Models for molybdenum coordination during the catalytic cycle of periplasmic nitrate reductase from Paracoccus denitrificans derived from EPR and EXAFS spectroscopy. Biochemistry 1999 Jul 13;38(28):9000-12
Date
07/22/1999Pubmed ID
10413473DOI
10.1021/bi990402nScopus ID
2-s2.0-0033551524 (requires institutional sign-in at Scopus site) 91 CitationsAbstract
The periplasmic nitrate reductase from Paracoccus denitrificans is a soluble two-subunit enzyme which binds two hemes (c-type), a [4Fe-4S] center, and a bis molybdopterin guanine dinucleotide cofactor (bis-MGD). A catalytic cycle for this enzyme is presented based on a study of these redox centers using electron paramagnetic resonance (EPR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The Mo(V) EPR signal of resting NAP (High g [resting]) has g(av) = 1.9898 is rhombic, exhibits low anisotropy, and is split by two weakly interacting protons which are not solvent-exchangeable. Addition of exogenous ligands to this resting state (e.g., nitrate, nitrite, azide) did not change the form of the signal. A distinct form of the High g Mo(V) signal, which has slightly lower anisotropy and higher rhombicity, was trapped during turnover of nitrate and may represent a catalytically relevant Mo(V) intermediate (High g [nitrate]). Mo K-edge EXAFS analysis was undertaken on the ferricyanide oxidized enzyme, a reduced sample frozen within 10 min of dithionite addition, and a nitrate-reoxidized form of the enzyme. The oxidized enzyme was fitted best as a di-oxo Mo(VI) species with 5 sulfur ligands (4 at 2. 43 A and 1 at 2.82 A), and the reduced form was fitted best as a mono-oxo Mo(IV) species with 3 sulfur ligands at 2.35 A. The addition of nitrate to the reduced enzyme resulted in reoxidation to a di-oxo Mo(VI) species similar to the resting enzyme. Prolonged incubation of NAP with dithionite in the absence of nitrate (i.e., nonturnover conditions) resulted in the formation of a species with a Mo(V) EPR signal that is quite distinct from the High g family and which has a g(av) = 1.973 (Low g [unsplit]). This signal resembles those of the mono-MGD xanthine oxidase family and is proposed to arise from an inactive form of the nitrate reductase in which the Mo(V) form is only coordinated by the dithiolene of one MGD. In samples of NAP that had been reduced with dithionite, treated with azide or cyanide, and then reoxidized with ferricyanide, two Mo(V) signals were detected with g(av) elevated compared to the High g signals. Kinetic analysis demonstrated that azide and cyanide displayed competitive and noncompetitive inhibition, respectively. EXAFS analysis of azide-treated samples show improvement to the fit when two nitrogens are included in the molybdenum coordination sphere at 2.52 A, suggesting that azide binds directly to Mo(IV). Based on these spectroscopic and kinetic data, models for Mo coordination during turnover have been proposed.
Author List
Butler CS, Charnock JM, Bennett B, Sears HJ, Reilly AJ, Ferguson SJ, Garner CD, Lowe DJ, Thomson AJ, Berks BC, Richardson DJAuthor
Brian Bennett D.Phil. Professor and Chair in the Physics department at Marquette UniversityMESH terms used to index this publication - Major topics in bold
AzidesCatalysis
Cyanides
Electron Spin Resonance Spectroscopy
Kinetics
Models, Chemical
Molybdenum
Nitrate Reductase
Nitrate Reductases
Oxidation-Reduction
Paracoccus denitrificans
Periplasm
Potentiometry
Spectrometry, Fluorescence
X-Rays
