XAS STUDIES OF NITRATE REDUCTASE

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Nitrate reductases (NRs) are mononuclear molybdenum enzymes coordinated by distinctive pyranopterin cofactors. The structurally characterized NRs have diverse sites, all of which accomplish the same function. Four distinct types of active site structures are predicted by sequence analysis. However, these need to be verified by biophysical methods. XAS has been used previously to characterize assimilatory NR from Arabidopsis Thaliana. This suggested an unexpected change of Mo-S from trans to cis relative to the Mo-O during the catalytic turnover. EXAFS results have also been reported for the penplasmic NR from Paracocus pantotrophus and P. denitrificans. These three different NR sites only share a common Mo=O. Given this demonstrated variability, XAS studies on the NR from S. Barnesii, which belongs to a different branch of the phylogenetic tree, are important. We will study the enzyme in the as-isolated state, in the nitrate oxidized state, in the fully reduced state, and in the one-electron reduced Mo(V) state.