Superoxide dismutases (SODs, EC 1.15.1.1) are ubiquitous enzymes that efficiently catalyze the dismutation of superoxide radical anions to protect biological molecules from oxidative damage. The crystal structure of nickel-containing SOD (NiSOD) from Streptomyces seoulensis was determined for the resting, x-ray-reduced, and thiosulfate-reduced enzyme state. NiSOD is a homohexamer consisting of four-helix-bundle subunits. The catalytic center resides in the N-terminal active-site loop, where a Ni(III) ion is coordinated by the amino group of His-1, the amide group of Cys-2, two thiolate groups of Cys-2 and Cys-6, and the imidazolate of His-1 as axial ligand that is lost in the chemically reduced state as well as after x-ray-induced reduction. This structure represents a third class of SODs concerning the catalytic metal species, subunit structure, and oligomeric organization. It adds a member to the small number of Ni-metalloenzymes and contributes with its Ni(III) active site to the general understanding of Ni-related biochemistry. NiSOD is shown to occur also in bacteria other than Streptomyces and is predicted to be present in some cyanobacteria.

Cobalamin (Cbl, vitamin B12) serves for two essential cofactors in mammals. The pathway for its intestinal absorption, plasma transport, and cellular uptake uses cell surface receptors and three Cbl-transporting proteins, haptocorrin, intrinsic factor, and transcobalamin (TC). We present the structure determination of a member of the mammalian Cbl-transporter family. The crystal structures of recombinant human and bovine holo-TCs reveal a two-domain architecture, with an N-terminal ␣6-␣6 barrel and a smaller C-terminal domain. One Cbl molecule in base-on conformation is buried inside the domain interface. Structural data combined with previous binding assays indicate a domain motion in the first step of Cbl binding. In a second step, the weakly coordinated ligand H 2O at the upper axial side of added H2O-Cbl is displaced by a histidine residue of the ␣6-␣6 barrel. Analysis of amino acid conservation on TC's surface in orthologous proteins suggests the location of the TC-receptor-recognition site in an extended region on the ␣6-␣6 barrel. The TC structure allows for the mapping of sites of amino acid variation due to polymorphisms of the human TC gene. Structural information is used to predict the overall fold of haptocorrin and intrinsic factor and permits a rational approach to the design of new Cbl-based bioconjugates for diagnostic or therapeutic drug delivery.cobalamin ͉ crystal structure ͉ P259R polymorphism ͉ transport protein

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Abstract:The chemistry and biochemistry of the vitamin B 12 compounds (cobalamins, XCbl) are described, with particular emphasis on their structural aspects and their relationships with properties and function. A brief history of B 12 , reveals how much the effort of chemists, biochemists and crystallographers have contributed in the past to understand the basic properties of this very complex vitamin. The properties of the two cobalamins, the two important B 12 cofactors Ado-and MeCbl are described, with particular emphasis on how the Co-C bond cleavage is involved in the enzymatic mechanisms. The main structural features of cobalamins are described, with particular reference to the axial fragment. The structure/property relationships in cobalamins are summarized. The recent studies on base-off/base-on equilibrium are emphasized for their relevance to the mode of binding of the cofactor to the protein scaffold. The absorption, transport and cellular uptake of cobalamins and the structure of the B 12 transport proteins, IF and TC, in mammals are reviewed. The B 12 transport in bacteria and the structure of the so far determined proteins are briefly described. The currently accepted mechanisms for the catalytic cycles of the AdoCbl and MeCbl enzymes are reported. The structure and function of B 12 enzymes, particularly the important mammalian enzymes methyltransferase (MetH) and methyl-malonyl-coenzymeA mutase (MMCM), are described and briefly discussed. Since fast proliferating cells require higher amount of vitamin B 12 than that required by normal cells, the study of B 12 conjugates as targeting agents has recently gained importance. Bioconjugates have been studied as potential agents for delivering radioisotopes and NMR probes or as various cytotoxic agents towards cancer cells in OPEN ACCESSMolecules 2010, 15 3229 humans and the most recent studies are described. Specifically, functionalized bioconjugates are used as "Trojan horses" to carry into the cell the appropriate antitumour or diagnostic label. Possible future developments of B 12 work are summarized.