LBD structures, suggesting that the mechanisms of RXR-mediated transcription regulation are very similar in mollusc and human. The structure shows further that binding of ligand and co-activator peptide does not necessarily lead to the typical holo-conformation in which helix 12 (H12) folds back and packs against the LBD. Within a canonical dimer, only one monomer is in this closed agonist conformation. The other monomer is in an open conformation with H12 protruding from the LBD core, occupying the H12 interaction groove of another open monomer in an adjacent dimer in a domain swapping fashion, thus resulting in a tetrameric association. Additional tetramer interfaces are formed between H11 of the closed LBD and H6 of the open LBD. This novel holo-tetramer configuration may have a biological role in activating genes whose promoters are poorly recognised by dimers but much more efficiently by the corresponding tetramers.
Asunto(s)
Biomphalaria/química , Receptores X Retinoide/química , Animales , Sitios de Unión , Cristalografía por Rayos X , Dimerización , Regulación de la Expresión Génica , Humanos , Ligandos , Receptores X Retinoide/agonistasRESUMEN
Recent experimental and theoretical studies have focused on the mechanism of the A-cluster active site of acetyl-CoA synthase that produces acetyl-CoA from a methyl group, carbon monoxide, and CoA. Several proposals have been made concerning the redox states of the (Ni-Ni) bimetallic center and the iron-sulfur cluster connected to one of the metals. Using hybrid density functional theory, we have investigated putative intermediate states from the catalytic cycle. Among our conclusions are the following: (i) the zerovalent state proposed for the proximal metal is unlikely if the charge on the iron-sulfur cluster is +2; (ii) a mononuclear mechanism in which both CO and CH(3) bind the proximal nickel is favored over the binuclear mechanism in which CO and CH(3) bind the proximal and distal nickel ions, respectively; (iii) the formation of a disulfide bond in the active site could provide the two electrons necessary for the reaction but only if methylation occurs simultaneously; and (iv) the crystallographic closed form of the active site needs to open to accommodate ligands in the equatorial site.
Asunto(s)
Acetato CoA Ligasa/química , Acetato CoA Ligasa/metabolismo , Sitios de Unión , Enlace de Hidrógeno , Modelos Químicos , Modelos Moleculares , Níquel/química , Teoría CuánticaRESUMEN
MAp19 is an alternative splicing product of the MASP-2 gene comprising the N-terminal CUB1-epidermal growth factor (EGF) segment of MASP-2, plus four additional residues at its C-terminal end. Like full-length MASP-2, it forms Ca(2+)-dependent complexes with mannan-binding lectin (MBL) and L-ficolin. The x-ray structure of human MAp19 was solved to a resolution of 2.5 A. It shows a head to tail homodimer held together by interactions between the CUB1 module of one monomer and the EGF module of its counterpart. A Ca(2+) ion bound to each EGF module stabilizes the dimer interfaces. A second Ca(2+) ion is bound to the distal end of each CUB1 module, through six ligands contributed by Glu(52), Asp(60), Asp(105), Ser(107), Asn(108), and a water molecule. Compared with its counterpart in human C1s, the N-terminal end of the MAp19 CUB1 module contains a 7-residue extension that forms additional inter-monomer contacts. To identify the residues involved in the interaction of MAp19 with MBL and L-ficolin, point mutants were generated and their binding ability was determined using surface plasmon resonance spectroscopy. Six mutations at Tyr(59), Asp(60), Glu(83), Asp(105), Tyr(106), and Glu(109) either strongly decreased or abolished interaction with both MBL and L-ficolin. These mutations map a common binding site for these proteins located at the distal end of each CUB1 module and stabilized by the Ca(2+) ion.
Asunto(s)
Proteínas Portadoras/metabolismo , Lectinas , Lectina de Unión a Manosa/metabolismo , Estructura Terciaria de Proteína , Serina Endopeptidasas/química , Secuencia de Aminoácidos , Sitios de Unión , Calcio/metabolismo , Dicroismo Circular , Cristalografía por Rayos X , Dimerización , Humanos , Serina Proteasas Asociadas a la Proteína de Unión a la Manosa , Modelos Moleculares , Datos de Secuencia Molecular , Mutagénesis Sitio-Dirigida , Estructura Secundaria de Proteína , Alineación de Secuencia , Serina Endopeptidasas/genética , Serina Endopeptidasas/metabolismo , Resonancia por Plasmón de Superficie , FicolinasRESUMEN
C1, the complex that triggers the classical pathway of complement, is assembled from two modular proteases C1r and C1s and a recognition protein C1q. The N-terminal CUB1-EGF segments of C1r and C1s are key elements of the C1 architecture, because they mediate both Ca2+-dependent C1r-C1s association and interaction with C1q. The crystal structure of the interaction domain of C1s has been solved and refined to 1.5 A resolution. The structure reveals a head-to-tail homodimer involving interactions between the CUB1 module of one monomer and the epidermal growth factor (EGF) module of its counterpart. A Ca2+ ion is bound to each EGF module and stabilizes both the intra- and inter-monomer interfaces. Unexpectedly, a second Ca2+ ion is bound to the distal end of each CUB1 module, through six ligands contributed by Glu45, Asp53, Asp98, and two water molecules. These acidic residues and Tyr17 are conserved in approximately two-thirds of the CUB repertoire and define a novel, Ca2+-binding CUB module subset. The C1s structure was used to build a model of the C1r-C1s CUB1-EGF heterodimer, which in C1 connects C1r to C1s and mediates interaction with C1q. A structural model of the C1q/C1r/C1s interface is proposed, where the rod-like collagen triple helix of C1q is accommodated into a groove along the transversal axis of the C1r-C1s heterodimer.
Asunto(s)
Calcio/metabolismo , Complemento C1/metabolismo , Secuencia de Aminoácidos , Animales , Secuencia de Bases , Sitios de Unión , Línea Celular , Complemento C1/química , Cristalografía por Rayos X , Cartilla de ADN , Dimerización , Electroforesis en Gel de Poliacrilamida , Modelos Moleculares , Datos de Secuencia Molecular , Unión Proteica , Conformación Proteica , Proteínas Recombinantes/química , Proteínas Recombinantes/metabolismo , Homología de Secuencia de Aminoácido , SpodopteraRESUMEN
T cell receptor (TCR) binding degeneracy lies at the heart of several physiological and pathological phenomena, yet its structural basis is poorly understood. We determined the crystal structure of a complex involving the BM3.3 TCR and an octapeptide (VSV8) bound to the H-2K(b) major histocompatibility complex molecule at a 2.7 A resolution, and compared it with the BM3.3 TCR bound to the H-2K(b) molecule loaded with a peptide that has no primary sequence identity with VSV8. Comparison of these structures showed that the BM3.3 TCR complementarity-determining region (CDR) 3alpha could undergo rearrangements to adapt to structurally different peptide residues. Therefore, CDR3 loop flexibility helps explain TCR binding cross-reactivity.
Asunto(s)
Regiones Determinantes de Complementariedad/química , Receptores de Antígenos de Linfocitos T/química , Linfocitos T/inmunología , Regiones Determinantes de Complementariedad/inmunología , Humanos , Ligandos , Unión Proteica/inmunología , Conformación Proteica , Estructura Terciaria de Proteína , Receptores de Antígenos de Linfocitos T/inmunología , Relación Estructura-Actividad , Linfocitos T/químicaRESUMEN
C1 is the multimolecular protease that triggers activation of the classical pathway of complement, a major element of antimicrobial host defense also involved in immune tolerance and various pathologies. This 790,000 Da complex is formed from the association of a recognition protein, C1q, and a catalytic subunit, the Ca2+-dependent tetramer C1s-C1r-C1r-C1s comprising two copies of each of the modular proteases C1r and C1s. Early studies mainly based on biochemical analysis and electron microscopy of C1 and its isolated components have allowed for characterization of their domain structure and led to a low-resolution model of the C1 complex in which the elongated C1s-C1r-C1r-C1s tetramer folds into a more compact, "8-shaped" conformation upon interaction with C1q. A major strategy used over the past years has been to dissect the C1 proteins into modular segments to characterize their function and solve their structure by either X-ray crystallography or nuclear magnetic resonance spectroscopy (NMR). The purpose of this review is to focus on this information, with particular emphasis on the architecture of the C1 complex and the mechanisms underlying its activation and proteolytic activity.
Asunto(s)
Activación de Complemento , Complemento C1/química , Animales , Dominio Catalítico , Complemento C1/fisiología , Complemento C1q/química , Cristalografía por Rayos X , Factor de Crecimiento Epidérmico/química , Humanos , Espectroscopía de Resonancia Magnética , Estructura Secundaria de Proteína , Serina Endopeptidasas/químicaRESUMEN
C1r is the serine protease (SP) that mediates autoactivation of C1, the complex that triggers the classical complement pathway. We have determined the crystal structure of two fragments from the human C1r catalytic domain, each encompassing the second complement control protein (CCP2) module and the SP domain. The wild-type species has an active structure, whereas the S637A mutant is a zymogen. The structures reveal a restricted hinge flexibility of the CCP2-SP interface, and both are characterized by the unique alpha-helical conformation of loop E. The zymogen activation domain exhibits high mobility, and the active structure shows a restricted access to most substrate binding subsites. Further implications relevant to the C1r self-activation process are derived from protein-protein interactions in the crystals.
Asunto(s)
Activación de Complemento , Complemento C1r/química , Precursores Enzimáticos/química , Sitios de Unión , Dominio Catalítico , Complemento C1r/metabolismo , Humanos , Modelos Moleculares , Unión Proteica , Conformación Proteica , Pliegue de Proteína , Estructura Secundaria de Proteína , Estructura Terciaria de Proteína , Especificidad por SustratoRESUMEN
C1r, the enzyme responsible for intrinsic activation of the C1 complex of complement, is a modular serine protease featuring an overall structural organization homologous to those of C1s and the mannan-binding lectin-associated serine proteases (MASPs). This review will initially summarize current information on the structure and function of C1r, with particular emphasis on the three-dimensional structure of its catalytic domain, which provides new insights into the activation mechanism of C1. The second part of this review will focus on recent discoveries dealing with a truncated, C1r-related protein, and the occurrence in the mouse of two isoforms, C1rA and C1rB, exhibiting tissue-specific expression patterns.
Asunto(s)
Complemento C1r/química , Complemento C1r/genética , Complemento C1r/fisiología , Vía Clásica del Complemento/fisiología , Animales , Complemento C1s/química , Complemento C1s/fisiología , Activación Enzimática , Humanos , Ratones , Isoformas de Proteínas , Estructura Secundaria de Proteína , Relación Estructura-ActividadRESUMEN
The elongated complementary-determining region (CDR) 3beta found in the unliganded KB5-C20 TCR protrudes from the antigen binding site and prevents its docking onto the peptide/MHC (pMHC) surface according to a canonical diagonal orientation. We now present the crystal structure of a complex involving the KB5-C20 TCR and an octapeptide bound to the allogeneic H-2K(b) MHC class I molecule. This structure reveals how a tremendously large CDR3beta conformational change allows the KB5-C20 TCR to adapt to the rather constrained pMHC surface and achieve a diagonal docking mode. This extreme case of induced fit also shows that TCR plasticity is primarily restricted to CDR3 loops and does not propagate away from the antigen binding site.