O -linked N -acetylglucosamine ( O -GlcNAc) is a reversible posttranslational modification of Ser and Thr residues on cytosolic and nuclear proteins of higher eukaryotes catalyzed by O -GlcNAc transferase (OGT). O -GlcNAc has recently been found on Notch1 extracellular domain catalyzed by EGF domain-specific OGT. Aberrant O -GlcNAc modification of brain proteins has been linked to Alzheimer's disease (AD). However, understanding specific functions of O -GlcNAcylation in AD has been impeded by the difficulty in characterization of O -GlcNAc sites on proteins. In this study, we modified a chemical/enzymatic photochemical cleavage approach for enriching O -GlcNAcylated peptides in samples containing ∼100 μg of tryptic peptides from mouse cerebrocortical brain tissue. A total of 274 O -GlcNAcylated proteins were identified. Of these, 168 were not previously known to be modified by O -GlcNAc. Overall, 458 O -GlcNAc sites in 195 proteins were identified. Many of the modified residues are either known phosphorylation sites or located proximal to known phosphorylation sites. These findings support the proposed regulatory cross-talk between O -GlcNAcylation and phosphorylation. This study produced the most comprehensive O -GlcNAc proteome of mammalian brain tissue with both protein identification and O -GlcNAc site assignment. Interestingly, we observed O -β-GlcNAc on EGF-like repeats in the extracellular domains of five membrane proteins, expanding the evidence for extracellular O -GlcNAcylation by the EGF domain-specific OGT. We also report a GlcNAc-β-1,3-Fuc-α-1- O -Thr modification on the EGF-like repeat of the versican core protein, a proposed substrate of Fringe β-1,3- N -acetylglucosaminyltransferases.

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The ability of Leishmania to survive in their insect or mammalian host is dependent upon an ability to sense and adapt to changes in the microenvironment. However, little is known about the molecular mechanisms underlying the parasite response to environmental changes, such as nutrient availability. To elucidate nutrient stress response pathways in Leishmania donovani, we have used purine starvation as the paradigm. The salvage of purines from the host milieu is obligatory for parasite replication; nevertheless, purine-starved parasites can persist in culture without supplementary purine for over three months, indicating that the response to purine starvation is robust and engenders parasite survival under conditions of extreme scarcity. To understand metabolic reprogramming during purine starvation we have employed global approaches. Whole proteome comparisons between purine-starved and purine-replete parasites over a 6–48 h span have revealed a temporal and coordinated response to purine starvation. Purine transporters and enzymes involved in acquisition at the cell surface are upregulated within a few hours of purine removal from the media, while other key purine salvage components are upregulated later in the time-course and more modestly. After 48 h, the proteome of purine-starved parasites is extensively remodeled and adaptations to purine stress appear tailored to deal with both purine deprivation and general stress. To probe the molecular mechanisms affecting proteome remodeling in response to purine starvation, comparative RNA-seq analyses, qRT-PCR, and luciferase reporter assays were performed on purine-starved versus purine-replete parasites. While the regulation of a minority of proteins tracked with changes at the mRNA level, for many regulated proteins it appears that proteome remodeling during purine stress occurs primarily via translational and/or post-translational mechanisms.

DFT calculations support a concerted mechanism for xanthine oxidase and aldehyde oxidase hydride displacement from the sp 2 carbon of 6-substituted 4-quinazolinones. The variations in transition state structure show that C-O bond formation is nearly complete in the transition state and the transition state changes are anti-Hammond with the C-H and C-O bond lengths being more product-like for the faster reactions. The C-O bond length in the transition state is around 90% formed. However, the C-H bond is only about 80% broken. This leads to a very tetrahedral transition state with an O-C-N angle of 109 degrees. Thus, while the mechanism is concerted, the anti-bonding orbital of the C-H bond that is broken is not directly attacked by the nucleophile and instead hydride displacement occurs after almost complete tetrahedral transition state formation. In support of this the C=N bond is lengthened in the transition state indicating that attack on the electrophilic carbon occurs by addition to the C=N bond with negative charge increasing on the nitrogen. Differences in experimental reaction rates are accurately reproduced by these calculations, and tend to support this mechanism.The mechanism(s) of molybdoenzymes such as aldehyde oxidase and xanthine oxidase are of interest since these enzymes are more frequently being linked to major metabolic pathways of drugs [1][2][3][4] and, in the case of xanthine oxidase, serve an important physiological function. 5 Recent studies by our lab have shown that the regioselectivity of aldehyde oxidase, but not xanthine oxidase can be predicted by relatively simple DFT calculations. 6 A better understanding of the overall mechanism should enhance our ability to control the metabolic properties of potential drug molecules metabolized by these enzymes.Both xanthine oxidase and aldehyde oxidase enzymes contain a molybdenum pyranopterin cofactor (MoCo) and oxidize electron deficient substrates via a nucleophilic oxidation. TheCorrespondence to: Jeffrey P. Jones. MoCo for these enzymes contains three sulfurs and two oxygens. The catalytically labile oxygen, which comes from water 7,8 , and the sulfur atom that accepts the hydride are in equatorial positions of the square-pyramidal molybdenum coordination complex. 9,10 The mechanism of the reaction could proceed either through a step-wise mechanism with a tetrahedral intermediate (Figure 1a) or a concerted mechanism as shown in Figure 1b. 7 Experimental evidence supports a concerted mechanism. 11 Skibo and coworkers used substituted quinazolinones as shown in Figure 1 as electronic probes and found electron withdrawing groups increase reaction rates, suggesting a negative charge build-up in the transition state, and that nucleophilic addition was rate determining. In addition, large deuterium isotope effects of up to 5.2 indicate that hydride transfer is also rate determining, consistent with a concerted mechanism. Without any other experimental data it is possible to conclude that the electronic effects are associated with an equi...

ABSTRACT:Aldehyde oxidase (AO) is a complex molybdo-flavoprotein that belongs to the xanthine oxidase family. AO is active as a homodimer, and each 150-kDa monomer binds two distinct [2Fe2S] clusters, FAD, and the molybdenum cofactor. AO has an important role in the metabolism of drugs based on its broad substrate specificity oxidizing aromatic aza-heterocycles, for example, was obtained with a purity of 95% and a yield of 50 g/l E. coli culture. Site-directed mutagenesis of the hAOX1 cDNA allowed the purification of protein variants bearing the amino acid changes R802C, R921H, N1135S, and H1297R, which correspond to some of the identified SNPs. The hAOX1 variants were purified and compared with the wild-type protein relative to activity, oligomerization state, and metal content. Our data show that the mutation of each amino acid residue has a variable impact on the ability of hAOX1 to metabolize selected substrates. Thus, the human population is characterized by the presence of functionally inactive hAOX1 allelic variants as well as variants encoding enzymes with different catalytic activities. Our results indicate that the presence of these allelic variants should be considered for the design of future drugs.