Structure of the Heme Prosthetic Group

Figure 14-2 Structures of the heme prosthetic groups of the cytochromes. In all cytochromes the heme Fe undergoes reversible oxidation and reduction between Fe2+ and Fe5+.

Fig. 1. Space filling model of yeast iso-1-cytochrome c. The edge of the heme prosthetic group is visible as a black linear structure in the center of the protein. Phe-82 is shaded a dark gray at the left upper side of the heme group...

It is quite evident that the ferrous complexes of porphyrins, both natural and synthetic, have extremely high affinities towards NO. A series of iron (II) porphyrin nitrosyls have been synthesized and their structural data [11, 27] revealed non-axial symmetry and the bent form of the Fe-N=0 moiety [112-116]. It has been found that the structure of the Fe-N-O unit in model porphyrin complexes is different from those observed in heme proteins [117]. The heme prosthetic group is chemically very similar, hence the conformational diversity was thought to arise from the steric and electronic interaction of NO with the protein residue. In order to resolve this issue femtosecond infrared polarization spectroscopy was used [118]. The results also provided evidence for the first time that a significant fraction (35%) of NO recombines with the heme-Fe(II) within the first 5 ps after the photolysis, making myoglobin an efficient N O scavenger. 

Perhaps the most fundamental fimctional property of a heme prosthetic group at the active site of a heme protein is the relative stability of the reduced and oxidized states of the heme iron. A number of structural characteristics of the heme binding environment provided by the apo-protein have been identified as contributing to the regulation of this equilibrium and have been reviewed elsewhere 82-84). Although a comprehensive discussion of these factors is not possible in the space available here, they can be summarized briefly. The two most significant influences of the reduction potential of the heme iron appear to be the dielectric constant of the heme environment 81, 83) and the chemical... 

Myoglobin was the first protein to have its three-dimensional structure solved by X-ray crystallography. It is a globular protein made up of a single polypeptide chain of 153 amino acid residues that is folded into eight a-helices. The heme prosthetic group is located within a hydrophobic cleft of the folded polypeptide chain. 

B. Redesign of Nonheme Iron Proteins. In heme protein redesign described above, the heme prosthetic group largely dictates the active site structure. Redesign focuses mainly on the proximal and distal sides of the heme, causing minimal effects on the overall protein scaffolds. This is not necessarily the case for nonheme metalloproteins in which metal sites are not as dominant and small changes may have more dramatic effects on the protein folds and stability. 

Porphyrins and their derivatives play critical roles in many biological functions. Some of the most remarkable examples are protoporphyrin IX and its iron complex that constitutes the heme prosthetic group, and the magnesium complexes of pheophytin a and bacteriopheophytin a that are known as chlorophyll a and bacteriochlorophyll a, respectively. These natural compounds are illustrated in Fig. 1 together with the structure of porphin,... 

Each monomer of COX consists of three structural domains a short N-terminal epidermal growth factor domain, a membranebinding domain, and a large, globular C-terminal catalytic domain (Fig. la) (3). The COX and POX active sites are located on opposite sides of the catalytic domain with the heme prosthetic group positioned at the base of the peroxidase... 

B enzyne and Cyclobutadlene Stereoelectronlc Activation - The catalytic interaction of cytochrome P-450 with 1-aminobenzotriazole (2), a compound that releases benzyne when chemically oxidized, results in time-dependent enzyme inactivation. 2,63 An unusual prosthetic heme adduct is formed in which two vicinal nitrogens of protoporphyrin IX are bridged by an ortho-substituted phenyl ring. The Inactivation reaction occurs even when substituents are placed on the exocyclic amino group of 1-amlnobenzo-triazole, although enzyme inactivation by substituted analogues has not been well characterized. The catalytic role of the enzyme and the structure of the heme adduct indicate that inactivation follows oxidative release of benzyne or a benzyne-like species within the active site of the enzyme. 

Independent evidence that olefin oxidation can proceed via a nonconcerted mechanism is provided by the fact that terminal olefins are not only oxidized to epoxides but, in many cases, simultaneously alkylate the P450 prosthetic heme group by covalently binding to one of its pyrrole nitrogen atoms (Fig. 4.29) [180]. It should be noted, however, that this heme alkylation process is relatively infrequent, with ratios of epoxidation to heme alkylation usually greater than 200. Despite the structures of the heme adducts, which nominally could arise by nucleophilic attack of the pyrrole nitrogen on the epoxide, epoxides are not involved in heme alkylation. This was definitely established by the fact that the synthetic epoxides do not react with the heme [181], and... 

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