Thiolate-Ligated Heme Systems

Cytochrome P-450 and chloroperoxidase have catalytic and spectral properties that differ significantly from those of other heme systems. As the current state of knowledge of these two proteins has been extensively reviewed [20-29], we focus here on the structural origin of their unique spectral properties and the correlation of these properties with the novel catalytic activities of the enzymes. As will be shown, both enzymes are now well established to be thiolate-ligated heme systems. 

There have been a number of recent review articles on the application of EXAFS spectroscopy to the study of metalloproteins [1-8]. The theory of EXAFS spectroscopy and the historical development of the field have also been extensively discussed [1-5,7-19]. Here we will briefly cover the practical aspects of data analysis for biological heme (iron porphyrin) systems eind the appropriate model complexes. We will then focus on the EXAFS of two types of biological heme system (a) thiolate-ligated heme enzymes and (b) oxo-ferryl [oxo-iron (IV), Fe =0] states of heme enzymes. All of the enzymes discussed herein have in common the iron protoporphyrin IX ( heme ) as the prosthetic group, Fig. 1. 

The study of model compounds or simply, models of heme proteins is very helpful in elucidating structure-function relationships. Models are compounds with low molecular weight that imitate struetural, spectroscopic, or functional details of the original enzymes. The latter are macromolecules and hence more difficult to study. Synthetic models for states 7-5 must be thiolate-ligated. Such models have been prepared and extensively characterized. The models from several laboratories have recently been reviewed [22]. A model system having a ferric-peroxide composition, as is present in 6, has also been described [40]. Relevant models are listed in the tables (see Sects. 3.1 and 4.1.1). Model chemistry has been extremely important in characterizing these intermediates. 

Spectral similarities between P-450 and chloroperoxidase originally led to suggestions that both enzymes had thiolate ligation [20, 22, 42]. However, the two systems displayed clear differences in their catalytic activities. Furthermore, at the time when EXAFS studies of chloroperoxidase were initiated, it was not clear whether the enzyme had a free (non-disulfide linked) cysteine available to coordinate to the heme iron [100]. Also, the unusually low pH optimum of the chloroperoxidase halogenation reaction, pH 3.0 for peroxidative formation of a carbon-halogen bond [42], raised questions concerning possible protonation of the axial heme ligand(s). 

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