Active site electronic structure sulfite oxidase

The initial contribution to this volume provides a detailed overview of how spectroscopy and computations have been used in concert to probe the canonical members of each pyranopterin Mo enzyme family, as well as the pyranopterin dithiolene ligand itself. The discussion focuses on how a combination of enzyme geometric structure, spectroscopy and biochemical data have been used to arrive at an understanding of electronic structure contributions to reactivity in all of the major pyranopterin Mo enzyme families. A unique aspect of this discussion is that spectroscopic studies on relevant small molecule model compounds have been melded with analogous studies on the enzyme systems to arrive at a sophisticated description of active site electronic structure. As the field moves forward, it will become increasingly important to understand the structure, function and reaction mechanisms for the numerous non-canonical [ie. beyond sulfite oxidase, xanthine oxidase, DMSO reductase) pyranopterin Mo enzymes. 

The photoelectron spectra of model complexes for these molybdenum enzyme active sites have been investigated to gain a better understanding of their basic electronic structure and the role of the ene-dithiolate hgand. For example, the metal coordination of model complexes such as Tp MoO(tdt) are similar to the molybdenum center of sulfite oxidase, which possesses the basic structural core of a terminal oxo group cis to a 1,2-dithiolate. 

Mo(Tp )(E)(bdt)j, [Mo(Tp )(E)(tdt)], [Mo(Tp )(E)(bdtCl2)]82 83 (E = O, NO), and [MoO(qdt)(Tp )]84 have been investigated as models for various pyranopterin Mo enzyme active sites, including sulfite oxidase. Solution redox potentials and heterogeneous electron transfer rate constants for these species have been also reported.85 The interactions between the sulfur tt-orbitals of arene dithiolates and high-valent Mo in [MoO(Tp )(bdt)] have been investigated by gas-phase photoelectron spectroscopy and DFT methods in order to understand the properties of the active site of pyranopterin Mo-W enzymes.86 Temperature-dependent measurements of potential and electron-transfer rate constants are also reported for electrochemical reduction of a series of [MoO(Tp )(X,Y)] complexes.87 The molecular and electronic structures of the SO active site [MoO(Tp )(bdt)] have been also reported.88... 

Figure 4.18 EXAFS Fourier transforms of the sulfite oxidase iodide complex. Transforms are Mo-S phase-corrected, with the solid line showing experimental data and the broken line the best fit. The weak EXAFS from the distant iodide can be seen as the small transform peak at 5 A. The DFT-computed active site structure, calculated using constraints based on the crystal structure, is shown in the inset with a computed total electron density iso-surface with a value of 0.02 electrons per cubic a.u.

Since mitochondrial cytochrome c was available commercially (horse heart muscle being the most common source) and could readily be purified to a high level, it formed the basic subject for most of the pioneering studies. Many ideas concerning the electrochemical mechanism, in particular, the mode of interaction with the electrode, have developed around the considerable wealth of information that is available [14, 18] on the structure and properties of the protein molecule. The extent to which the metal centre is buried is illustrated well in Fig. 1 which shows the 3D structure [19] of yeast (iso-1) cytochrome c and a view of the exposed active site. The major function of cytochrome c is as electron donor to cytochrome c oxidase (Complex IV), the membrane-bound enzyme that is the terminus of the aerobic respiratory chain and a site for proton translocation. Another physiological oxidant of cytochrome c (in yeasts) is cytochrome c peroxidase, a soluble enzyme whose crystal structure is known (see Sect. 7). The most important reduc-tant of cytochrome c is the cytochrome Cj component of the membrane-bound hcj complex (Complex III), but others (see Sect. 6, Scheme 5) include cytochrome b, sulfite oxidase, and flavocytochrome (lactate dehydrogenase, found in yeasts). 

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