Transport

• Solute moves to lower concentration to approach equilibrium in concentration gradients
• A transmembrane potential is created by a charge separation across the membrane. The charged particles move to region of opposite charge
• The electrochemical gradient is the combination of the concentration gradient and the transmembrane potential
• The concentration of the molecules being transported and the nature of the process is what determines rate or kinetics of transport
• Small non-polar molecules diffuse freely
• Inophores and channels allow molecules to move based on concentration gradients
• Carriers move molecules and the rate depends on gradients involved and transporter kinetics for active and passive transporters
• Enzymes work to reduce the large energy barrier
• Active transport moves solute against the gradient. Change in free energy is greater than 0 so have to have energy input with exergonic process. have two types Primary and Secondary
• Primary Active transport: Exergonic chemical reaction
• Exergonic chemical reaction is an energy favourable reaction the change in free energy or energy input provides enough to make it favourable
• Secondary Active transport is exergonic solute ion transport meaning it uses another solutes. The sum of both the solute and second aspect have to be negative
• Passive transport have solute movement determined by electrochemical gradient with change in free energy negative. May be specific or non-specific depending on transporter structure
• Ionophores are small peptide like molecules that shuttle ions across the membrane down the concentration gradient. Also called Facilitated Diffusion. Can have Channel ionophores or carrier ionophores
• Ionophores actions destroy transmembrane electrochemical gradients affecting secondary active transport processes. The destruction can be lethal as it prevents organisms from carrying out necessary processes
• Valinomycin is a neutral peptide like carrier ionophore specific to K+ ions. Six carbonyl groups form stable interactions with K+ ions. It is lipid soluble bound and unbound so it can move across the membrane and is poisonous to any cell
• Valinomycin likes to coordinate with 6 H2O but can replace all water interactions with comparable reactions
• Gramicidin is a peptide based channel ionophore that allows K+ to flow freely. A, B and C are linear structures that form beta helix by alternating L and D amino acids. Form a dimer that creates membrane spanning channel. It is a Monovalent cation ionophore
• Gramicidin B alternates D and L amino acids curving its polypeptide backbone to a helix. All side chains are orientated out to hydrophobic core. The carbonyl group forms interactions that replace bond with water
• Porins are beta barrel transmembrane proteins that can be selective or non selective. They have hydrophobic outside inside the membrane and aqueous internal channel that is polar with polar amino acids
• Maltoporin transports maltose which is disaccharide of D-glucose. It interacts with OH groups
• Maltoporin is:

  • Homotrimer
  • C3 symmetry
  • each subunit has 18 stranded beta barrel
  • Openings have left handed curvature
• Ion-selective Channel:

  • Allows for rapid ion movememnt across the membrane
  • Is a passive transport process if its not gated
  • Channel selectivity due to functional group interactions with specific molecules or other molecules that are repelled
  • Highly selective
  • Motion is down concentration gradient and rate that is determined by free diffusion
• K+ channel:

  • 4 identical subunits- 2 transmembrane helices and one core helix
  • The channel is formed between subunits
  • Is single pass channel spanning to either bilayer with many K+ binding sites along inside
• The K+ channel allows for ion binding because openings on either end are negatively charged. The third helix subunit is orientated with negative dipole towards channel opening to draw in the K+
• A molecule binds to the first of the 4 K+ binding sites and move downward each time a new ion enters alternating binding sites.
• Chloride Channelis an:

  • Anion Channel with the channel through center of each subunit
  • Homodimer structure
  • 18 transmembrane alpha helices tilted relative to membrane
  • Selectivity filter is created by alpha helices N-terminal dipoles and hydroxyl containing amino acids like Serine or Tyrosine. Not positive charged
  • If there was a positive charge interaction it would stop going through channel due to attraction
• Aquaporin:

  • allows for cross-membrane movement of water but excludes solutes and Hydronium ions
  • Homotetramer with multiple transmembrane helices
  • Has water channel at center of each subunit
  • Hydrogen bonding interactions in the channel prevent proton jumping movement
• The aquaporin has size restrictions, water dipole re-orientation and electrostatic repulsion by positively charged amino acids
• Glucose transporter of erythrocytes GLUT1:

  • Polytopic membrane protein
  • Major facilitator superfamily
  • 12 transmembrane helices and contains polar amino acids
  • Passive carrier for D-glucose
  • Faster movement across membrane than simple unassisted diffusion
• GLUT1 has glucose bind to central cavity midway due to conformation change that occurs
• In helixes polar residues and non polar residues are on opposite faces
• Groups of helices can create a polar area near the center and non polar regions orientate toward lipid core
• Conformational shift in GLUT1- Rocking Banana

  • It is carrier protein so doesn't have continuous passage
  • Binding of glucose and a conformational shift allow binding site to open on the interior
  • Transport depends on rate of each step with a Kt similar to Km
  • Glucose binds to middle of GLUT1 outside cell and it has conformational change that allows it to be released into cell and then it flips back to outside to allow more to bind
• GLUT1 has specificity to D-glucose if you test the rates of diffusion with epimers it is nowhere near as fast
• For primary active transport negative free energy comes from chemical reaction but secondary active transport uses another solute
• P-type ATPases

  • Primary active transporter
  • Hydrolysis of ATP gives enough energy to move solute up concentration gradient
  • Flippases, proton pumps etc
• Serca pump:

  • Sarcoplasmic and Endoplasmic Reticulum Calcium pumps
  • Ca2+ ATPase
  • Transports calcium out of cytoplasm and into Sarcoplasmic reticulum during muscle relaxation
• Sarcoplasmic Reticulum Ca2+ pump has 4 domains:

  • Transmembrane M domain- has 10 transmembrane helixes and 2 calcium binding sites near center
  • Phosphorylation P domain- ATP-binding domains
  • Nucleotide binding N domain- Asp side chain phosphorylated by ATP induces a conformational change to transmembrane domain
  • Actuator A domain- connects conformational change between N and P domains to M
• SERCA pump mechanism:

  • Has two conformations E1 that has high affinity for calcium and is cytosol exposed (inside) and E2 that has low affinity to calcium and is lumen exposed (outside)
  • E1 releases ADP when 2 Ca2+ bind and form high energy intermediate
  • Conformational change occurs making it E2 and 2 Ca2+ leave
  • Hydrolysis occurs to dephosphoylate E2 but occurs in cytosol and ATP then binds and repeats