Metal Ion Transport and Storage - PowerPoint PPT Presentation

metal ion transport and storage n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Metal Ion Transport and Storage PowerPoint Presentation
Download Presentation
Metal Ion Transport and Storage

play fullscreen
1 / 53
Metal Ion Transport and Storage
404 Views
Download Presentation
january
Download Presentation

Metal Ion Transport and Storage

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. Metal Ion Transport and Storage Tim Hubin March 3, 1998

  2. References • J. J. R. Frausto da Silva and R. J. P. Williams The Biological Chemistry of the Elements, Clarendon Press, Oxford, 1991. • J. A. Cowan Inorganic Biochemistry: An Introduction VCH Publishers, 1994. • S. J. Lippard and J. M. Berg Principles of Bioinorganic Chemistry, University Science Books, 1994. • M. D. Yudkin and R. E. Offord A Guidebook to Biochemistry, Cambridge University Press, 1980. • CHM 986, Spring 1997, Prof. Grover Everett, University of Kansas.

  3. Outline • General Concepts • Abundance of Metal Ions in Biology • Challenges in Transport and Storage of Metal Ions • Membrane Transport • Specific Metal Ions • Sodium and Potassium • Calcium • Iron • Copper • Zinc

  4. Need for Metal Ions • Metal ions must be obtained for growth and development

  5. General Transport/Storage Problems • Capture of Trace Ions from the Environment • Homeostatic Control of Concentration is essential for life • Bulk ions present in high concentration • Trace ions must be actively accumulated • Trace ions are often insoluble • Selectivity of Ion Uptake is Essential • Toxic ions must be excluded • Beneficial ions must be accumulated • Specialized Molecules have evolved

  6. General Transport/Storage Problems • Charged Ions must pass through a Hydrophobic Membrane • Neutral gases (O2, CO2) and low charge density ions (anions) can move directly through the membrane • High charge density cations require help • Once inside the cell, metal ions must be transported to the location of their use, then released or stored for later • Release from ligand is often not trivial • Storage requires additional molecules

  7. Mechanisms for Membrane Transport • Ionophores: special carrier molecules that wrap around metal ions so they can pass through the membrane by diffusion • Ion Channels: large, membrane-spanning molecules that form a hydrophilic path for diffusion • Ion Pumps: molecules using energy to transport ions in one direction through a membrane

  8. Mechanisms for Membrane Transport • Passive Transport: moves ions down the concentration gradient, requiring no energy source • Ionophores and Ion Channels are Passive • Active Transport: moves ions against the concentration gradient, requiring energy from ATP hydrolysis • Ion Pumps are Active • Choice of Transport Mechanism • Charge • Size • Ligand Preference

  9. Sodium and Potassium • Function: • Simple Electrolytes to create potentials (along with Cl-) • Provide counter ions for DNA, membranes, etc... • Nerve action • Concentrations: [Na+] outside cells, [K+] inside cells • Inside Red Blood cells: [Na+] = 0.01 M [K+] = 0.09 M • Outside (Blood Plasma): [Na+] = 0.16 M [K+] = 0.01 M • Ion Pump is required to maintain concentration gradients

  10. Sodium and Potassium--Ionophore • Nonactin: microbial Na+ and K+ ionophore • Makes Na+ and K+ membrane soluble when complexed • Oxygen Donors can be modeled by Crown Ethers Nonactin

  11. Sodium and Potassium--Ion Channel • Gramicidin: ion channel-forming molecule • Helical peptide dimer • Hydrophobic outer surface interacts with membrane • Carbonyls and Nitrogens on inner surface can interact with cations as they pass through • Potassium selective: pore size and ligands select for K+ • Channels can be Voltage-Gated or activated by the binding of a Chemical Effector which changes the conformation • 107-108 ion/second may pass (Emem = 100 mV)

  12. Active Form Inactive Form

  13. Sodium and Potassium--Ion Pump • Na+/K+-ATPase • Membrane-Spanning Protein Ion Pump • a2b2 tetrameric 294,000 dalton protein • Conformational changes pump the ions: one conformation binds Na+ best, the other binds K+ best • Hydrolysis of ATP provides the energy for conformational changes (30% of a mammal’s ATP is used in this reaction) • Antiport transport: like charged ions are transported in opposite directions • Reversing the normal reaction can generate ATP • Reaction can occur 100 time per second 3Na+in + 2Kout+ + ATP4- + H2O 3Na+out + 2K+in + ADP3- + HPO42- + H+

  14. Calcium • Function: • Signal pathways (Ex: Muscle Contraction) • Skeletal Material • Concentration: • Outside of Cell [Ca2+] = 0.001 M • Inside Cell [Ca2+] = 10-7 M • Ca2+-ATPase in Cell Membrane controls concentration

  15. Calcium--Muscle Contraction • Muscle Cells • Sarcoplasmic Reticulum(SR): muscle cell organelle • Ca2+-ATPase pumps Ca2+ into SR to concentrations up to 0.03 M • Inside SR, Ca2+ is bound by Calsequestrin, a 40,000 dalton protein (50 Ca2+ per molecule) • Hormone induced stimulation of ion channels releases Ca2+ from the SR into the muscle cell causing contraction

  16. Calcium--Storage • CaCO3 in a protein matrix makes up egg shells and coral skeletons • Calcium Hydroxyapatite in a collagen framework is the stored form of Ca2+ in bones and teeth: Ca10(PO4)6(OH)2 • Collagen: triple helix fibrous protein • Hydroxyapatite crystallizes around the collagen • Replacement of OH- by F- prevents tooth decay because F- is a weaker base • When needed, Ca2+ can be released and reabsorbed

  17. Iron • Iron is the most abundant transition metal ion in biological systems--almost all organisms use it • Availability: • Most abundant transition metal on the Earth’s crust • Nuclear Binding Energy is maximized at 56Fe • Versatility: • Fe2+/Fe3+ • High Spin/Low Spin • Hard/Soft • Labile/Inert • Coordination Number: 4,5,6

  18. Iron--Evolution • When life began: • Reducing Atmosphere: H2, H2S, CH4, NH3---> Fe2+ used • Ksp(Fe(OH)2) = 4.9 x 10-17 [Fe2+] = 5.0 x 10-3 • After Photosynthesis: • Oxidizing Atmosphere: O2---> Fe3+ used • Ksp(Fe(OH)3) = 2.6 x 10-39 [Fe3+] = 2.6 x 10-18 • Specialized Molecules were developed to solubilize Fe3+ and protect Fe2+ from oxidation • Functions:O2 transport, electron transfer, metabolism

  19. Iron--Siderophores • Siderophores: class of bacterial ionophores specific to Fe3+ • Small molecules released into the environment • Complexation of Fe3+ solubilizes it for uptake • Ligands are Catechol and Hydroxamic Acid chelates • Enterobactins: 3 catechols • Ferrichromes: 3 hydroxamic acids, cyclic peptide • Ferrioxamines: 3 hydroxamic acids, acyclic peptide Catechol Hydroxamic Acid

  20. Iron-Enterobactin • Structure: 3 catechol chelates bound to a 12-membered ring • Kf = [Fe(ent)3-]/[Fe3+][ent6-] = 1049 • Complex anion is soluble • Spectroscopy: • UV-Vis: like [Fe(cat)33-] • D structure assigned by [Cr(ent)3-] circular dichroism • Crystal Structure: [V(ent)2-]

  21. Iron-Enterobactin • Getting Fe3+ into the cell • [Fe(ent)3-] binds to an uncharacterized receptor on cell surface • Active transport process takes the complex inside • Mechanism of iron release is still unknown • Hydrolysis of ligand • Reduction to Fe2+ would labilize ion • Ered = -750 mV vs NHE at pH = 7 • Lowering pH would facilitate reduction • Intracellular ligand

  22. Iron-Transferrin • Transferrin: Mammalian transport ab dimer protein • 80,000 dalton protein carries 2 Fe3+ ions in serum • Iron captured as Fe2+ and oxidized to Fe3+ • CO32- must bind at same time: Synergism • Taking Iron into the cell--Endocytosis

  23. Iron--Ferritin • Family of protein found in animals, plants, and bacteria • Structure: • symmetric, spherical protein coat of 24 subunits • Subunits are 175 amino acids, 18,500 daltons each • Channels on 3-fold axes are hydrophilic: iron entry • Inside surface is also hydrophilic • Inner cavity • 75 Å inner diameter holds 4500 iron atoms • Iron stored as Ferrihydrate Phosphate [(Fe(O)OH)8(FeOPO3H2) . nH2PO4] • Iron-protein interface: binding of core to protein is believed to be through oxy- or hydroxy- bridges

  24. Ferritin The Gene Pool

  25. Iron-Ferritin • Iron thought to enter as soluble Fe2+, then undergo oxidation by O2 in channels or inside the cavity • Biomineralization: synthesis of minerals by organisms • Ferritin is synthesized as needed • Normal iron load is 3-5 grams in a human • Ferritin is stored in cells in the bone marrow, liver, and spleen • Siderosis: iron overload (60 g can be accumulated) • doposits in liver, kidneys, and heart • treated by Chelation Therapy (desferrioxamine)

  26. Copper • Function • O2 transport (hemocyanin in crustacean and mollusks) • O2 activation (Cu oxidases) • electron transfer (plastocyanin) • Availability • Third most abundant transition metal ion in organisms • 300 mg in a human body • Ksp(Cu(OH)2) = 2.6 x 10-19 [Cu2+] = 2.6 x 10-5 • Solubility means less specialized transport and storage

  27. Copper--Transport • Ceruloplasmin • 132,000 dalton glycoprotien (7% carbohydrate) • Binds 95% of the Cu2+ in human plasma • 6 Cu2+ sites: 1 Type I, 1 Type II, 4 Type III

  28. Copper--Transport • Ceruloplasmin • Biological role not fully understood • transport • oxygen metabolism • Wilson’s Disease • genetic disorder of low ceruloplasmin levels • Cu2+ accumulates in the brain and liver • treated by chelation therapy (EDTA)

  29. Copper--Storage • Metallothioneins • Small (6000 dalton) metal storage protein family • 20 cysteine residues select for soft metals: • Cu+, Zn2+, Cd2+, Hg2+, Pb2+ • X-Ray structure of Cd2+/Zn2+ complex shows tetrahedrally coordinated metal clusters • Up to 20 Cu+ can bind • Mechanism of Cu+ and Zn2+ homeostasis • Detoxification by removal of soft ions: Cd2+, Hg2+, Pb2+