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Function of Metal Ions in Proteins

Function of Metal Ions in Proteins. Structural Binding site Control mechanism Lewis acid Redox catalyst. Enzyme-metal-ligand interaction modes. I. Ligand bridged binding. II. Metal bridged binding. III. Enzyme bridged binding. Lectins.

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Function of Metal Ions in Proteins

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  1. Function of Metal Ions in Proteins • Structural • Binding site • Control mechanism • Lewis acid • Redox catalyst

  2. Enzyme-metal-ligand interaction modes I. Ligand bridged binding II. Metal bridged binding III. Enzyme bridged binding

  3. Lectins Lectins are metalloproteins, first identified in plants, that are involved in binding carbohydrates They are proposed to function in cell recognition through binding to cell surface glucoproteins plant lectin Each subunit in the dimer contains a bimetallic binding site for Mn(II) and Ca(II) plant lectin Each subunit in the tetramer contains a bimetallic binding site for Mn(II) and Ca(II)

  4. Lectins While most lectins have a bimetallic site the identity and the role of the metal ions can vary Bacterial lectin Contains a bimetallic Ca(II) site that directly interacts with the carbohydrate Ficolin Contains multiple Ca(II) sites, but no direct carbohydrate interactions

  5. Concanavalin A Concanavalin A is a plant lectin that binds carbohydrates with high specificity Metal ions are required for carbohydrate binding The Mn(II) and Ca(II) sites do share common ligands However, NMR relaxation studies show that the sugar is not bound in the same site Con A is a tetramer and early structural studies suggested that the metals and sugar occupy a common site

  6. Concanavalin Ametal ion binding sites Tetramer of Con A Monomer showing the binding site proximity The carbohydrate is bound in the vicinity of the metal ion sites There are no direct interactions between the metal ions and the carbohydrate However, it is quite clear from these structures why Concanavalin A cannot bind carbohydrates in the absence of bound Mn(II) and Ca(II)

  7. Oxygen transport proteins Depending on the species, several different types of metalloproteins can be involved in oxygen transport Hemoglobin is the primary transporter of oxygen in vertebrates Hemerythrin transports oxygen in marine invertebrates Hemocyanin carries out oxygen transport in molluscs and arthropods

  8. Oxygen transport proteinsheme proteins Iron-heme proteins are the primary oxygen transporters in mammals Iron heme Myoglobin

  9. Oxygen transport proteinshemoglobin Hemoglobin is a tetramer composed of two α and two β subunits Each subunit contains an iron-porphyrin ring that binds oxygen Oxygen binding is highly cooperative between each subunit

  10. Oxygen transport proteinsporphyrin models There are several types of iron-porphyrin model compounds that can reversibly bind oxygen These models are useful to study the details of oxygen binding and release

  11. Oxygen transport proteinshemerythrin Hemerythrin contains a binuclear iron site that reversibly binds oxygen

  12. Oxygen transport proteinshemocyanin Hemocyanin contains a binuclear copper site that reversibly binds oxygen

  13. Oxygen transport proteinshemocyanin For both hemerythrin and hemocyanin oxygen binds by bridging between the metal ions However the interactions with oxygen are different for Cu(II) and Fe(III)

  14. lactoferrin Functions as an antimicrobial because of its very high affinity for iron Lactoferrin contains a 6-coordinate octahedral iron binding site Two tyrosine hydoxyl groups are involved in iron binding

  15. Diphtheria toxin repressor A DNA binding protein that prevent expression of diphtheria toxin Co(II)-DTR with sulfate bound in the anion site Zn(II)-DTR with phosphate bound in the anion site

  16. Anion binding proteinsModA ModA is a molybdate binding protein that functions in the uptake of the essential trace element molybdenum The protein exists as a trimer each subunit containing a highly specific molybdate binding site The oxygens of MoO4 are bound through hydrogen bonds to side chain hydroxyl groups and backbone amide nitrogens

  17. Anion binding proteinsSulfate Binding Protein The sulfate binding protein is an iron-metalloprotein that contains several bound sulfates However, only one of these sites is a high affinity binding site Most of the interactions with the sulfate oxygens come from backbone amide nitrogens

  18. Anion binding proteinsModA and SBP The anion binding sites of ModA and SBP are quite similar However, the binding specificity of these sites are quite different ModA has over a 1000-fold preference for molybdate over sulfate SBP has over a 20,000-fold preference for sulfate over molybdate Neither protein shows a high affinity for phosphate

  19. Summary • Metal ions can fulfill a variety of functions in proteins • Metal ions are essential for carbohydrate binding in lectins • Metalloproteins are responsible for oxygen transport in all aerobic species, but different types of metals and metal ion binding sites are used • Protein binding sites can distinguish between even closely related anions, such as sulfate, phosphate and molybdate

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