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1.  Structure of a bacterial putative acetyltransferase defines the fold of the human O-GlcNAcase C-terminal domain 
Open Biology  2013;3(10):130021.
The dynamic modification of proteins by O-linked N-acetylglucosamine (O-GlcNAc) is an essential posttranslational modification present in higher eukaryotes. Removal of O-GlcNAc is catalysed by O-GlcNAcase, a multi-domain enzyme that has been reported to be bifunctional, possessing both glycoside hydrolase and histone acetyltransferase (AT) activity. Insights into the mechanism, protein substrate recognition and inhibition of the hydrolase domain of human OGA (hOGA) have been obtained via the use of the structures of bacterial homologues. However, the molecular basis of AT activity of OGA, which has only been reported in vitro, is not presently understood. Here, we describe the crystal structure of a putative acetyltransferase (OgpAT) that we identified in the genome of the marine bacterium Oceanicola granulosus, showing homology to the hOGA C-terminal AT domain (hOGA-AT). The structure of OgpAT in complex with acetyl coenzyme A (AcCoA) reveals that, by homology modelling, hOGA-AT adopts a variant AT fold with a unique loop creating a deep tunnel. The structures, together with mutagenesis and surface plasmon resonance data, reveal that while the bacterial OgpAT binds AcCoA, the hOGA-AT does not, as explained by the lack of key residues normally required to bind AcCoA. Thus, the C-terminal domain of hOGA is a catalytically incompetent ‘pseudo’-AT.
PMCID: PMC3814719  PMID: 24088714
signalling; O-GlcNAc; glycobiology; protein structure
2.  Structural and biochemical characterization of a trapped coenzyme A adduct of Caenorhabditis elegans glucosamine-6-phosphate N-acetyltransferase 1 
Glucosamine-6-phosphate N-acetyltransferase is an essential enzyme of the eukaryotic UDP-GlcNAc biosynthetic pathway. A crystal structure at 1.55 Å resolution revealed a highly unusual covalent product complex and biochemical studies investigated the function of a fully conserved active-site cysteine.
Glucosamine-6-phosphate N-acetyltransferase 1 (GNA1) produces GlcNAc-6-phosphate from GlcN-6-phosphate and acetyl coenzyme A. Early mercury-labelling experiments implicated a conserved cysteine in the reaction mechanism, whereas recent structural data appear to support a mechanism in which this cysteine plays no role. Here, two crystal structures of Caenorhabditis elegans GNA1 are reported, revealing an unusual covalent complex between this cysteine and the coenzyme A product. Mass-spectrometric and reduction studies showed that this inactive covalent complex can be reactivated through reduction, yet mutagenesis of the cysteine supports a previously reported bi-bi mechanism. The data unify the apparently contradictory earlier reports on the role of a cysteine in the GNA1 active site.
PMCID: PMC3413214  PMID: 22868768
carbohydrates; glycobiology; Caenorhabditis elegans; glucosamine-6-phosphate N-acetyltransferase; coenzyme A adduct; mechanism
3.  Bisdionin C—A Rationally Designed, Submicromolar Inhibitor of Family 18 Chitinases 
ACS Medicinal Chemistry Letters  2011;2(6):428-432.
Chitinases of the GH18 family play important roles in a variety of pathogenic organisms and have also been shown to be involved in human asthma progression, making these enzymes potential drug targets. While a number of potent GH18 chitinase inhibitors have been described, in general, these compounds suffer from limited synthetic accessibility or unfavorable medicinal-chemical properties, making them poor starting points for the development of chitinase-targeted drugs. Exploiting available structural data, we have rationally designed bisdionin C, a submicromolar inhibitor of GH18 enzymes, that possesses desirable druglike properties and tractable chemical synthesis. A crystallographic structure of a chitinase-bisdionin C complex shows the two aromatic systems of the ligand interacting with two conserved tryptophan residues exposed in the active site cleft of the enzyme, while at the same time forming extensive hydrogen-bonding interactions with the catalytic machinery. The observed mode of binding, together with inhibition data, suggests that bisdionin C presents an attractive starting point for the development of specific inhibitors of bacterial-type, but not plant-type, GH 18 chitinases.
PMCID: PMC4018065  PMID: 24900325
GH18 Chitinase; xanthine; ligand design

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