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Bioinorganic chemistry Editorial overview Lawrence Que Jr and Lucia Banci Current Opinion in Chemical Biology 2002, 6:169–170 1367-5931/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
Lawrence Que Jr Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, 207 Pleasant St SE, Minneapolis, Minnesota 55455, USA; e-mail:
[email protected]
Larry’s research focuses on understanding the detailed mechanisms of oxygen activation by iron enzymes, particularly those with nonheme ligand environments, using biomimetic as well as biophysical and spectroscopic approaches. Lucia Banci Centro Risonanze Magnetiche, University of Florence, Via Luigi Sacconi 6, 50019 Sesto Fiorentino, Florence, Italy; e-mail:
[email protected]
Lucia’s research is mainly dedicated to the structural biology of metalloproteins, with the aim of determining their structural and dynamic properties and correlating them to their biological functions. She has also started a major structural genomic project devoted to the localization and structural characterization of all the proteins involved in metal homeostasis.
Bioinorganic chemistry as an interdisciplinary field that connects inorganic chemistry with biology and health continues to fascinate scientists in a wide range of disciplines. The interaction of scientists from different fields has stimulated cross fertilization and the development of new ideas that invigorate discussions at workshops and conferences, and in the published scientific literature. The tremendous success of the Tenth International Conference on Bioinorganic Chemistry (ICBIC-10) held in August 2001 in Florence is a testament to the strong interest in this field and the excitement it has generated. For this year’s bioinorganic section of Current Opinion in Chemical Biology, we have commissioned seven articles that address recent developments in this active area of research. In the past few years, metal homeostasis and the proteins involved in these processes have received and continue to receive tremendous attention by a steadily increasing number of research groups. New classes of proteins have been discovered in recent years that have attracted the attention of several groups. The article by Puig and Thiele (pp 171–180) gives an overview of the mechanisms of copper transport in the cell and of copper distribution to the various copper proteins within cells. In recent years, exciting advances have been made toward understanding how copper is transferred and distributed. The determination of several three-dimensional structures of these coppertransporting proteins provides a starting point for developing a unifying picture for copper transport processes. The studies available on some protein–protein complexes reveal highly specific interactions between proteins and the features of the metal-transfer processes. Finally, the use of mouse knockout models has shown the importance of copper uptake and distribution, through both predicted and new ways, in the growth of living organisms. Within this general biochemical problem (i.e. the transfer and distribution of copper), the chapter by Opella, DeSilva and Veglia (pp 217–223) reviews the structural analysis of the cysteine-rich metal-binding domains present in several metal transporter proteins. They particularly focus on the bacterial mercury detoxification system and on the copper-transporting proteins in eukaryotes and bacteria. Through a combined use of solution NMR structures and X-ray crystallographic data with EXAFS measurements, the mechanisms for protein-mediated heavy metal ion homeostasis are analyzed. The most striking finding from this analysis is the remarkable similarities of the metalbinding domains across different species and towards different metal ions. Alteration of the metal transport processes can produce severe health problems. These issues are addressed in the article by Andrews (pp 181–186). Iron and copper are essential nutrients, whose concentration in the cell must be meticulously regulated to optimize their function in biological processes and to protect living organisms from their tendency to promote formation of toxic free radicals. Over the years, it is becoming increasingly evident that the metabolism of iron is tightly connected with that of copper ions. Alteration in metal transporters for a particular ion is reflected in altered metabolism of the
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other. This article describes how defects in the various steps of iron and of copper transport lead to human diseases such as homochromatosis, Menkes disease, Wilson disease and others. Along the same lines, the article by Lehman (pp 187–192) reviews the interconnection between metal ions and neurodegenerative disorders. Particular attention is devoted to the prion protein, whose misfolding leads to the Creuzfeldt–Jacob disease. In recent years, it has become well established that the prion protein can bind copper with high affinity, which then modifies the structural and biochemical properties of the protein. Links between prion protein expression and the expression of the other copper-dependent proteins have also been found, which reinforce the idea that the copper transport processes are strongly inter-regulated. Still, many questions remain, including the exact involvement of metal ions in the conformational modifications of the prion protein and in the pathology occurring in prion diseases. Non-heme iron oxygenases represent a diverse class of iron enzymes with a broad range of functions. Progress in our understanding of these enzymes has accelerated in the past decade as more investigators have joined in this effort. Ryle and Hausinger (pp 193–201) bring us up to date on the latest developments in this important subclass of enzymes. Genomic sequencing data and sequence comparisons have identified a number of new enzymes with mononuclear or dinuclear non-heme iron active sites. Of particular recent interest is the identification and characterization of a conserved family of α-ketoglutarate-dependent prolyl-4-hydroxylases; these enzymes specifically hydroxylate a particular prolyl residue in one subunit of the mammalian hypoxia-inducible factor and are thus involved in oxygen sensing in the cell. X-ray crystallography continues to play a big role in providing new insight into metalloenzyme structure and, more recently, into catalytic mechanisms as interactions with substrates and their conversion into products are investigated in the crystalline state. There is also progress in our understanding of how these nonheme iron active sites work. Examples of post-translational oxidation of side chain residues have been reported for ribonucleotide reductase (Phe208 to m-tyrosine) and TfdA, an α-ketoglutarate-dependent dioxygenase that degrades the herbicide 2,4-D (Trp112 to hydroxytryptophan). These results
demonstrate that the oxidizing species formed in these nonheme iron active sites can be redirected under appropriate conditions to attack other targets in the course of oxygen activation. New insights into the hydroxylation of aliphatic C–H bonds by soluble methane monooxygenase and memabranebound alkane monooxygenase have also been obtained. Cryocrystallography is another powerful approach for elucidating the mechanisms of metalloenzymes by the use of low temperature to stabilize reactive intermediates. Wilmot and Pearson (pp 202–207) provide an update of the progress in freeze-trapping reaction intermediates in crystals of metalloenzymes. They comment on strategies for minimizing X-ray radiation damage in the crystals and on methods for monitoring changes in the crystal in the course of the diffraction experiment. With these techniques, the structures of several proteins with bound gases were obtained. These include the NO adducts to the endothelial nitric oxide synthase and nitrophorin 4, a protein that transports NO in blood sucking insects, an O2-bound intermediate of cytochrome cd1 nitrite reductase, and soluble methane monooxygenase with a xenon atom trapped in the putative substrate-binding cavity. These techniques also work for enzymes with nongaseous substrates. For example, insights into the DNA polymerase β mechanism have been obtained with the solution of structures of an early intermediate in the nucleotidyl transferase pathway and of a post-nucleotidyl transfer intermediate. It is expected that these techniques, which were developed using metalloenzymes as prototypes, will find application in the study of other enzymes, even those without metal cofactors. The rational design and redesign of enzymes is an area of active research as a means of taking the evolutionary advantages built into an enzyme but targeting other substrates or reactivities. The superlative properties of heme enzymes and the depth of knowledge in these systems have made them a logical target for such efforts. Watanabe (pp 208–216) has reviewed recent developments in constructing new heme enzymes that involve re-engineering the active site by site-directed mutagenesis, directed evolution strategies, exon shuffling, or chemical modification of the heme prosthetic group. These approaches show promise either in providing a fuller understanding of heme enzyme reaction mechanisms or in new reagents for biocatalysis.