Étude structure-fonction des fructose-1,6-bisphosphate aldolases métallo-dépendantes : mécanisme catalytique et développement d’antimicrobiens

Les fructose-1,6-bisphosphate aldolases (FBPA) sont des enzymes glycolytiques (EC 4.1.2.13) qui catalysent la transformation réversible du fructose-1,6-bisphosphate (FBP) en deux trioses-phosphates, le glycéraldéhyde-3-phosphate (G3P) et le dihydroxyacétone phosphate (DHAP). Il existe deux classes de FBPA qui diffèrent au niveau de leur mécanisme catalytique. Les classes I passent par la formation d’un intermédiaire covalent de type iminium alors que les classes II, métallodépendantes, utilisent généralement un zinc catalytique. Contrairement au mécanisme des classes I qui a été très étudié, de nombreuses interrogations subsistent au sujet de celui des classes II. Nous avons donc entrepris une analyse détaillée de leur mécanisme réactionnel en nous basant principalement sur la résolution de structures cristallographiques. De nombreux complexes à haute résolution furent obtenus et ont permis de détailler le rôle de plusieurs résidus du site actif de l’enzyme. Nous avons ainsi corrigé l’identification du résidu responsable de l’abstraction du proton de l’O4 du FBP, une étape cruciale du mécanisme. Ce rôle, faussement attribué à l’Asp82 (chez Helicobacter pylori), est en fait rempli par l’His180, un des résidus coordonant le zinc. L’Asp82 n’en demeure pas moins essentiel car il oriente, active et stabilise les substrats. Enfin, notre étude met en évidence le caractère dynamique de notre enzyme dont la catalyse nécessite la relocalisation du zinc et de nombreux résidus. La dynamique de la protéine ne permet pas d’étudier tous les aspects du mécanisme uniquement par l’approche cristallographique. En particulier, le résidu effectuant le transfert stéréospécifique du proton pro(S) sur le carbone 3 (C3) du DHAP est situé sur une boucle qui n’est visible dans aucune de nos structures. Nous avons donc développé un protocole de dynamique moléculaire afin d’étudier sa dynamique. Validé par l’étude d’inhibiteurs de la classe I, l’application de notre protocole aux FBPA de classe II a confirmé l’identification du résidu responsable de cette abstraction chez Escherichia coli (Glu182) mais pointe vers un résidu diffèrent chez H. pylori (Glu149 au lieu de Glu142). Nos validations expérimentales confirment ces observations et seront consolidées dans le futur. Les FBPA de classe II sont absentes du protéome humain mais sont retrouvées chez de nombreux pathogènes, pouvant même s'y révéler essentielles. Elles apparaissent donc comme étant une cible idéale pour le développement de nouveaux agents anti-microbiens. L’obtention de nouveaux analogues des substrats pour ces enzymes a donc un double intérêt, obtenir de nouveaux outils d’étude du mécanisme mais aussi développer des molécules à visée pharmacologique. En collaboration avec un groupe de chimistes, nous avons optimisé le seul inhibiteur connu des FBPA de classe II. Les composés obtenus, à la fois plus spécifiques et plus puissants, permettent d’envisager une utilisation pharmacologique. En somme, c’est par l’utilisation de techniques complémentaires que de nouveaux détails moléculaires de la catalyse des FBPA de classe II ont pu être étudiés. Ces techniques permettront d’approfondir la compréhension fine du mécanisme catalytique de l’enzyme et offrent aussi de nouvelles perspectives thérapeutiques.Fructose-1,6-bisphosphate aldolases (FBPA) are glycolytic enzymes (EC 4.1.2.13) that catalyze the reversible cleavage of fructose-1,6-bisphosphate (FBP) into the triose phosphates, glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). There are two classes of FBPAs that differ at the level of their mechanism. Class I FBPAs form a covalent iminium intermediate whereas class II FBPAs, being metalloenzymes, generally use a catalytic zinc in their reaction mechanism. In contrast to the mechanism of the class I FBPAs, which has been thoroughly studied, there are several unresolved inquiries as to the mechanism of class II FBPAs. We have therefore pursued a detailed analysis of the reaction mechanism using as a primary tool the elucidation of crystallographic structures. Several high resolution complexes have been resolved and have provided critical evidence to help us suggest the implication and role of several key residues in the active site. Consequently, we have correctly identified the residue which is responsible for the abstraction of the O4 proton from FBP, a vital step in the reaction mechanism. The residue responsible for this abstraction, which had incorrectly been assigned to Asp82 (in Helicobacter pylori), has been appropriately consigned to His180, a residue which is involved in coordinating the zinc molecule. Nevertheless, Asp82 remains an important residue as it orients, activates and stabilizes substrates. Finally, our study brings to evidence the dynamic character of our enzyme in which catalysis entails the relocalization of the catalytic zinc and several residues. The complexity of this reaction, notably one of the proton exchanges in the mechanism, could not be resolved solely by crystallographic means. In fact, the residue responsible for the stereospecific transfer of the pro(S) proton on carbon 3 (C3) of DHAP is situated on a loop that was not resolved in any of our structures. We therefore developed a molecular dynamics approach to study this intricate movement. After preliminary validation by inhibitor studies with class I FBPAs, the protocol was applied to class II FBPAs and several remarkable observations emerged: the residue responsible for this abstraction in Escherichia coli is Glu182 whereas a different residue, Glu149 (instead of Glu142) appears to assume this role in H. pylori. Our preliminary validations have confirmed this observation and shall be further consolidated in the future. Class II FBP aldolases, although absent from the human proteome, are prevalently found in several pathogens, and have further been found to be essential to a number of these organisms. As such, they are ideal targets for the development of novel anti-microbial agents. Developing new analogues of the cognate substrates of these enzymes is therefore not only advantageous for mechanistic studies, but has endless pharmacological potential. In the context of a collaborative effort involving a group of chemists, a compound that initially had an inhibition constant in the millimolar range was optimized and produced a series of compounds that inhibit in the nanomolar range

Glycolytic and Non-glycolytic Functions of Mycobacterium tuberculosis Fructose-1,6-bisphosphate Aldolase, an Essential Enzyme Produced by Replicating and Non-replicating Bacilli

The search for antituberculosis drugs active against persistent bacilli has led to our interest in metallodependent class II fructose- 1,6-bisphosphate aldolase (FBA-tb), a key enzyme of gluconeogenesis absent from mammalian cells. Knock-out experiments at the fba-tb locus indicated that this gene is required for the growth of Mycobacterium tuberculosis on gluconeogenetic substrates and in glucose-containing medium. Surface labeling and enzymatic activity measurements revealed that this enzyme was exported to the cell surface of M. tuberculosis and produced under various axenic growth conditions including oxygen depletion and hence by non-replicating bacilli. Importantly, FBA-tb was also produced in vivo in the lungs of infected guinea pigs and mice. FBA-tb bound human plasmin(ogen) and protected FBA-tb-bound plasmin from regulation by α 2-antiplasmin, suggestive of an involvement of this enzyme in host/pathogen interactions. The crystal structures of FBA-tb in the native form and in complex with a hydroxamate substrate analog were determined to 2.35- and 1.9-Å resolution, respectively. Whereas inhibitor attachment had no effect on the plasminogen binding activity of FBA-tb, it competed with the natural substrate of the enzyme, fructose 1,6-bisphosphate, and substantiated a previously unknown reaction mechanism associated with metallodependent aldolases involving recruitment of the catalytic zinc ion by the substrate upon active site binding. Altogether, our results highlight the potential of FBA-tb as a novel therapeutic target against both replicating and non-replicating bacilli.Fil: Santangelo, María de la Paz. State University of Colorado - Fort Collins; Estados Unidos. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigación en Ciencias Veterinarias y Agronómicas. Instituto de Biotecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Gest, Petra M.. State University of Colorado - Fort Collins; Estados UnidosFil: Guerin, Marcelo E.. Universidad del País Vasco; EspañaFil: Coinçon, Mathieu. University of Montreal; CanadáFil: Pham, Ha. State University of Colorado - Fort Collins; Estados UnidosFil: Ryan, Gavin. State University of Colorado - Fort Collins; Estados UnidosFil: Puckett, Susan E.. Cornell University; Estados UnidosFil: Spencer, John S.. State University of Colorado - Fort Collins; Estados UnidosFil: Gonzalez Juarrero, Mercedes. State University of Colorado - Fort Collins; Estados UnidosFil: Daher, Racha. Universite de Paris XI. Institut de Chimie Moléculaire et des Matériaux d'Orsay; FranciaFil: Lenaerts, Anne J.. State University of Colorado - Fort Collins; Estados UnidosFil: Schnappinger, Dirk. Cornell University; Estados UnidosFil: Therisod, Michel. Universite de Paris XI. Institut de Chimie Moléculaire et des Matériaux d'Orsay; FranciaFil: Ehrt, Sabine. Cornell University; Estados UnidosFil: Sygusch, Jurgen. University of Montreal; CanadáFil: Jackson, Mary. State University of Colorado - Fort Collins; Estados Unido

Visualizing drug binding interactions using microcrystal electron diffraction

Visualizing ligand binding interactions is important for structure-based drug design and fragment-based screening methods. Rapid and uniform soaking with potentially reduced lattice defects make small macromolecular crystals attractive targets for studying drug binding using microcrystal electron diffraction (MicroED). However, so far no drug binding interactions could unambiguously be resolved by electron diffraction alone. Here, we use MicroED to study the binding of a sulfonamide inhibitor to human carbonic anhydrase isoform II (HCA II). We show that MicroED data can efficiently be collected on a conventional transmission electron microscope from thin hydrated microcrystals soaked with the clinical drug acetazolamide (AZM). The data are of high enough quality to unequivocally fit and resolve the bound inhibitor. We anticipate MicroED can play an important role in facilitating in-house fragment screening for drug discovery, complementing existing methods in structural biology such as X-ray and neutron diffraction

Dissecting the proton transport pathway in electrogenic Na + /H + antiporters

Sodium/proton exchangers of the SLC9 family mediate the transport of protons in exchange for sodium to help regulate intracellular pH, sodium levels, and cell volume. In electrogenic Na(+)/H(+) antiporters, it has been assumed that two ion-binding aspartate residues transport the two protons that are later exchanged for one sodium ion. However, here we show that we can switch the antiport activity of the bacterial Na(+)/H(+) antiporter NapA from being electrogenic to electroneutral by the mutation of a single lysine residue (K305). Electroneutral lysine mutants show similar ion affinities when driven by [Formula: see text] pH, but no longer respond to either an electrochemical potential ([Formula: see text]) or could generate one when driven by ion gradients. We further show that the exchange activity of the human Na(+)/H(+) exchanger NHA2 (SLC9B2) is electroneutral, despite harboring the two conserved aspartic acid residues found in NapA and other bacterial homologues. Consistently, the equivalent residue to K305 in human NHA2 has been replaced with arginine, which is a mutation that makes NapA electroneutral. We conclude that a transmembrane embedded lysine residue is essential for electrogenic transport in Na(+)/H(+) antiporters