Protoheme structure

The simplest example is the water-soluble complex of poly(l-vinyl-2-methylimidazole) and iron-protoporphyrin IX [protoheme Structure (3)]. Protoheme itself is easily isolated from bovine and human hemoglobin and is commercially available [10]. The polymer ligand forms primarily the five-... 

The heme of catalases is deeply buried within the core of the catalase subunit. Protoheme IX or heme b is found in all small-subunit catalases so far characterized. The two large-subunit enzymes HPII and PVC have been characterized biochemically, spectrally, and structurally 91) as containing heme d in which ring HI is oxidized to a cis-hydroxyspirolactone. Heme b is initially bound to both enzymes during assembly, and it is subsequently oxidized by the catalase itself during the early rounds of catalysis 92). 

The work of Gadher et al and the awareness of the diversity of the biological activity of FBI fungicides suggests that the structure of the fungitoxicant must satisfy more than one requirement. The ability to bind to the protoheme iron of cytochrome P-450 appears to be a required function for activity. The fitness to bind could be dependent upon several facets such as ... 

Fig. 2. Structure of some porphyrin-iron complexes. Protoheme IX (Proto) R = —CH=CH2 Deuteroheme IX (Deut) R = —H Mesoheme IX (Meso) R =...

Blumberg WE, Peisach J, Wittenberg BA et al (1968) The electronic structure of protoheme proteins. I. An electron paramagnetic resonance and optical study of horseradish peroxidase and its derivatives. J Biol Chem 243 1854-1862... 

One of the variables in the structures of the porphyrins present in heme proteins is the presence or absence of vinyl substituents on the periphery of the macrocycle. For example, b hemes have vinyl substituents whereas c hemes do not. Because of the sensitivity of such vinyl substituents during synthetic transformations, it has often been desirable to use octa-alkyl porphyrins in model studies of the spectroscopic properties of heme systems. The development of improved methods for the preparation of octa-alkyl porphyrins has likewise increased the availability of such porphyrins for model studies (20, 21). To assess the effect that replacement of the two vinyl substituents in protoporphyrin IX with alkyl (ethyl) groups has on the MCD properties of the heme system, an extensive and systematic study of the MCD properties of mesoheme IX-reconstituted myoglobin and horseradish peroxidase in comparison with the spectra of the native protoheme-bound proteins has been carried out (22). The structures of these two porphyrins are shown in Figure 3. 

Figure 3. Structures of iron protoporphyrin IX (protoheme IX) (left) and iron mesoporphyrin IX (mesoheme IX) (right).

Of the two catalases found in E. coli, one has a protoheme prosthetic group whereas the other, the HPII catalase, contains an iron chlorin prosthetic group, the proposed structure of which is displayed in Figure 5 (24). Although the nature of the prosthetic group has been established,... 

Figure 1 Three-dimensional structures of myoglobin and hemoglobin. (a) Deoxymyoglobin from sperm whale (1A6N) and (b) deoxy-hemoglobin from human (IBZO). Black and gray stick molecules are protoheme-lX. Hemoglobm consists of two af dimers, dark and light gray pairs...

Figure 1. Structure of protoheme IX (iron protoporphyrin IX, heme b).

Fig 1. Structure of the flavocytochrome prosthetic groups—protoheme IX and flavin mononucleotide. 

Fig. 5. Schematic representation of the structure of a flavocytochrome 62 subunit. A, The heme domain B, the flavodehydrogenase domain C, the C-terminal tail D, the hinge region linking the two domains E, the proteolytically sensitive loop F, flavin mononucleotide G, protoheme IX.

Fig. 5. Model for the protein binding site for protoheme in cytochrome b559 (A) and modei for the orientation of the two histidine imidazole rings in cytochrome bS59 (B). See text for discussion. Figures adapted from Babcock, Widger, Cramer, Oertling and Metz (1985) Axial ligands of chloroplast cytochrome b559 identification and requirement for a heme-cross-linked polypeptide structure. Biochemistry 24 3643.

The terminal oxidase in an energy-transducing, cytochrome-based electron-transport system maintains electron flow by coupling cytochrome oxidation to dioxygen (O2) reduction. Members of this protein class are referred to as cytochrome oxidases they carry out Oj-binding and redox chemistry at transition metal-containing active sites. Although iron is the most commonly used metal and may occur as a protoheme or iron-chlorin species in the protein, this section is concerned only with mitochondrial cytochrome oxidase, which contains 2 mol of Cn and 2 mol of heme a bound Fe per function unit. Biochemistry of the protein will not be considered here, instead the focus will be on the structure of the metal centers, on the reactions they catalyze and on models for these centers. 

Many type b cytochromes associated with the classic cytochrome oxidase (a and 03) show their a peaks at 561-563 nm, and they are usually designated as cytochrome b. The name cytochrome 61 was originally used for the pigments with a peak at 557-560 nm, but it is now generally applied to the cytochrome in the nitrate reductase system. Cytochrome 62 (a, 557 nm) is the entity of yeast lactate dehydrogenase which contains FMN and protoheme. Cytochromes 63 (a, 559 nm) and 65 (a, 556 nm) participate in the microsomal electron transport system in plants and animals, respectively. Both the primary and ternary structures of cytochrome 65 are known. Cytochromes bg (a, 563 nm) and 6-559 are the components of the photosynthetic electron transport system in plant. 

Fig. 6.16. Structures of the iron (II) hemes complexes protoheme dimethyl ester (PDME) and monochelated protoheme (MCPH).

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