Insights into the Mechanism of Bovine CD38/NAD+Glycohydrolase from the X-Ray Structures of Its Michaelis Complex and Covalently-Trapped Intermediates

Bovine CD38/NAD+glycohydrolase (bCD38) catalyses the hydrolysis of NAD+ into nicotinamide and ADP-ribose and the formation of cyclic ADP-ribose (cADPR). We solved the crystal structures of the mono N-glycosylated forms of the ecto-domain of bCD38 or the catalytic residue mutant Glu218Gln in their apo state or bound to aFNAD or rFNAD, two 2′-fluorinated analogs of NAD+. Both compounds behave as mechanism-based inhibitors, allowing the trapping of a reaction intermediate covalently linked to Glu218. Compared to the non-covalent (Michaelis) complex, the ligands adopt a more folded conformation in the covalent complexes. Altogether these crystallographic snapshots along the reaction pathway reveal the drastic conformational rearrangements undergone by the ligand during catalysis with the repositioning of its adenine ring from a solvent-exposed position stacked against Trp168 to a more buried position stacked against Trp181. This adenine flipping between conserved tryptophans is a prerequisite for the proper positioning of the N1 of the adenine ring to perform the nucleophilic attack on the C1′ of the ribofuranoside ring ultimately yielding cADPR. In all structures, however, the adenine ring adopts the most thermodynamically favorable anti conformation, explaining why cyclization, which requires a syn conformation, remains a rare alternate event in the reactions catalyzed by bCD38 (cADPR represents only 1% of the reaction products). In the Michaelis complex, the substrate is bound in a constrained conformation; the enzyme uses this ground-state destabilization, in addition to a hydrophobic environment and desolvation of the nicotinamide-ribosyl bond, to destabilize the scissile bond leading to the formation of a ribooxocarbenium ion intermediate. The Glu218 side chain stabilizes this reaction intermediate and plays another important role during catalysis by polarizing the 2′-OH of the substrate NAD+. Based on our structural analysis and data on active site mutants, we propose a detailed analysis of the catalytic mechanism

Potent, Selective, and Orally Efficacious Antagonists of Melanin-Concentrating Hormone Receptor 1

The high expression of MCH in the hypothalamus with the lean hypophagic phenotype coupled with increased resting metabolic rate and resistance to high fat diet-induced obesity of MCH KO mice has spurred considerable efforts to develop small molecule MCHR1 antagonists. Starting from a lead thienopyrimidinone series, structure−activity studies at the 3- and 6-positions of the thienopyrimidinone core afforded potent and selective MCHR1 antagonists with representative examples having suitable pharmacokinetic properties. Based on structure−activity relationships, a structural model for MCHR1 was constructed to explain the binding mode of these antagonists. In general, a good correlation was observed between pKas and activity in the right-hand side of the template, with Asp123 playing an important role in the enhancement of binding affinity. A representative example when evaluated chronically in diet-induced obese mice resulted in good weight loss effects. These antagonists provide a viable lead series in the discovery of new therapies for the treatment of obesity

Facile Site-Specific Multiconjugation Strategies in Recombinant Proteins Produced in Bacteria.

For biomedical applications, proteins may require conjugation to small and large molecules. Typical examples are dyes for imaging, cytotoxic effector molecules for cell killing, or half-life extension modules for optimized pharmacokinetics. Although many conjugation strategies are straightforward to apply, most of them do not enable site-specific and orthogonal conjugation, and do not yield a defined stoichiometry. Moreover, techniques offering these desirable features often rely on complex expression procedures and suffer from low production yields. A more promising manufacturing strategy for flexible, site-specific and stoichiometrically defined payloading of proteins is the combination of click chemistry and thiol-maleimide conjugation, which even enables dual labeling when used consecutively. Here, we describe as an example the production of Designed Ankyrin Repeat Proteins (DARPins), a non-IgG binding scaffold, in a specific E. coli strain to obtain high yields of protein carrying both a thiol and an azide group. We provide straightforward protocols for strain-promoted azide-alkyne cycloaddition (SPAAC) and thiol-maleimide conjugation, and furthermore compare these conjugation chemistries with existing alternatives like copper-catalyzed azide-alkyne cycloaddition (CuAAC). Finally, detailed instructions for reactivity analysis and yield estimations of the reactions are provided