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PseH alone and in complex with AcCoA, to 1.95 A resolution. PseH folds into a single-domain structure of a central beta-sheet decorated by four alpha-helices with two continuously connected grooves. A deep groove accommodates the AcCoA molecule. The acetyl end of AcCoA points toward an open space in a neighboring shallow groove, which is occupied by extra electron density that potentially serves as a pseudosubstrate. PseH may utilize a catalytic mechanism of acetylation different from other glycosylation-associated acetyltransferases
in complex with cofactor acetyl-CoA, to 2.3 A resolution. PseH is a homodimer in the crystal, each subunit of which has a central twisted beta-sheet flanked by five alpha-helices. The cofactor-binding site is located between the splayed strands beta4 and beta5. The catalytic mechanism involves direct acetyl transfer from AcCoA without an acetylated enzyme intermediate. Modeling of the Michaelis complex suggests that the nucleotide- and 4-amino-4,6-dideoxy-beta-L-AltNAc-binding pockets form extensive interactions with the substrate and are thus the most significant determinants of substrate specificity. A hydrophobic pocket accommodating the 6'-methyl group of the altrose dictates preference to the methyl over the hydroxyl group