Metal loproteins

MIR), requires the introduction of new x-ray scatterers into the unit cell of the crystal. These additions should be heavy atoms (so that they make a significant contribution to the diffraction pattern) there should not be too many of them (so that their positions can be located) and they should not change the structure of the molecule or of the crystal cell—in other words, the crystals should be isomorphous. In practice, isomorphous replacement is usually done by diffusing different heavy-metal complexes into the channels of preformed protein crystals. With luck the protein molecules expose side chains in these solvent channels, such as SH groups, that are able to bind heavy metals. It is also possible to replace endogenous light metals in metal-loproteins with heavier ones, e.g., zinc by mercury or calcium by samarium. 

Lewis, C.D. and Laemmli, U.K. (1982). Higher order metaphase chromosome structure evidence for metal-loprotein interactions. Cell 29, 171-181. 

Hagen, W.R. 1981. Dislocation strain broadening as a source of anisotropic linewidth and asymmetrical lineshape in the electron paramagnetic resonance spectrum of metal-loproteins and related systems. Journal of Magnetic Resonance 44 447-469. 

Biosynthesis of the polypeptide chain is realised by a complicated process called translation. The basic polypeptide chain is subsequently chemically modified by the so-called posttranslational modifications. During this sequence of events the peptide chain can be cleaved by directed proteolysis, some of the amino acids can be covalently modified (hydroxylated, dehydrogenated, amidated, etc.) or different so-called prosthetic groups such as haem (haemoproteins), phosphate residues (phosphoproteins), metal ions (metal-loproteins) or (oligo)saccharide chains (glycoproteins) can be attached to the molecule by covalent bonds. Naturally, one protein molecule can be modified by more means. 

Interest in this class of coordination compounds was sparked and fueled by the discovery that radical cofactors such as tyrosyl radicals play an important role in a rapidly growing number of metalloproteins. Thus, in 1972 Ehrenberg and Reichard (1) discovered that the R2 subunit of ribonucleotide reductase, a non-heme metal-loprotein, contains an uncoordinated, very stable tyrosyl radical in its active site. In contrast, Whittaker and Whittaker (2) showed that the active site of the copper containing enzyme galactose oxidase (GO) contains a radical cofactor where a Cu(II) ion is coordinated to a tyrosyl radical. 

A wealth of spectroscopic methods are available for the study of metal-loproteins. Most require some level of sophistication in physics for rigor-... 

The replacement of native haem with metal-substituted porphyrin can be performed in two ways. In the first, iron ions are removed from the protein by treating native protein with anhydrous HF, followed by insertion of the appropriate metal to metal-free protein [57-59]. In the second, haem is removed either chemically or by recombination (preparing a proper recombinant protein), and then protein is reconstituted with metal-substituted porphyrins [60-64]. The Zn-substituted metal-loproteins such as cytochrome c [65-67], myoglobin [59,61, 62, 64, 68, 69], and haemoglobin [64, 68,70] have been extensively used to study photoinduced ET (PET) between modified proteins and their physiological redox partners. Interestingly, in haemoglobin with a and /3 subunits it was possible to determine ET parameters for... 

Studies of the infrared spectra carbon monoxide inhibited forms of [FeNi]-hydrogenase revealed that the native protein exhibits absorbances at energies not normally associated with protein samples. These absorbances in the 1900-2100 cm region are also observed in [Fe]-hydrogenase samples. This absorption is attributed to the cyanide and carbonyl ligands at the active site metal centers. Cyanide and carbon monoxide are not commonly found as ligands to metal centers in metal-loproteins. The different redox states of the enzymes give rise to different absorption spectra. 

Most recently, the first all-electron density functional calculation of a metal-loprotein was reported, made possible by massively parallel distributed computing... 

We prepared three bifunctional redox protein maquettes based on 12 16-, and 20-mer three-helix bundles. In each case, the helix was capped with a Co(III) tris-bipyridyl electron acceptor and also functionalized with a C-terminal viologen (l-ethyl-V-ethyl-4,4 -bipyridinium) donor. Electron transfer (ET) was initiated by pulse radiolysis and flash photolysis and followed spectrometrically to determine average, concentration-independent, first-order rates for the 16-mer and 20-mer maquettes. For the 16-mer bundle, the a-helical content was adjusted by the addition of urea or trifluoroethanol to solutions containing the metal-loprotein. This conformational flexibility under different solvent conditions was exploited to probe the effects of helical secondary structure on ET rates. In addition to describing experimental results from these helical systems, this chapter discusses several additional metalloprotein models from the recent literature. 

Blyumenfeld, L.A., R.M. Davydov, S.P. Kuprin, and S.V Stepanov (1977). Chemical characteristics of non-equilibrium conformational states of metal-loproteins. Biqfizika 22, 977-994. 

The nuclear magnetic resonance technique is very useful for the study of a metal-loprotein, namely to obtain information about the changes of conformation, redox states, assignment of axial ligands in heme proteins and detection of different spin states. H-NMR was used to study the oxidized and the half-reduced states of P. denitrificans CCP in the presence and absence of calcium. The results have confirmed the hypothesis based on interpretation of the UV/visible spectra and have also contributed to add new data to the mechanism of activation of the protein. Figure 6-5 shows the 300 MHz H-NMR of the oxidized and half-reduced states... 

This first decade of crystallographic study of enzymes helped to create two new fields bio-inorganic chemistry and mechanistic bio-organic chemistry. Harry B. Gray (1935- ), at Caltech, began to make fundamental contributions to the chemistry and chemical physics of metal-loproteins. Unlike reactions between transition metal ions and their complexes, which involve intimate contact and electron-transfer upon collision, electron transfer between metals on different proteins is not so intimate and may require several collisions and may proceed through... 

Often metals of constant valency compounds are used in combination with redox-active transition-metal complexes to promote a variety of reactions involving the transfer of electrons [10], This effect is typified in metal-loproteins such as the copper zinc superoxide dismutase, in which both metal ions have been proposed to be functionally active [11],... 

Copper plays a most important role in living systems, copper-containing proteins being widely distributed both in plants and animals, including man. The biological functions of these proteins include, for instance, dioxygen reduction and electron transport, and are closely connected with the oxidation state and coordination sphere of copper in the metal-loproteins. 

Element Metal loprotein Occurrence Element Metalloproteln Occurrence... 

Lindqvist Y. 2001. stearoyl-acyl carrier protein desaturase. In Handbook of metal-loproteins, pp. 725-737. Ed A Messerschmidt. Chichester John Wiley Sons. 

---