RESUMO
The dynamic, intracellular, O-GlcNAc modification is of continuing interest and one whose study through targeted "chemical genetics" approaches is set to increase. Of particular importance is the inhibition of the O-GlcNAc hydrolase, O-GlcNAcase (OGA), since this provides a route to elevate cellular O-GlcNAc levels, and subsequent phenotypic evaluation. Such a small molecule approach complements other methods and potentially avoids changes in protein-protein interactions that manifest themselves in molecular biological approaches to O-GlcNAc transferase knockout or over-expression. Here we describe the kinetic, thermodynamic and three-dimensional structural analysis of a bacterial OGA analogue from Bacteroides thetaiotaomicron, BtGH84, in complex with a lactone oxime (LOGNAc) and a lactam form of N-acetylglucosamine and compare their binding signatures with that of the more potent inhibitor O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino N-phenyl carbamate (PUGNAc). We show that both LOGNAc and the N-acetyl gluconolactam are significantly poorer inhibitors than PUGNAc, which may reflect poorer mimicry of transition state geometry and steric clashes with the enzyme upon binding; drawbacks that the phenyl carbamate adornment of PUGNAc helps mitigate. Implications for the design of future generations of inhibitors are discussed.
Assuntos
Proteínas de Bactérias/antagonistas & inibidores , Bacteroides/enzimologia , Inibidores Enzimáticos/química , Lactamas/química , Lactonas/química , N-Acetilglucosaminiltransferases/antagonistas & inibidores , Proteínas de Bactérias/química , Proteínas de Bactérias/genética , Proteínas de Bactérias/metabolismo , Bacteroides/química , Inibidores Enzimáticos/farmacologia , Cinética , Lactamas/farmacologia , Lactonas/farmacologia , Modelos Moleculares , N-Acetilglucosaminiltransferases/química , N-Acetilglucosaminiltransferases/genética , N-Acetilglucosaminiltransferases/metabolismo , Ligação Proteica , TermodinâmicaRESUMO
Despite decades of its use in diabetes research, the mechanism of cytotoxicity of streptozotocin (STZ) toward pancreatic beta-islet cells has remained a topic of discussion. Although STZ toxicity is likely a function of its capacity to promote DNA alkylation, it has been proposed that STZ induces pancreatic beta-cell death through O-GlcNAcase inhibition. In this report, we explore the binding mode of STZ to a close homolog of human O-GlcNAcase, BtGH84 from Bacteroides thetaiotaomicron. Our results show that STZ binds in the enzyme active site in its intact form, without the formation of a covalent adduct, consistent with solution studies on BtGH84 and human O-GlcNAcase, as well as with structural work on a homolog from Clostridium perfringens. The active site of the BtGH84 is considerably deformed upon STZ binding and as a result the catalytic machinery is expelled from the binding cavity.
Assuntos
Acetilglucosaminidase/antagonistas & inibidores , Acetilglucosaminidase/química , Inibidores Enzimáticos/farmacologia , Estreptozocina/química , Estreptozocina/farmacologia , Bacteroides/enzimologia , Calorimetria , Clostridium perfringens/enzimologia , Cristalografia por Raios X/métodos , Humanos , Modelos Moleculares , Ligação ProteicaRESUMO
The O-GlcNAc post-translational modification is considered to act as a sensor of nutrient flux through the hexosamine biosynthetic pathway. A cornerstone of this hypothesis is that global elevation of protein O-GlcNAc levels, typically induced with the non-selective O-GlcNAcase inhibitor PUGNAc (O-(2-acetamido-2-deoxy-D-glycopyranosylidene) amino-N-phenylcarbamate), causes insulin resistance in adipocytes. Here we address the potential link between elevated O-GlcNAc and insulin resistance by using a potent and selective inhibitor of O-GlcNAcase (NButGT (1,2-dideoxy-2'-propyl-alpha-D-glucopyranoso-[2,1-D]-Delta 2'-thiazoline), 1200-fold selectivity). A comparison of the structures of a bacterial homologue of O-GlcNAcase in complex with PUGNAc or NButGT reveals that these inhibitors bind to the same region of the active site, underscoring the competitive nature of their inhibition of O-GlcNAcase and the molecular basis of selectivity. Treating 3T3-L1 adipocytes with NButGT induces rapid increases in global O-GlcNAc levels, but strikingly, NButGT treatment does not replicate the insulin desensitizing effects of the non-selective O-GlcNAcase inhibitor PUGNAc. Consistent with these observations, NButGT also does not recapitulate the impaired insulin-mediated phosphorylation of Akt that is induced by treatment with PUGNAc. Collectively, these results suggest that increases in global levels of O-GlcNAc-modified proteins of cultured adipocytes do not, on their own, cause insulin resistance.
