229981 lecture 25
- 1. HBC1011 Biochemistry I Lecture 25 – Lipids and Cell Membranes Ng Chong Han, PhD ITAR1010, 06-2523751 chng@mmu.edu.my
- 2. Overview • Active and passive molecule transport across a membrane • P-type ATPase and ATP-binding cassette (ABC) transporter/pump • Ion channels • Gap junctions & aquaporin 2
- 3. Introduction • Lipid bilayer is intrinsically impermeable to ions to polar molecules, yet certain molecules must be able to enter for cellular functions. • Permeability is conferred by membrane proteins, eg pumps/transporter and channels. 3
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- 5. Protein-free lipid bilayers are impermeable to ions • The rate of molecule diffusion depends partly on the size of the molecule but mostly on its relative hydrophobicity. • In general, the smaller the molecule and the more hydrophobic it is, the more easily it will diffuse across a lipid bilayer. • Small nonpolar molecules, such as O2 and CO2, readily dissolve in lipid bilayers and therefore diffuse rapidly across them. Small uncharged polar molecules, such as water or urea, also diffuse across a bilayer, albeit much more slower. 5
- 6. Transport of molecules across a membrane may be active/passive • Many molecules require protein transporters to cross membranes. • 2 factors determine whether a molecule will cross a membrane: 1. Permeability of the molecule in a lipid bilayer - The molecule must be able to cross a hydrophobic barrier. 2. Availability of energy source - An energy source must power the movement 6
- 7. Membrane Transport Proteins: Carriers and Channels • Transporters (also called carriers, or permeases) bind the specific solute to be transported and undergo a series of conformational changes that facilitate the molecule transport. • Channels interact with the solute to be transported much more weakly. They form continuous pores that extend across the lipid bilayer. When open, these pores allow specific solutes (such as inorganic ions) to pass through them. Transport through channels occurs at a much faster rate than transport mediated by transporters. 7
- 8. Differences between channels and transporters (a) In an ion channel, a transmembrane pore is either open or closed, depending on the position of the single gate. When it is open, ions move through at a rate limited only by the maximum rate of diffusion. 8
- 9. Differences between channels and transporters (b) Transporters have two gates, and both are never open at the same time. Movement of a substrate through the membrane is therefore limited by the time needed for one gate to open and close and the second gate to open. Rates of movement through ion channels can be orders of magnitude greater than rates through transporters, but channels simply allow the ion to flow down the electrochemical gradient, whereas active transporters can move a substrate against its concentration gradient. 9
- 10. Transporters can function as uniports, symports, or antiports. • Transporters that carry a single solute across the membrane are called uniports. transporters that move multiple solutes are called coupled transporters. In coupled transport, the solutes can be transferred either in the same direction, by symports, or in the opposite direction, by antiports. Uniports, symports, and antiports can be used for either passive or active transport. Some coupled transporters, for example, act as pumps, coupling the uphill transport of one solute to the downhill transport of another. 10
- 11. Transport of molecules across a membrane may be active/passive 11 Some small nonpolar molecules such as CO2 can move passively down their concentration gradient across the lipid bilayer by simple diffusion, without the help of a transport protein. Most solutes, however, require the assistance of a channel or transporter. Passive transport, which allows molecules to move down their concentration gradients, occurs spontaneously; whereas active transport against a concentration gradient requires an input of energy. Only transporters can carry out active transport.
- 12. Ion gradient 12 The electrochemical gradient of a charged solute (an ion) affects its transport. This gradient combines the membrane potential and the concentration gradient of the solute. The electrical and chemical gradients can work additively to increase the driving force on an ion across the membrane (middle, eg Na+) or can work against each other (right, eg K+).
- 13. Concentration gradient, membrane potential and electrochemical gradient 13 • Membrane potential/membrane voltage: the difference in electric potential between the interior and the exterior of a biological cell. • Concentration gradient: The movement of solutes through a membrane from an area of higher concentration to an area of lower concentration (energy is generated). Electrochemical gradient = concentration gradient + membrane potential
- 14. Passive transport • Passive transport is a movement of ions and other molecules across cell membranes without need of energy input. • Unlike active transport, it does not require an input of cellular energy because the substance go down its concentration gradient across a membrane, eg glucose transporters. 14 channel transporter Passive transport • Simple diffusion • Facilitated diffusion
- 15. Passive transport - Simple diffusion • Some molecules can pass through cell membranes without interacting with another molecule. • Dissolve in lipid bilayer: lipophilic molecule, i.e.: steroid hormones (cholesterol) • Pass through a membrane down their concentration gradient: simple diffusion. • Molecule spontaneously move from a region of higher concentration to one of lower concentration. 15
- 16. Passive transport - Facilitated diffusion • Uniporter can be either ion channels or carrier proteins • Uniporter carrier proteins transport one molecule across a membrane down its concentration gradient. • Uniporter channels open in response to a stimulus and allow the free flow of specific molecules. • Both rely on passive transport, as they do not directly require cellular energy to function. 16
- 17. Active transport: Ion-concentration gradients • Active transport occur when a molecule moves against a concentration gradient with external energy source. The electrochemical potential measures the combined ability of a concentration gradient and an uneven distribution of charge to drive species across a membrane. 17 Orange: Actively transported molecules Red: Energy source
- 18. Active transport: Ion-concentration gradients • Coupled transporters harness the energy stored in concentration gradients to couple the uphill transport of one solute across the membrane to the downhill transport of another. • ATP-driven pumps couple uphill transport to the hydrolysis of ATP. • Light- or redox-driven pumps, which are known in bacteria, archaea, mitochondria, and chloroplasts, couple uphill transport to an input of energy from light, as with bacteriorhodopsin. 18 Orange: Actively transported molecules Red: Energy source
- 19. Primary active transport • Primary active transport: use energy source from ATP hydrolysis or light absorption to directly drive the transport of a solute against its concentration gradient • Undergo sequential conformational changes to transport specific small molecules across membranes, • Energy transducer, convert one form of free energy into another. • 2 types of ATP driven pumps: – P-type ATPases – ATP-binding cassette (ABC) transporters 19
- 20. Primary active transport P-type pumps are structurally and functionally related to multipass transmembrane proteins. They are called “P-type” because they phosphorylate themselves during the pumping cycle. Responsible for setting up and maintaining gradients of Na+, K+, H+, and Ca2+ across cell membranes. 20
- 21. Na+ pump in animal cells uses ATP to expel Na+ and bring in K+ • The concentration of K+ is typically 10–30 times higher inside cells than outside, whereas the reverse is true of Na+. A P-type Na+-K+ pump/Na+ -K+ ATPase, found in the plasma membrane of virtually all animal cells maintains these concentration differences. 21
- 22. Plasma membrane Na+-K+ pump • The Na+-K+ pump operates as an ATP-driven antiporter, actively pumping Na+ out of the cell against its steep electrochemical gradient and pumping K+ in. 22 Functions Controls cell volume, Renders neurons & muscle cells electrically excitable, Drives the active transport of sugars & amino acids.
- 23. Primary active transport • ABC transporters (ATP- Binding Cassette transporters) differ structurally from P-type ATPases and primarily pump small molecules across cell membranes. 23
- 24. ABC transporter • ABC transporters harvest the energy released upon ATP binding and hydrolysis to drive transport of solutes across the bilayer. The transport is directional toward inside or toward outside, depending on the particular conformational change in the solute binding site that is linked to ATP hydrolysis 24
- 25. ABC transporter • The superfamily of ABC transporters is the largest family of membrane transport proteins and is especially important clinically. Example- Proteins that are responsible for cystic fibrosis (mutation in cystic fibrosis transmembrane conductance protein – CFTR causes misregulation of ion concentration in the extracellular fluid, especially in the lung). Drug resistance in cancer cells (multidrug resistance (MDR) protein- pumping out drugs efficiently before the drugs can exert their effects ). 25
- 26. Secondary active transport • Secondary active transport: Mediate the transport of ions and small molecules across the membrane using electrochemical gradient without direct coupling of ATP. eg E. coli lactose transporter. 26 The free energy released during the movement of an ion down an electrochemical gradient is used as the driving force to pump other solutes uphill, against their electrochemical gradient.
- 27. Secondary active transport -Symporter • A glucose–Na+ symport protein uses the electrochemical Na+ gradient (produce energy, high to low) to drive the active import of glucose (low to high). Both molecules are transported in the same direction. • Example: the glucose symporter SGLT1, which co-transports one glucose molecule into the cell for every two sodium ions it imports into the cell. 27
- 28. Secondary active transport - Antiporter • Two species of molecules are pumped in opposite directions across a membrane. • One of these species move from from high to low concentration , producing energy to drive the transport of the other solute from a low concentration to a high one. • An example is the sodium- calcium exchanger or antiporter, which allows three sodium ions into the cell to transport one calcium out. 28
- 29. Specific channels can rapidly transport ions across membranes • For transport efficiency, ion channels have an advantage over transporters, in that they can pass up to 100 million ions through one open channel each second—a rate 105 times greater than the fastest rate of transport mediated by any known transporter. • However, channels cannot be coupled to an energy source to perform active transport, so the transport they mediate is always passive. • Thus, the function of ion channels is to allow specific inorganic ions—primarily Na+, K+, Ca2+, or Cl–—to diffuse rapidly down their electrochemical gradients across the lipid bilayer. • Examples: Gap Junction and Aquaporin 29
- 30. Ion channels are ion-selective and gated Properties: Ion selectivity and Gated channel • Ion selectivity - Selectivity filter permits some inorganic ions to pass, but not others. This suggests that their pores must be narrow enough in places to force permeating ions to shed most of their associated water molecules so that only ions of appropriate size and charge can pass. Thus, as the ion concentration increases, the flux of the ion through a channel increases proportionally but then levels off (saturates) at a maximum rate. 30
- 31. Ion channels are ion-selective and gated • Ion channels are not continuously open. Instead, they are gated, which allows them to open briefly and then close again. • Moreover, with prolonged stimulation, most ion channels go into a closed “desensitized,” or “inactivated,” state, in which they are refractory (resistant) to further opening until the stimulus has been removed. In most cases, the gate opens in response to a specific stimulus. 31 Eg. Olfactory fatigue is the temporary, normal inability to distinguish a particular odor after a prolonged exposure to that airborne compound due to desensitization of a cation channel.
- 32. Different types of gated ion channels respond to different types of stimuli Depending on the type of channel, the probability of gate opening is controlled by (a) a change in the voltage difference across the membrane, (B) the binding of a chemical ligand to the extracellular face of a channel, (c) ligand binding to the intracellular face of a channel, or (D) mechanical stress. 32
- 33. Potassium (K+) channel • Many ion channels have a common structural framework. In regards to K+ channels, hydrated potassium ions must transiently lose their coordinated water molecules as they move to the narrowest part of the channel, termed the selectivity filter. 33 In the selectivity filter, peptide carbonyl groups coordinate the ions. Rapid ion flow through the selectivity filter is facilitated by ion-ion repulsion, with one ion pushing the next ion through the channel.
- 34. Potassium (K+) channel • Many channels spontaneously inactivate after having been open for a short period of time. • Some ion channels are voltage gated: changes in the membrane potential induce conformational changes that open these channels. 34 Voltage-gated channels work in concert to generate action potentials.
- 35. Aquaporins are permeable to water but impermeable to ions • In addition to the direct diffusion of water across the lipid bilayer, some prokaryotic and eukaryotic cells have water channels, or aquaporins, embedded in their plasma membrane to allow water to move more rapidly. 35 Aquaporins are particularly abundant in animal cells that must transport water at high rates, such as the epithelial cells of the kidney.
- 36. Aquaporins are permeable to water but impermeable to ions • Aquaporins do not transport protons. • To avoid disrupting ion gradients across membranes, they have to allow the rapid passage of water molecules while completely blocking the passage of ions. 36
- 37. Gap junction • Cell junction with adjacent cells that are connected through protein channels. These channels connect the cytoplasm of each cell and allow molecules, ions, and electrical signals to pass between them. • Small hydrophilic molecules/ions can pass through. • Inorganic ions & most metabolites (sugars, a.a, nucleotides) can flow between the interiors of cell joined by gap junctions. • Proteins, nucleic acids and polysaccharides are too large to traverse these channels. 37
- 38. Gap junction Functions: – allows for direct cell-to-cell communication without having to go through the extracellular fluid surrounding the cells, eg. cells in some excitable tissues: heart muscle • Essential for the nourishment of cells that are distant from blood vessels: lens & bone. 38 Communicating channels are important in development and differentiation.
- 39. Summary 1. The transport of molecules across a membrane may be active or passive. 2. Two families of membrane proteins use ATP hydrolysis to pump ions across membranes. 3. Secondary transporters use one concentration gradient to power the formation of another. 4. Specific channels can rapidly transport ions across membranes. 5. Gap junctions allow ions and small molecules to flow between communicating cells. 6. Aquaporin increase the permeability of some membranes to water. 39