Heme proteins, catalytic properties

Compared with the heme proteins discussed in Section 2.2, the non-heme iron proteins presented here have a much more flexible coordination geometry. Taken together with the differences in electronic properties - heme enzymes contain mostly low-spin iron whereas non-heme enzymes contain mostly a high-spin iron - this is responsible for the more diverse chemistry found for the non-heme iron proteins. The great versatility of these enzymes makes them a treasure trove for the development of iron-based catalysts. Inspired by their biological archetypes, numerous catalytic reactions await to be reproduced by iron catalysts in organic synthesis. 

Zederbauer M, Furtmuller PG, Brogioni S et al (2007) Heme to protein linkages in mammalian peroxidases impact on spectroscopic, redox and catalytic properties. Nat Prod Rep 24 571-584... 

MPO is a covalently linked dimer which is ellipsoidal in shape with overall dimensions of 110 x 60 x 50 A. The dimer can be cleaved by reduction of a disulfide bond into two identical halves. Each half of the dimer termed hemi-MPO has the same optical and catalytic properties of the dimer. Hemi-MPO consists of two polypeptides of466 and 108 amino acid residues, and a heme prosthetic group covalently bound to the large polypeptide. Like CcP and LIP, MPO is largely a helical bundle protein with very little -sheet stmcture. The bulk of the large polypeptide folds into five separate domains and one... 

Little is known of the catalytic properties of cytochrome b investigators must rely on the spectrum characterization of cytochrome b rather than on its enzymic activity to evaluate the purity of the preparation. Little is known of the heme group of mammalian cytochrome b. It is significant that cytochrome b extracted from Micrococcus denitrificans and Pseudomonas denitrificans contains protoheme, the same heme as is in cytochrome c. The nature of the binding between the prosthetic group and the protein is not known. The firm thioester bond in cytochrome c probably is not present in cytochrome b. 

In the hemoproteins, the central ion is iron. In protoporphyrins, central ion and ligand form the heme. The same heme is inserted into different protein molecules, yielding different molecular complexes with different catalytic properties. For example, cytochromes facilitate electron transfer, catalase converts H2O2 into H2O, and hemoglobin stores and transports oxygen. The same chelates (central ion and ligand) can perform... 

C. Comparison op Certain Catalytic Properties op Heme Proteins... 

This net transformation, however, encompasses a more complex mechanism that can be visualized in the generic catalytic cycle shown schematically in Fig. 5.2. Compounds I and II, the critical catalytic intermediates, are readily distinguished from the resting ferric state of the protein by their UV-visible absorption spectra (Table 5.1) [1-4]. Although the exact positions of the maxima show small variations, the spectroscopic properties of HRP are representative of those of all the peroxidases in which the heme iron atom is coordinated to a histidine nitrogen atom. The individual stages of the catalytic cycle are considered below. 

The last systematic description of heme peroxidases was published in 1999 by Brian Dunford, from the University of Alberta in Canada. The book Heme peroxidases covers discussion on three-dimensional structure, reaction mechanism, kinetics, and spectral properties of representative enzymes from bacterial, plant, fungal, and animal origin. Since 1999, vast information on basic but also applied aspects of heme peroxidases has been generated. We believe fusion of these two aspects will benefit research of those dedicated to development of biocatalytic process. The aim of this book is to present recent advances on basic aspects such as evolution, structure-function relation, and catalytic mechanism, as well as applied aspects, such as bioreactor and protein engineering, in order to provide the tools for rational design of enhanced biocatalysts and biocatalytic processes. The book does not include an exhaustive listing of references but rather a selected collection to enrich discussion and to allow envisioning future directions for research. 

The crystal structure of canine MPO has recently been determined to 3-A resolution (7, 14). This mammalian enzyme is a covalently linked dimer of molecular weight 140 kDa that can be cleaved into two identical halves by reduction of a single disulfide bond. Each half of the dimer, termed hemi-MPO, has the same optical properties and catalytic activity as the parent. Hemi-MPO consists of two polypeptides of 466 and 108 amino acid residues, and a heme-type prosthetic group is covalently bound to the larger polypeptide in a crevice 15 A below the protein surface. Like CCP and LIP, the secondary structure of MPO is dominated by a-helices with relatively little /3-sheet structure (Fig. 

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