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The biological reduction of dinitrogen (N2) to ammonia is catalyzed by the complex metalloenzyme nitrogenase. Structures of the nitrogenase component proteins, Iron (Fe) protein and Molybdenum-iron (MoFe) protein, and the stabilized complexes these component proteins, have been determined, providing a foundation for a number of fundamental aspects of the complicated catalytic mechanism. The reduction of dinitrogen to ammonia is a complex process that involves the binding of N2followed by reduction with multiple electrons and protons. Electron transfer into nitrogenase is typically constrained to the unique electron donor, the Fe protein. These constraints have prevented structural characterization of the active site with bound substrate. Recently it has been realized that selected amino acid substitutions in the environment of the active site metal cluster (Iron-Mo molybdenum cofactor, FeMo-co) allow substrates to persist even in the resting state. Reported here is a 1.70 Ã crystal structure of a nitrogenase MoFe protein alfa-96Arg -> Gln variant with the alternative substrate acetylene trapped in a channel in close proximity to FeMo-co. Complementary theoretical calculations support the validity of the acetylene interaction at this site and is also consistent with more favorable interactions in the variant MoFe protein compared to the native MoFe protein. This work represents the first structural evidence of a substrate trapped in the nitrogenase MoFe protein and is consistent with earlier assignments of proposed substrate pathways and substrate binding sites deduced from biochemical, spectroscopic, and theoretical studies.

Web-based learning management systems hold considerable promise in supporting approaches to teaching that are well-informed, both practically and theoretically. Unfortunately, this promise has often remained unrealised, especially in higher education contexts. Various reasons may be proffered including: monolithic approaches to system development, mismatches between software and pedagogic models, or attenuated communication between system developers and practitioners. This paper examines one case study involving the concurrent development (simultaneous deployment and refinement) of a general on-line collaborative tool for students and the co- development of an instructional approach based on authentic constructivist principles. This case study is used to abstract some tentative principles that relate to the effective development of learning management systems. These principles include: * Some pedagogical designs, while productive in terms of student learning, rely on complex interactions that are difficult to support technically. * It is possible to set up conditions for a productive participant-developer dialogue in order to co-inform technical and pedagogical design. * Non-traditional methods of system development are appropriate in order to support innovation, especially if measures are taken to handle the risks involved. The results should be of interest to teachers in higher education, administrators and developers of web-based learning management systems.