We are just beginning to understand the metabolism of heavy metals and to use their metabolic functions in biotechnology, although heavy metals comprise the major part of the elements in the periodic table. Because they can form complex compounds, some heavy metal ions are essential trace elements, but, essential or not, most heavy metals are toxic at higher concentrations. This review describes the workings of known metal-resistance systems in microorganisms. After an account of the basic principles of homoeostasis for all heavy-metal ions, the transport of the 17 most important (heavy metal) elements is compared.

Microbial heavy-metal resistance

For present purposes, a protein-bound metal site consists of one or more metal ions and all protein side chain and exogenous bridging and terminal ligands that define the first coordination sphere of each metal ion. Such sites can be classified into five basic types with the indicated functions: (1) structural -- configuration (in part) of protein tertiary and/or quaternary structure; (2) storage -- uptake, binding, and release of metals in soluble form: (3) electron transfer -- uptake, release, and storage of electrons; (4) dioxygen binding -- metal-O{sub 2} coordination and decoordination; and (5) catalytic -- substrate binding, activation, and turnover. The authors present here a classification and structure/function analysis of native metal sites based on these functions, where 5 is an extensive class subdivided by the type of reaction catalyzed. Within this purview, coverage of the various site types is extensive, but not exhaustive. The purpose of this exposition is to present examples of all types of sites and to relate, insofar as is currently feasible, the structure and function of selected types. The authors largely confine their considerations to the sites themselves, with due recognition that these site features are coupled to protein structure at all levels. In themore » next section, the coordination chemistry of metalloprotein sites and the unique properties of a protein as a ligand are briefly summarized. Structure/function relationships are systematically explored and tabulations of structurally defined sites presented. Finally, future directions in bioinorganic research in the context of metal site chemistry are considered. 620 refs.« less

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Structural and Functional Aspects of Metal Sites in Biology

Iron-sulfur proteins are found in all life forms. Most frequently, they contain Fe2S2, Fe3S4, and Fe4S4 clusters. These modular clusters undergo oxidation-reduction reactions, may be inserted or removed from proteins, can influence protein structure by preferential side chain ligation, and can be interconverted. In addition to their electron transfer function, iron-sulfur clusters act as catalytic centers and sensors of iron and oxygen. Their most common oxidation states are paramagnetic and present significant challenges for understanding the magnetic properties of mixed valence systems. Iron-sulfur clusters now rank with such biological prosthetic groups as hemes and flavins in pervasive occurrence and multiplicity of function.

http://science.sciencemag.org/content/sci/277/5326/653.full.pdf

Iron-Sulfur Clusters: Nature's Modular, Multipurpose Structures

Mechanisms and applications of plant growth promoting rhizobacteria: Current perspective

Mechanism of Molybdenum Nitrogenase.

Structural models for the nitrogenase FeMo-cofactor and P-clusters are proposed based on crystallographic analysis of the nitrogenase molybdenum-iron (MoFe)-protein from Azotobacter vinelandii at 2.7 angstrom resolution. Each center consists of two bridged clusters; the FeMo-cofactor has 4Fe:3S and 1Mo:3Fe:3S clusters bridged by three non-protein ligands, and the P-clusters contain two 4Fe:4S clusters bridged by two cysteine thiol ligands. Six of the seven Fe sites in the FeMo-cofactor appear to have trigonal coordination geometry, including one ligand provided by a bridging group. The remaining Fe site has tetrahedral geometry and is liganded to the side chain of Cys alpha 275. The Mo site exhibits approximate octahedral coordination geometry and is liganded by three sulfurs in the cofactor, two oxygens from homocitrate, and the imidazole side chain of His alpha 442. The P-clusters are liganded by six cysteine thiol groups, two which bridge the two clusters, alpha 88 and beta 95, and four which singly coordinate the remaining Fe sites, alpha 62, alpha 154, beta 70, and beta 153. The side chain of Ser beta 188 may also coordinate one iron. The polypeptide folds of the homologous alpha and beta subunits surrounding the P-clusters are approximately related by a twofold rotation that may be utilized in the binding interactions between the MoFe-protein and the nitrogenase Fe-protein. Neither the FeMo-cofactor nor the P-clusters are exposed to the surface, suggesting that substrate entry, electron transfer, and product release must involve a carefully regulated sequence of interactions between the MoFe-protein and Fe-protein of nitrogenase.

https://science.sciencemag.org/content/sci/257/5077/1677.full.pdf

Structural models for the metal centers in the nitrogenase molybdenum-iron protein

The crystal structure of the nitrogenase molybdenum–iron protein from Azotobacter vinelandii has been determined at 2.7 A resolution. The α- and β-subunits in this α_2β_2 tetramer have similar polypeptide folds. The FeMo-cofactor is completely encompassed by the α-subunit, whereas the P-cluster pair occurs at the interface between α- and β-subunits. Structural similarities are apparent between nitrogenase and other electron transfer systems, including hydrogenases and the photosynthetic reaction centre.

Crystallographic structure and functional implications of the nitrogenase molybdenum-iron protein from azotobacter vinelandii.

Structures recently proposed for the FeMo-cofactor and P-cluster pair of the nitrogenase molybdenum-iron (MoFe)-protein from Azotobacter vinelandii have been crystallographically verified at 2.2 angstrom resolution. Significantly, no hexacoordinate sulfur atoms are observed in either type of metal center. Consequently, the six bridged iron atoms in the FeMo-cofactor are trigonally coordinated by nonprotein ligands, although there may be some iron-iron bonding interactions that could provide a fourth coordination interaction for these sites. Two of the cluster sulfurs in the P-cluster pair are very close together (approximately 2.1 angstroms), indicating that they form a disulfide bond. These findings indicate that a cavity exists in the interior of the FeMo-cofactor that could be involved in substrate binding and suggest that redox reactions at the P-cluster pair may be linked to transitions of two cluster-bound sulfurs between disulfide and sulfide oxidation states.

https://science.sciencemag.org/content/sci/260/5109/792.full.pdf

The nitrogenase FeMo-cofactor and P-cluster pair: 2.2 A resolution structures.

Biological nitrogen fixation is catalyzed by the nitrogenase enzyme system which consists of
two metalloproteins, the iron (Fe-) protein and the molybdenum-iron (MoFe-) protein. Together, these
proteins mediate the ATP-dependent reduction of dinitrogen to ammonia. Recent crystallographic analyses
of Fe-protein and MoFe-protein have revealed the polypeptide fold and the structure and organization of
the unusual metal centers in nitrogenase. These structure provide a molecular framework for addressing
the mechanism of the nitrogenase-catalyzed reaction. General features of the nitrogenase system, including
conformational coupling of nucleotide hydrolysis, aspects of the cluster structures, and the general spatial
organization of redox centers within the protein subunits, are relevant to a wide range of biochemical
systems.

Nitrogenase and biological nitrogen fixation.

The crystal structure of the nitrogenase molybdenum-iron (MoFe) protein from Clostridium
pasteurianum (Cp1) has been determined at 3.0-A resolution by a combination of isomorphous replacement,
molecular replacement, and noncrystallographic symmetry averaging. The structure of Cp1, including the
two types of metal centers associated with the protein (the FeMo-cofactor and the P-cluster pair), is similar
to that previously described for the MoFe-protein from Azotobacter vinelandii (Av1). Unique features of
the Cpl structure arise from the presence of an ~50-residue insertion in the α subunit and an ~50-residue
deletion in the β subunit. As a consequence, the FeMo-cofactor is more buried in Cp1 than in Av1, since
the insertion is located on the surface above the FeMo-cofactor. The location of this insertion near the
putative nitrogenase iron protein binding site provides a structural basis for the observation that the nitrogenase
proteins from C. pasteurianum have low activity with complementary nitrogenase proteins isolated from
other organisms. Mechanistic implications of the Cp1 structure for substrate entry /product release,
substrate binding to the FeMo-cofactor, and electron- and proton-transfer reactions of nitrogenase are
discussed.

Jongsun Kim

Papers

Structural models for the metal centers in the nitrogenase molybdenum-iron protein

Crystallographic structure and functional implications of the nitrogenase molybdenum-iron protein from azotobacter vinelandii.

The nitrogenase FeMo-cofactor and P-cluster pair: 2.2 A resolution structures.

Nitrogenase and biological nitrogen fixation.

X-ray crystal structure of the nitrogenase molybdenum-iron protein from Clostridium pasteurianum at 3.0-A resolution.