The X-ray crystal structure of the molecular complex of penicillin G with a deacylation-defective mutant of the RTEM-1 beta-lactamase from Escherichia coli shows how these antibiotics are recognized and destroyed. Penicillin G is covalently bound to Ser 70 0 gamma as an acyl-enzyme intermediate. The deduced catalytic mechanism uses Ser 70 0 gamma as the attacking nucleophile during acylation. Lys 73 N zeta acts as a general base in abstracting a proton from Ser 70 and transferring it to the thiazolidine ring nitrogen atom via Ser 130 0 gamma. Deacylation is accomplished by nucleophilic attack on the penicilloyl carbonyl carbon by a water molecule assisted by the general base, Glu 166.

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Crystals of turkey skeletal muscle troponin-C reveal a molecule of two domains with an unusual structure. Two Ca2+ ions are bound to the C-terminal domain. The two cation-binding sites of the regulatory (N-terminal) domain are Ca2+ free; this domain adopts a markedly different conformation from the C-terminal domain. The two domains are connected by a long nine-turn alpha-helix; three of these turns are exposed fully to solvent.

Prion infections cause lethal neurodegeneration. This process requires the cellular prion protein (PrP(C); ref. 1), which contains a globular domain hinged to a long amino-proximal flexible tail. Here we describe rapid neurotoxicity in mice and cerebellar organotypic cultured slices exposed to ligands targeting the 1 and 3 helices of the PrP(C) globular domain. Ligands included seven distinct monoclonal antibodies, monovalent Fab1 fragments and recombinant single-chain variable fragment miniantibodies. Similar to prion infections, the toxicity of globular domain ligands required neuronal PrP(C), was exacerbated by PrP(C) overexpression, was associated with calpain activation and was antagonized by calpain inhibitors. Neurodegeneration was accompanied by a burst of reactive oxygen species, and was suppressed by antioxidants. Furthermore, genetic ablation of the superoxide-producing enzyme NOX2 (also known as CYBB) protected mice from globular domain ligand toxicity. We also found that neurotoxicity was prevented by deletions of the octapeptide repeats within the flexible tail. These deletions did not appreciably compromise globular domain antibody binding, suggesting that the flexible tail is required to transmit toxic signals that originate from the globular domain and trigger oxidative stress and calpain activation. Supporting this view, various octapeptide ligands were not only innocuous to both cerebellar organotypic cultured slices and mice, but also prevented the toxicity of globular domain ligands while not interfering with their binding. We conclude that PrP(C) consists of two functionally distinct modules, with the globular domain and the flexible tail exerting regulatory and executive functions, respectively. Octapeptide ligands also prolonged the life of mice expressing the toxic PrP(C) mutant, PrP(Δ94-134), indicating that the flexible tail mediates toxicity in two distinct PrP(C)-related conditions. Flexible tail-mediated toxicity may conceivably play a role in further prion pathologies, such as familial Creutzfeldt-Jakob disease in humans bearing supernumerary octapeptides. (Fig. 1b). None of three high-affinity antibodies to the octapeptide repeats (OR, residues 50-90 embedded within the FT) were neurotoxic (Fig. 1b). Antibodies POM3 and D13, which bind the "charged cluster-2" 11 (CC2, residues 95-110), were innocuous at 67 nM but neurotoxic at 200 nM (Fig. 1b). None of the tested antibodies were toxic to Prnp o/o COCS ( Supplementary Fig. 2a). The identity of the targeted epitopes appeared to be a better predictor of PrP C antibody toxicity than their affinity to PrP C , suggesting that neurotoxicity resulted from the interaction of antibodies with specific PrP C domains (Supplementary Table 2).The mechanisms of neurotoxicity were further explored using POM1, a highly toxic antibody targeting the GD. Wild-type (wt) and tga20 COCS lost most granule cells (CGC) within 28 and 14 days post-exposure (dpe) to POM1, respectively (Fig. 2a-c). Controls included POM1-treated Prnp o/o COCS 12 , t...

Several serotypes of Escherichia coli produce protein toxins closely related to Shiga toxin (Stx) from Shigella dysenteriae serotype 1. These Stx-producing E. coli cause outbreaks of hemorrhagic colitis and hemolytic uremic syndrome in humans, with the latter being more likely if the E. coli produce Stx2 than if they only produce Stx1. To investigate the differences among the Stxs, which are all AB 5 toxins, the crystal structure of Stx2 from E. coli O157:H7 was determined at 1.8-Å resolution and compared with the known structure of Stx. Our major finding was that, in contrast to Stx, the active site of the A-subunit of Stx2 is accessible in the holotoxin, and a molecule of formic acid and a water molecule mimic the binding of the adenine base of the substrate. Further, the A-subunit adopts a different orientation with respect to the B-subunits in Stx2 than in Stx, due to interactions between the carboxyl termini of the B-subunits and neighboring regions of the A-subunit. Of the three types of receptor-binding sites in the B-pentamer, one has a different conformation in Stx2 than in Stx, and the carboxyl terminus of the A-subunit binds at another. Any of these structural differences might result in different mechanisms of action of the two toxins and the development of hemolytic uremic syndrome upon exposure to Stx2.

Carbohydrates are involved in many diverse biological functions including cell structural integrity, energy storage, pathogen defense and invasion mechanisms, viral penetration, and cellular signaling. Therefore, a large number of enzymes dedicated to carbohydrate metabolism have evolved. Enzymes specifically responsible for carbohydrate catabolism are collectively referred to as glycosyl hydrolases and have been classified into 77 families based on amino acid sequence similarity (1-3). Three-dimensional structures are known for representatives of 30 of the families. Although there are differences in chain length and domain structure between proteins of a single family, all proteins of a family hydrolyze the glycosidic bond with the same stereochemical outcome (4).Family 20 includes the ␤-N-acetylhexosaminidases (␤-hexosaminidases) 1 (EC 3.2.1.52), enzymes that catalyze the removal of terminal ␤-1,4 linked N-acetylhexosamine residues from the nonreducing ends of oligosaccharides and their conjugates. In humans, there are two major ␤-hexosaminidase isoforms: HexA and HexB. HexA is a heterodimer of subunits ␣ (encoded by HEXA) and ␤ (encoded by HEXB), whereas HexB is a homodimer of ␤ subunits. HexA is essential for degrading GalNAc-␤(1,4)-[N-acetylneuraminic acid (2,3)]-Gal-␤(1,4)-Glcceramide ganglioside; the biological importance of HexA activity is illustrated by the fatal neurodegenerative disorders that result from its heritable deficiency (5). Mutations in HEXA or HEXB cause Tay-Sachs and Sandhoff disease, respectively. These genetic diseases have made the human ␤-hexosaminidase isoforms the subject of much research. A substantial amount of genetic and biochemical information is available for these isozymes (5), but detailed information about their catalytic mechanism is limited. Mechanistic studies have been primarily limited by the difficulties in producing sufficient amounts of recombinant enzyme needed for kinetic analysis (6, 7); however, recent improvements in expression and purification procedures have allowed more accurate kinetic measurements to be made (8). Crystals of human HexB have been grown (9); however, attempts at solving its three-dimensional structure have not been successful. Nonetheless, much insight into the mechanism of human HexA and HexB has been provided by structural and functional studies carried out on related family 20 glycosyl hydrolases (10 -12).Stereochemical outcome studies on the family 20 chitobiase from Serratia marcescens (13) and human ␤-hexosaminidase (14) demonstrated that this family operates via a retaining * This work was supported in part by the

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