Structure of the Prosthetic Group

IRON-CONTAINING OXYGEN CARRIERS AND THEIR SYNTHETIC MODELS 

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Koda, P., and Lee, J. (1979). Separation and structure of the prosthetic group of the blue fluorescence protein from the bioluminescent bacterium Photobacterium phosphoreum. Proc. Natl. Acad. Sci. USA 76 3068-3072. 

A. Gottschalk, The structure of the prosthetic group of bovine submaxillary gland mucopro-tein, Biochim. Biophys. Acta, 24 (1957) 649-650. 

Fig. 15. Structure of the prosthetic group in (a) rhodopsin, where the retinal Schiff base is in an 1 l-co,6-j-cir-l2-s-ew conformation (b) delocalized excited singlet state (c) bathorhodopsin. where the chromophore is a hexaene amine-imidazole complex. From van der Meer et al. [127],...

The probable axial ligands of common haemoproteins are given in Table 2. Even in cytochromes, where the protein may occupy both axial positions, one of the axial ligands can be replaced by anions. The effects of changing the axial field in this way are useful in probing the electronic structure of the prosthetic group. 

Vanadium nitrogenase is produced by certain bacteria grown in molybdenum-deficient environments. It is effective in the reduction of N2 and other nitrogenase substrates, although with less activity than the Mo—Nase. The enzyme resembles the Mo analogue (see Sections 17-E-10 and 18-C-13) in the construction and structure of the prosthetic groups, as well as in its functions.101 It consists of a FeV protein, FeVco, and an iron protein (a 4Fe—4S ferredoxin). 

In order to estabHsh the relationship between the structure of the prosthetic group and its reactivity toward NO in various hemoproteins in vivo, many efforts have been devoted to reveal the influence of the porphyrin microenvironment in the iron(III) porphyrin systems (i.e., the identity and charge of the substituents in the porphyrin periphery) on the dynamics of both the binding and release of nitric oxide. In this context, several new water-soluble iron(III) porphyrin models, i.e., a highly negatively charged ((P )Fe = [5", 115", 20" -tetra-ief i-butyl-5, 5 , 15, 15 -tetrakis-(2,2-biscarboxylato-ethyl)-... 

Cytochrome Ci can be reduced by succinate, and its reduction is inhibited by antimycin A. Its oxidation potential is probably of a value intermediate between the oxidation potential of cytochrome c and cytochrome b. It can now be prepared from submitochon-drial fragments and purified by treatments of the preparation with butanol and deoxycholate. Such a preparation yields a polymer, the monomer of which contains one hemin. The exact molecular weight of the unit is not clear, but the proposed values range between 37,000 and 70,000. The structure of the prosthetic group of cytochrome Cl has not been established. 

The role of SODs in ageing and disease has been reviewed, as has been the relation between superoxide and iron in produdng cellular damage. For the well studied eukaryotic Cu/Zn protein the dismutation reaction is beheved to involve reduction and reoxidation of Cu(II) in a consecutive two-step mechanism. For FeSOD (Figure 1), which is often treated in parallel with MnSOD, there are some basic questions still open. One concerns the metal spedfidty. Typically, one can substitute native MnSOD with Fe but the catalytic activity then is reduced or lost. In contrast, the X-ray structures of the prosthetic groups of Mn-and Fe-SODs are hardly distinguishable and, in addition, there are cambialistic... 

The propionic acid side chains of rings III and IV have been retained, but those of rings I and II have been converted to vinyl groups. In a few cytochromes the side chains are converted further by a few more steps in some instances, the structure of the prosthetic group is still unknown (cf. also Table XIII, Section 3). 

FIGURE 16.3 Aminotransferase reactions, (a) Outline of the reaction, (b) Structures of the prosthetic group and parent compound, (c) Covalent linkage between enzyme and pyridoxal phosphate is replaced by covalent linkage between incoming amino acid and pyridoxal phosphate... 

Porphyrins and corrins form part of the prosthetic groups in a large variety of vitally important natural products. As a result of intense interest in the structure, function and mechanistic details of the biological systems containing porphyrins and chlorins, the whole field has developed in a most remarkable way over the past 50 or more years. There can be few other research areas which possess such a rich, eventful and informative historical literature, and yet continue to produce important and exciting revelations in current journals. 

Another class of compounds that readily combine reversibly with molecular oxygen are the metal phthalocyanines.188,189 A number of oxidative enzymes contain the metal-porphyrin structure as the prosthetic group. In view of the... 

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,... 

The electronic properties of haemoproteins have been measured and discussed in recent years by workers whose primary interests cover a wide range of scientific disciplines, from theoretical physics to medicine and biology. In fact there can be few other fields in which so many disciplines have pooled their resources, both experimental and theoretical. In spite of the prodigious development of other physical methods electronic absorption spectroscopy remains the most widely-used tool in the study of these proteins. A proper understanding of their spectra is clearly of the greatest importance in the investigation of the molecular electronic structure of the haem chromophore, and of the effects of the structure and conformation of the polypeptide chain on the properties of the prosthetic groups derived from it. 

This chapter will review the general features of the enzymology of methanol dehydrogenase, its structure as determined by X-ray crystallography, the properties of the prosthetic group (PQQ), the mechanism of its reduction by substrate, its oxidation by its specific cytochrome electron acceptor, and processes involved in its synthesis. 

Fig. 4. Arrangement of the prosthetic groups in the Rp. viridis. reaction center, redrawn from Ref. 101. Ob is shown at the site identified by Deisenhofcr et al. [102], but the orientation of the quinone in this site is drawn arbitrarily the exact orientation of Ob in the crystal structure has not been described. The four hemes at the top of the figure are in the cytochrome subunit the other components are in the L-M complex. As in Fig. 3, the normal to the chromatophore membrane is approximately vertical and the periplasmic side of the complex is at the top.

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