Metallo-proteins

The side chains of the 20 different amino acids listed in Panel 1.1 (pp. 6-7) have very different chemical properties and are utilized for a wide variety of biological functions. However, their chemical versatility is not unlimited, and for some functions metal atoms are more suitable and more efficient. Electron-transfer reactions are an important example. Fortunately the side chains of histidine, cysteine, aspartic acid, and glutamic acid are excellent metal ligands, and a fairly large number of proteins have recruited metal atoms as intrinsic parts of their structures among the frequently used metals are iron, zinc, magnesium, and calcium. Several metallo proteins are discussed in detail in later chapters and it suffices here to mention briefly a few examples of iron and zinc proteins. 

Weser, U. Structural Aspects and Biochemical Function of Erythrocuprein. Vol. 17, pp. 1-65. Weser, U. Redox Reactions of Sulphur-Containing Amino-Acid Residues in Proteins and Metallo-proteins, an XPS-Study. Vol. 61, pp. 145-160. 

In Table 10 there are some examples of EPR signals obtained from metallo-proteins. Most of this work has been conducted at low temperatures and it is now necessary to develop probes which contain metals and which will give signals at room temperature, e.g. d1 complexes. Such a label could be of use in many explorations of activity in biological systems. 

Source From Messerschmidt, A., Bode, W. and Cygler, M. (2004). Handbook of Metallo-Proteins Vol. 3, John Wiley Sons, Chichester, UK. 

C.H. Lochmiiller, J. Galbraith, R. Willis and R. Walter, Metal-ion distribution in metallo-proteins by proton-induced x-ray emission analysis, Anal. Biochem., 57 (1974) 618. 

This pedagogical account is intended to provide a brief introduction for the non-specialist, to the theoretical and experimental aspects of variable temperature MCD spectroscopy that are applicable in the study of metallopro-teins. This is followed by some individual examples of MCD studies of metallo-proteins that have been chosen to illustrate the utility of the technique and the type of information that is available. 

By use of appropriate sterically demanding carboxylates it is possible to generate four-, five-, and six-coordinated mononuclear iron(III) complexes. Ligand flexibility and electronic properties provide fine-tuning. These complexes are subunits of the models for di-iron(II) sites in metallo-proteins mentioned in the following section. 

Vedani, A., Dobler, M., and Dunitz, J. D. (1986). An empirical potential function for metal centers Application to molecular mechanics calculations on metallo proteins.y. Corn-put. Chem. 7, 701-710. 

The amino acid side chains which serve as zinc ligands in a metallo-protein are densely packed within the protein structure and often make hydrogen bond contacts with other residues. Argos et al. (1978) pointed out that such interactions orient the metal ligands, and these interactions may also enhance the electrostatic interaction between the metal ion and its ligands. 

Polyphenol oxidase (PPO) (EC 1.14.18.1 monophenol monooxygenase [tyrosinase] or EC 1.10.3.2 0-diphenol 02-oxidoreductase) is one of the more important enzymes involved in the formation of black tea polyphenols. The enzyme is a metallo-protein thought to contain a binudear copper active site. The substance PPO is an oligomeric particulate protein thought to be bound to the plant membranes. The bound form of the enzyme is latent and activation is likely to be dependent upon solubilization of the protein (35). PPO is distributed throughout the plant (35) and is localized within in the mitochondria (36), the cholorplasts (37), and the peroxisomes (38). Using antibody techniques, polyphenol oxidase activity has also been localized in the epidermis palisade cells (39). Reviews on the subject of PPO are available (40—42). 

In summary, organisms can use biological reductants such as NAD(P)H, capable of hydride transfer (two electron transfer), and reduced flavoproteins and metallo-proteins, capable of single electron donation. Although not necessarily intended to interact with xenobiotic organic compounds, when such organic chemicals come in contact with suitably reactive bioreductants in vivo, reductions can occur. 

Catalases and peroxidases. Many iron and copper proteins do not bind 02 reversibly but "activate" it for further reaction. We will look at such metallo-protein oxidases in Chapter 18. Here we will consider heme enzymes that react not with 02 but with peroxides. The peroxidases,1943 which occur in plants, animals, and fungi, catalyze the following reactions (Eq. 16-6,16-7) ... 

The L-rhamnulose 1-phosphate aldolase (RhuA EC 4.1.2.19) is found in the microbial degradation of L-rhamnose which, after conversion into the corresponding ketose 1-phosphate 44, is cleaved into 41 and L-lactaldehyde (l-16). The RhuA has been isolated from E. coli [336-339], and characterized as a metallo-protein [194,340,341]. Cloning was reported for the E. coli [342,343] and Salmonella typhimurium [344] genes, and construction of an efficient overexpression system [195,220] has set the stage for crystallization of the homotetrameric E. coli protein for the purposes of an X-ray structure analysis [345]. 

Direct evidence for long range electron-transfer in biological systems was first observed by Gray et al.50,51) and Isied et al.481 using [Ru(NH3)5]3+ substituted metallo protein. Histidine-83 of blue copper (azurin) was labeled with Ru(III)(NH3)5 50). Flash photolysis reduction of the His-83 bound Ru(III) followed by electron-transfer from the Ru(II) to Cu2+ was observed with a rate constant of 1.9 s 1. The result shows that intramolecular long distance (approx. 1 nm) electron-transfer from the Ru(II) to the Cu2 + of the azurin takes place rapidly. 

Structural information on hemoproteins can also be obtained from investigations of the enhancement of the nuclear spin relaxation in the bulk water. In this section a qualitative discussion is presented of relaxation mechanisms in solutions of diamagnetic and paramagnetic metallo-proteins, followed by a brief survey of experiments with hemoproteins. 

The pH dependence of cytochrome c oxidation-reduction reactions and the studies of modified cytochrome c thus demonstrate that the coordination environment of the iron and the conformation of the protein are relatively labile and strongly influence the reactivity of the metallo-protein toward oxidation and reduction. The effects seen may originate chiefly from alterations in the thermodynamic barriers to electron transfer, but the conformation changes are expected to affect the intrinsic barriers also. One such conformation change is the opening of the heme crevice referred to above. The anation and Cr(II) reduction studies provide an estimate of 60 sec 1 for this process in Hh(III) at 25°C (59). To date, no evidence has been found for a rapid heme-crevice opening step in ferrocytochrome c. 

In this volume, we attempt to bring together topics that span the breadth of metal-catalyzed reactions in biological systems and have accordingly selected chapters not only on catalytic reaction of metallo-proteins, but also on other reactions of metal ions in biological systems, such as electron transfer and nucleic acid scission. Of course, an emphasis on mechanisms does not mean that structure and electronic properties can be ignored. In fact, a mechanistic emphasis requires a detailed knowledge of how structure and electronic properties influence reactivity, and these subjects are also explored here. 

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