Protein lysine posttranslational modification by an increasing number of different acyl groups is becoming appreciated as a regulatory mechanism in cellular biology. Sirtuins are class III histone deacylases that use NAD ؉ as a co-substrate during amide bond hydrolysis. Several studies have described the sirtuins as sensors of the NAD ؉ /NADH ratio, but it has not been formally tested for all the mammalian sirtuins in vitro. To address this problem, we first synthesized a wide variety of peptide-based probes, which were used to identify the range of hydrolytic activities of human sirtuins. These probes included aliphatic ⑀-N-acyllysine modifications with hydrocarbon lengths ranging from formyl (C 1 ) to palmitoyl (C 16 ) as well as negatively charged dicarboxyl-derived modifications. In addition to the well established activities of the sirtuins, "long chain" acyllysine modifications were also shown to be prone to hydrolytic cleavage by SIRT1-3 and SIRT6, supporting recent findings. We then tested the ability of NADH, ADP-ribose, and nicotinamide to inhibit these NAD ؉ -dependent deacylase activities of the sirtuins. In the commonly used 7-amino-4-methylcoumarin-coupled fluorescence-based assay, the fluorophore has significant spectral overlap with NADH and therefore cannot be used to measure inhibition by NADH. Therefore, we turned to an HPLC-MS-based assay to directly monitor the conversion of acylated peptides to their deacylated forms. All tested sirtuin deacylase activities showed sensitivity to NADH in this assay. However, the inhibitory concentrations of NADH in these assays are far greater than the predicted concentrations of NADH in cells; therefore, our data indicate that NADH is unlikely to inhibit sirtuins in vivo. These data suggest a re-evaluation of the sirtuins as direct sensors of the NAD ؉ /NADH ratio.

The design of covalent inhibitors in glycoscience research is important for the development of chemical biology probes. Here we report the synthesis of a new carbocyclic mechanism-based covalent inhibitor of an α-glucosidase. The enzyme efficiently catalyzes its alkylation via either an allylic cation or a cationic transition state. We show this allylic covalent inhibitor has different catalytic proficiencies for pseudoglycosylation and deglycosylation. Such inhibitors have the potential to be useful chemical biology tools.

The design of mechanism-based enzyme inactivators to generate chemical probes for biological research is an important challenge in carbohydrate chemistry. Here we describe the synthesis and biological evaluation of a novel carbocyclic mechanism-based inactivator of galactosidases (glycoside hydrolase families 27 and 36). Upon catalysis of this unnatural substrate, a transient non-classical carbocation forms within the enzyme active site. We show that the inactivation event, which proceeds via a bicyclobutonium ion intermediate, leads to a single alkylation event that occurs on the enzymatic nucleophile, an aspartic acid residue in this case. We also show that the catalytic proficiencies for enzymatic hydrolysis of substrates and inactivation by our bicyclo[4.1.0]heptyl analogue of galactose differ by only a factor of 20. This inactivator has the potential for further development as a useful biological research tool for both basic research and biotechnological applications.

Mechanism-based glycoside hydrolase inhibitors are carbohydrate analogs that mimic the natural substrate’s structure. Their covalent bond formation with the glycoside hydrolase makes these compounds excellent tools for chemical biology and potential drug candidates. Here we report the synthesis of cyclohexene-based α-galactopyranoside mimics and the kinetic and structural characterization of their inhibitory activity toward an α-galactosidase from Thermotoga maritima (TmGalA). By solving the structures of several enzyme-bound species during mechanism-based covalent inhibition of TmGalA, we show that the Michaelis complexes for intact inhibitor and product have half-chair (2H3) conformations for the cyclohexene fragment, while the covalently linked intermediate adopts a flattened half-chair (2H3) conformation. Hybrid QM/MM calculations confirm the structural and electronic properties of the enzyme-bound species and provide insight into key interactions in the enzyme-active site. These insights should stimulate the design of mechanism-based glycoside hydrolase inhibitors with tailored chemical properties.

The MelA gene from Citrobacter freundii, which encodes a glycosyl hydrolase family 4 (GH4) α-galactosidase, has been cloned and expressed in Escherichia coli. The recombinant enzyme catalyzes the hydrolysis of phenyl α-galactosides via a redox elimination-addition mechanism involving oxidation of the hydroxyl group at C-3 and elimination of phenol across the C-1-C-2 bond to give an enzyme-bound glycal intermediate. For optimal activity, the MelA enzyme requires two cofactors, NAD(+) and Mn(2+), and the addition of a reducing agent, such as mercaptoethanol. To delineate the mechanism of action for this GH4 enzyme, we measured leaving group effects, and the derived β(lg) values on V and V/K are indistinguishable from zero (-0.01 ± 0.02 and 0.02 ± 0.04, respectively). Deuterium kinetic isotope effects (KIEs) were measured for the weakly activated substrate phenyl α-D-galactopyranoside in which isotopic substitution was incorporated at C-1, C-2, or C-3. KIEs of 1.06 ± 0.07, 0.91 ± 0.04, and 1.02 ± 0.06 were measured on V for the 1-(2)H, 2-(2)H, and 3-(2)H isotopic substrates, respectively. The corresponding values on V/K were 1.13 ± 0.07, 1.74 ± 0.06, and 1.74 ± 0.05, respectively. To determine if the KIEs report on a single step or on a virtual transition state, we measured KIEs using doubly deuterated substrates. The measured (D)V/K KIEs for MelA-catalyzed hydrolysis of phenyl α-D-galactopyranoside on the dideuterated substrates, (D)V/K((3-D)/(2-D,3-D)) and (D)V/K((2-D)/(2-D,3-D)), are 1.71 ± 0.12 and 1.71 ± 0.13, respectively. In addition, the corresponding values on V, (D)V((3-D)/(2-D,3-D)) and (D)V((2-D)/(2-D,3-D)), are 0.91 ± 0.06 and 1.01 ± 0.06, respectively. These observations are consistent with oxidation at C-3, which occurs via the transfer of a hydride to the on-board NAD(+), being concerted with proton removal at C-2 and the fact that this step is the first irreversible step for the MelA α-galactosidase-catalyzed reactions of aryl substrates. In addition, the rate-limiting step for V(max) must come after this irreversible step in the reaction mechanism.