Biochemistry 2e Garrett Grisham Chapter 10 Membrane Transport

Biochemistry 2/e - Garrett & Grisham Chapter 10 Membrane Transport to accompany Biochemistry, 2/e by Reginald Garrett and Charles Grisham All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887 -6777 Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Outline • • • 10. 1 Passive Diffusion 10. 2 Facilitated Diffusion 10. 3 Active Transport 10. 4 - 10. 6 Transport Driven by ATP, light, etc. 10. 7 Group Translocation 10. 8 Specialized Membrane Pores • 10. 9 Ionophore Antibiotics Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Passive Diffusion No special proteins needed • Transported species simply moves down its concentration gradient - from high [c] to low [c] • Be able to use Eq. 10. 1 and 10. 2 • High permeability coefficients usually mean that passive diffusion is not the whole story Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Facilitated Diffusion G negative, but proteins assist • Solutes only move in thermodynamically favored direction • But proteins may "facilitate" transport, increasing the rates of transport • Understand plots in Figure 10. 3 • Two important distinguising features: – solute flows only in the favored direction – transport displays saturation kinetics Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Active Transport Systems • • Energy input drives transport Some transport must occur such that solutes flow against thermodynamic potential Energy input drives transport Energy source and transport machinery are "coupled" Energy source may be ATP, light or a concentration gradient Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The Sodium Pump • • • aka Na, K-ATPase Large protein - 120 k. D and 35 k. D subunits Maintains intracellular Na low and K high Crucial for all organs, but especially for neural tissue and the brain ATP hydrolysis drives Na out and K in Alpha subunit has ten transmembrane helices with large cytoplasmic domain Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Na, K Transport • ATP hydrolysis occurs via an E-P intermediate • Mechanism involves two enzyme conformations known as E 1 and E 2 • Cardiac glycosides inhibit by binding to outside Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Na, K Transport • Hypertension involves apparent inhibition of sodium pump. Inhibition in cells lining blood • Vessel walls results in Na, Ca accumulation • Studies show this inhibitor to be ouabain! Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Calcium Transport in Muscle A process akin to Na, K transport • Calcium levels in resting muscle cytoplasm are maintained low by Ca-ATPase - a Ca pump • Calcium is pumped into the sarcoplasmic reticulum (SR) by a 110 k. D protein that is very similar to the alpha subunit of Na, K-ATPase • Aspartyl phosphate E-P intermediate is at Asp 351 and Ca-pump also fits the E 1 -E 2 model Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The Gastric H, K-ATPase The enzyme that keeps the stomach at p. H 0. 8 • The parietal cells of the gastric mucosa (lining of the stomach) have an internal p. H of 7. 4 • H, K-ATPase pumps protons from these cells into the stomach to maintain a p. H difference across a single plasma membrane of 6. 6! Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The Gastric H, K-ATPase • This is the largest concentration gradient across a membrane in eukaryotic organisms! • H, K-ATPase is similar in many respects to Na, K-ATPase and Ca-ATPase Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Osteoclast Proton Pumps How your body takes your bones apart! • Bone material undergoes ongoing remodeling – osteoclasts tear down bone tissue – osteoblasts build it back up • Osteoclasts function by secreting acid into the space between the osteoclast membrane and the bone surface - acid dissolves the Caphosphate matrix of the bone • An ATP-driven proton pump in the membrane does this! • Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The MDR ATPase aka the P-glycoprotein • Animal cells have a transport system that is designed to recognize foreign organic molecules • This organic molecule pump recognizes a broad variety of molecules and transports them out of the cell using the hydrolytic energy of ATP Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The MDR ATPase • MDR ATPase is a member of a "superfamily" of genes/proteins that appear to have arisen as a "tandem repeat" • MDR ATPase defeats efforts of chemotherapy Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Light-Driven H + Transport The Bacteriorhodopsin story • Halobacterium halobium, the salt-loving bacterium, carries out normal respiration if O 2 and substrates are plentiful • But when substrates are lacking, it can survive by using bacteriorhodopsin and halorhodopsin to capture light energy • Purple patches of H. halobium are 75% b. R and 25% lipid - a "2 D crystal" of b. R - ideal for structural studies Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Bacteriorhodopsin Protein opsin and retinal chromophore • Retinal is bound to opsin via a Schiff base link • The Schiff base (at Lys-216) can be protonated, and this site is one of the sites that participate in H+ transport Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Bacteriorhodopsin • Lys-216 is buried in the middle of the 7 TMS structure of b. R, and retinal lies mostly parallel to the membrane and between the helices • Light absorption converts all-trans retinal to 13 -cis configuration - see Figure 10. 22 Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Bacteriorhodopsin The protons visit the aspartates. . • Asp-85 and Asp-96 lie on opposite sides of a membrane-spanning helix • These remarkable aspartates have p. Ka values around 11! (Why? ) • Protons are driven from Asp-96 to the Schiff base at Lys-216 to Asp-85 and out of the cell Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Halorhodopsin • Halorhodopsin transports Cl - instead of H + • Halorhodopsin has Lys-242 Schiff base but no aspartates and no deprotonation of Schiff base during the transport cycle Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Secondary Active Transport processes driven by ion gradients • Many amino acids and sugars are accumulated by cells in transport processes driven by ion gradients Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Secondary Active Transport • Symport - ion and the amino acid or sugar are transported in the same direction across the membrane • Antiport - ion and transported species move in opposite directions • Several examples are described in Table 10. 2 Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Group Translocation The phosphotransferase system (PTS) • Discovered by Saul Roseman in 1964 • Sugars are phosphorylated from PEP during transport into E. coli cells • Four proteins required: EI, HPr, EII, and EIII Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Group Translocation • EI and HPr are universal and work for all sugars • EII and EIII are specific for each sugar • Mechanism involves transfer of P from PEP to EI and then to HPr and then to 2 sites on EIII and then finally phosphorylation of sugar Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Porins • • • Found both in Gram-negative bacteria and in mitochondrial outer membrane Porins are pore-forming proteins - 30 -50 k. D General or specific - exclusion limits 600 -6000 Most arrange in membrane as trimers High homology between various porins Porin from Rhodobacter capsulatus has 16 stranded beta barrel that traverses the membrane to form the pore (with eyelet!) Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Why Beta Sheets? • • for membrane proteins? ? Genetic economy Alpha helix requires 21 -25 residues per transmembrane strand Beta-strand requires only 9 -11 residues per transmembrane strand Thus, with beta strands , a given amount of genetic material can make a larger number of trans-membrane segments Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The Pore-Forming Toxins • Lethal molecules produced by many organisms • They insert themselves into the host cell plasma membrane • They kill by collapsing ion gradients, facilitating entry by toxic agents, or introducing a harmful catalytic activity Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Colicins • Produced by E. coli • Inhibit growth of other bacteria (even other strains of E. coli) • Single colicin molecule can kill a host! • Three domains: translocation (T), receptor-binding (R), and channelforming (C) Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Clues to Channel Formation • C-domain: 10 -helix bundle, with H 8 and H 9 forming a hydrophobic hairpin • Other helices amphipathic (Fig. 10. 30) • H 8 and H 9 insert, with others splayed on the membrane surface • A transmembrane potential causes the amphipathic helices to insert! Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Other Pore-Forming Toxins • Delta endotoxin also possesses a helixbundle and may work the same way • There are other mechanisms at work in other toxins • Hemolysin from Staphylococcus aureus forms a symmetrical pore • Aerolysin may form a heptameric pore with each monomer providing 3 beta strands to a membrane-spanning barrel Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Amphiphilic Helices form Transmembrane Ion Channels • Many natural peptides form oligomeric transmembrane channels • The peptides form amphiphilic -helices • Aggregates of these helices form channels that have a hydrophobic surface and a polar center • Melittin (bee venom), magainins (frogs) and cecropin (from cecropia moths) are examples Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Amphipathic Helices • Melittin - bee venom toxin - 26 residues • Cecropin A - cecropia moths - 37 residues • Magainin 2 amide - frogs - 23 residues • See Figure 10. 35 to appreciate helical wheel presentation of the amphipathic helix Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The Magainin Peptides • Discovered by Michael Zasloff • He noticed that incisions on Xenopus laevis (African clawed frog) healed without infection, even in bacteria-filled aquarium water • He deduced that the frogs produced a substance that protected them from infection! Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The Cecropins • Produced by Hyalophora cecropia (the cecropia moth - see Figure 10. 36) • Induced when the moth is challenged by bacterial infections • These peptides are thought to form helical aggregates in membranes, creating an ion channel in the center of the aggregate Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Gap Junctions • • Vital connections for animal cells Provide metabolic connections Provide a means of chemical transfer Provide a means of communication Permit large number of cells to act in synchrony Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Gap Junctions • Hexameric arrays of a single 32 k. D protein • Subunits are tilted with respect to central axis • Pore in center can be opened or closed by the tilting of the subunits, e. g. as response to stress Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Ionophore Antibiotics Mobile carrier or pore (channel) • How to distinguish? Temperature! • Pores will not be greatly affected by temperature, so transport rates are approximately constant over large temperature ranges • Carriers depend on the fluidity of the membrane, so transport rates are highly sensitive to temperature, especially near the phase transition of the membrane lipids Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Valinomycin • • • A classic mobile carrier A depsipeptide - a molecule with both peptide and ester bonds Valinomycin is a dodecadepsipeptide The structure places several carbonyl oxygens in the center of the ring structure Potassium and other ions coordinate the oxygens Valinomycin-potassium complex diffuses freely and rapid across membranes Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Selectivity of Valinomycin • • Why? K + and Rb + bind tightly, but affinities for Na + and Li + are about a thousand-fold lower Radius of the ions is one consideration Hydration is another - see page 324 for solvation energies It "costs more" energetically to desolvate Na+ and Li+ than K+ Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Gramicidin • • A classic channel ionophore Linear 15 -residue peptide - alternating D & L Structure in organic solvents is double helical Structure in water is end-to-end helical dimer Unusual helix - 6. 3 residues per turn with a central hole - 0. 4 nm or 4 A diameter • Ions migrate through the central pore Copyright © 1999 by Harcourt Brace & Company