Lecture 6: Transporters Flashcards
Understand the difference between passive transport and active transport.
Passive transport uses gradient energy. Transport with a gradient cannot be used to concentrate nutrients like active transport can. Examples are Aquaporin and glyceroporin.
Active transport has uniports that use electrical potential (negative inside) like a magnet to pull positively charged ions into the cell. It requires energy and often uses the proton motive force (PMF).
Understand the difference between primary (ATP) and secondary (PMF) active transport.
Primary active transport requires the hydrolysis of ATP to drive the movement of the different substances across a membrane. These substances move against their concentration gradients and therefore requires energy input. Example: Na-K pump.
Explain why passive transport requires energy, but cannot concentrate a molecule against its gradient.
Passive transport uses gradient energy. Gradient of the ligand causes the rock to switch from outward to inward or vice versa. If there is no gradient then there will be no net movement. Gradient energy is the only energy provided therefore it cannot move molecules against their gradients.
Understand what part of the PMF is used by uniports to concentrate a molecule against a gradient.
Uniport uses electrical potential (negative inside the cell) like a magnet to pull positively charged ions into the cell or to push negatively charged ions out of the cell. An exmaple is the ammonia-proton symport. Proteins use energy from the gradient of one ion (H+) to move another ion or uncharged solute (NH3) against its gradient.
Differentiate between symports and antiports.
Symporter moves two molecules with one another. An example is the ammonia-proton symport.
An antiporter moves two molecules against one another. An example is the ammonium-proton antiporter.
Explain how symports and antiports can use both the delta pH and the delta upsilon to concentrate a molecule against its gradient.
Electrical potential (negative inside) like a magnet to pull positively charged ions into the cell and to push negatively charged ions out of the cell. Ions will also move based on concentration (pH) of molecules. Delta pH + delta upsilon = 100 accumulation ratio.
Understand how the carefully regulated movement of protein structures is critical for protein function.
Protein structures must face inward or outward in order to bind with the correct ion. If protein is dysfunctional, the gate will not open correctly and let the ions inside or outside the cell.
Explain the rocker-switch mechanism for the function of MFS-type transporters.
Major facilitator superfamily (MFS) is when proteins used energy from the gradient of one ion (H+) to move another ion or uncharged solute against its gradient. Protons bind in order to hold the transporter in the outward facing position. The molecule or ligand will bind, destabilizing the outward conformation (breaking ionic bonds), and the transporter will switch to the inside conformation. The molecule or ligand will be released inside the cell and the switch will face back outward.
Explain the mechanism for the action of ABC importers that use a periplasmic binding protein, and explain why such transporters are essentially irreversible.
ATP binding cassette (ABC) are importers that use ATP and receive a signal from a periplasmic binding protein to open a transport channel and import a solute. Periplasmic binding proteins bind to ligand and pushes down alpha-helix and activates an ATPase. Periplasmic proteins remain bound and prevent molecules from coming back out of the cell. Conformation change of the gate allows the molecule to enter the cell and the periplasmic binding protein can leave the ligand, alpha helix goes back up and the gate closes.
Understand why PTS import of sugars is more efficient than other transporters.
The phosphotransferase system (PTS) uses a phosphate “relay” to transport glucose. Transporting and phosphorylating the ligand makes the ligand more hydrophilic and less likely to leave the cell. Glucose is made into G-6-P when it enters the cell. Increasing [G6P]in does not alter [Glu]in and more G6P is allowed to enter the cell.
Describe the 3 basic categories of export transporters: Export through both membranes simultaneously
Type 1 export allows unfolded proteins to go through the ABC transporter and through the inner membrane, periplasm, and outer membrane. It then folds outside of the cell.
Type 3 export allows unfolded proteins to go straight into the target cell. Needle complex must come in contact with target cell before excretion.
Describe the 3 basic categories of export transporters: Export with a periplasmic intermediate
The periplasmic intermediate is found in gram positive bacteria (Type 2 export). Protein goes through SecA/TAT/SecYEG. folds in periplasm, and leaves outer membrane through gap secreation.
Describe the 3 basic categories of export transporters: Export of fully folded proteins
Type 3 exporter allows proteins to go through needle complexes and straight into the target cell.
Explain the additional importance of type III secretion systems that makes them “hot topics” for study.
Proteins are only secreated once bacterium has contacted a host cell. Protein is secreated directly into host (like syringe). It may be used to inject anti-cancer proteins directly into human host cells. The presence of T3SS is often associated with pathogens (Shigella, yersinia, and many others). Many components of T3SS also used in flagella (intermediate evolutionary step).
