Two distinct catalytic pathways for GH43 xylanolytic enzymes unveiled by X-ray and QM/MM simulations

Xylanolytic enzymes from glycoside hydrolase family 43 (GH43) are involved in the breakdown of hemicellulose, the second most abundant carbohydrate in plants. Here, we kinetically and mechanistically describe the non-reducing-end xylose-releasing exo-oligoxylanase activity and report the crystal structure of a native GH43 Michaelis complex with its substrate prior to hydrolysis. Two distinct calcium-stabilized conformations of the active site xylosyl unit are found, suggesting two alternative catalytic routes. These results are confirmed by QM/MM simulations that unveil the complete hydrolysis mechanism and identify two possible reaction pathways, involving different transition state conformations for the cleavage of xylooligosaccharides. Such catalytic conformational promiscuity in glycosidases is related to the open architecture of the active site and thus might be extended to other exo-acting enzymes. These findings expand the current general model of catalytic mechanism of glycosidases, a main reaction in nature, and impact on our understanding about their interaction with substrates and inhibitors

The Roots of Educational Inequality: Germantown High School, 1907-2011

The Roots of Educational Inequality: Germantown High School, 1907 - 2011 Erika M. Kitzmiller Dr. Michael B. Katz (History) Dr. Stanton E.F. Wortham (Education) This study, The Roots of Educational Inequality, examines the political, economic, and social factors that led to the transformation of Germantown High School and its urban community throughout the twentieth century. This longitudinal study, accomplished through a careful analysis of daily events rather than sampling key turning points, maximizes the benefits of a case study approach by connecting local conditions to the larger transformation of urban schools, urban communities, and the social welfare state. Using a variety of quantitative and qualitative methods and source materials, this dissertation links the school\u27s history to the community and city\u27s history to demonstrate how the influx of working class residents, the escalation of residential segregation, and the failures of urban renewal efforts affected the high school. This dissertation suggests that white flight, alone, did not lead to the school\u27s transformation. Rather the deterioration of this American high school is connected, at least in part, to the dramatic decrease in the levels of private funding that residents contributed to the high school and charitable organizations during the twentieth century. The availability of charitable funding supplemented government aid and enhanced the opportunities and support available to Germantown youth--this ensured the high school\u27s early success and legitimacy. As the demographics of the community changed, this funding dwindled and the infrastructure that had supported the high school and its youth quickly deteriorated. By tracing this history over the course of entire century from the school\u27s glorious promises to its current challenges, this dissertation provides a fresh understanding about the transformation of American public high schools, urban communities, and the social welfare state over the past 100 years

Cryptographic key distribution in wireless sensor networks: a hardware perspective

In this work the suitability of different methods of symmetric key distribution for application in wireless sensor networks are discussed. Each method is considered in terms of its security implications for the network. It is concluded that an asymmetric scheme is the optimum choice for key distribution. In particular, Identity-Based Cryptography (IBC) is proposed as the most suitable of the various asymmetric approaches. A protocol for key distribution using identity based Non-Interactive Key Distribution Scheme (NIKDS) and Identity-Based Signature (IBS) scheme is presented. The protocol is analysed on the ARM920T processor and measurements were taken for the run time and energy of its components parts. It was found that the Tate pairing component of the NIKDS consumes significants amounts of energy, and so it should be ported to hardware. An accelerator was implemented in 65nm Complementary Metal Oxide Silicon (CMOS) technology and area, timing and energy figures have been obtained for the design. Initial results indicate that a hardware implementation of IBC would meet the strict energy constraint of a wireless sensor network node