Crystallography metalloproteins

Kenneth Suslick has written several pertinent statements about the study of metalloproteins and their model compounds. This paradigm will guide the study of the varied bioinorganic systems described in this chapter. Additions to Suslick s statement in point 2 stress the importance of modern structural analysis (X-ray crystallography, EXAFS, EPR, and two- and three-dimensional high-held NMR) of the purihed metalloproteins and their model compounds. As paraphrased from reference 19, the following statements outline the steps involved in the study of metalloproteins ... 

Table I. Selected Metalloprotein Metal Coordination Site Structure Based on X-Ray Crystallography...

Armstrong, W. H. (1988). Metalloprotein crystallography. Survey of recent results and relationships to model studies. ACS Symp. Ser. 372, 1-27. 

Hemocyanin [30,31], tyrosinase [32] and catechol oxidase (2) [33] comprise this class of proteins. Their active sites are very similar and contain a dicopper core in which both Cu ions are ligated by three N-bound histidine residues. All three proteins are capable of binding dioxygen reversibly at ambient conditions. However, whereas hemocyanin is responsible for O2 transport in certain mollusks and arthropods, catechol oxidase and tyrosinase are enzymes that have vital catalytic functions in a variety of natural systems, namely the oxidation of phenolic substrates to catechols (Scheme 1) (tyrosinase) and the oxidation of catechols to o-quinones (tyrosinase and catechol oxidase). Antiferromagnetic coupling of the two Cu ions in the oxy state of these metalloproteins leads to ESR-silent behavior. Structural insight from X-ray crystallography is now available for all three enzymes, but details... 

Stellacyanin, the plastocyanins, and the azurins are the most widely studied copper-containing metalloproteins of the next active-site class, the Blue Copper sites. These proteins, which generally appear to be involved in redox chemistry, have quite unique spectral features32,33). The potential for complementary interaction between inorganic spectroscopy and protein crystallography is well demonstrated by the roles that they have played in generating fairly detailed geometric and electronic structural pictures of the Blue Copper metal centers. 

The metalloproteins that have attracted most attention are those whose properties are most obviously different from those observed in the normal classical aqueous coordination chemistry of the metal ions. The challenge is to account for (initially) unique spectral or chemical properties in terms of the coordination chemistry of the metalloprotein active site, as moderated by the protein environment. With increasing frequency, as in the case of type 1 copper (Section IIIB), crystallography reveals the active site structure with sufficient clarity to provide strong clues as to the origin of the unusual spectroscopy. However, an important test of the structural and spectroscopic analyses is to reproduce the same effects in a model complex. On other occasions, as with the [4Fe-4S] proteins (Section VC), many questions remained even after the structures were known. In spite of the very impressive achievements of protein crystallography, there remain many metalloproteins for which structural data are either not available or inconclusive. 

The principal impetus to the study of zinc thiolates has undoubtedly been the search for structural models for the metal coordination in zinc metalloproteins such as the Zn(S-Cys)2(His)2 center in the transcription factor IIIA and other zinc fingers that feature in protein-DNA interactions (93, 94). The 2 1 complexes [M(SR)2l (M = Zn, Cd SR = TIPT, TEMT) were used as the principle precursors for the synthesis of such models for the Zn(S-Cys)2(His)2. Thus complexes of the types [Cd(TIPT)2(l-CH3-imid)2], [Zn(TIPT)2(bipy)], and [Zn(TEMT)2(l-CH3-imid)2l (imid = imidazole, bipy = bipyridine) were ssmthesized by addition of a nitrogeneous base to the precursor in MeCN. All of them have been characterized by X-ray crystallography and were found to have distorted tetrahedral geometries. It was noticed that the cobalt analogue binds a solvent molecule MeCN to form five-coordinate [Co(TIPT)2(bipy)(CH3CN)], whereas [Zn(TIPT)2(bipy)] does not 96,97). 

X-ray crystallography for detection of metalloproteins accuracy problems, 66 determination of geometric parameters DFT-the Hohenberg-Kohn theorem approach, 67 QM/S techniques, 67 QSOR approach, Karplus relationship, 67 refinement of, linear-scaling quantum chemical methods, 66-67... 

Albrecht Messerschmidt is the editor of the three volumes of the Handbook of Metalloproteins published by Wiley, Chichester editor of Multicopper Oxidases from World Scientific Publishing, Singapore and the author of the textbook X-Ray Crystallography of Biomacromolecules puhYisheA by Wiley-VCH, Weinheim. 

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