Lecture 9: Membrane protein function

What are the main functions of membrane proteins

Needed for movement of water-soluble, organic molecules and inorganic ions that aren’t able to diffuse into the membrane

Which type of membrane proteins allows solutes to cross, by which processes

Carrier/transport, by active/passive transport

What is used to transport ions

Ion channels (ether selective or gated)

What are the two classes of membrane transport protein

Transporters and channel proteins

What are the key characteristics of transporter proteins

Have a solute binding site, and do not have a continuous channel for the membrane

What are the key characteristics of a channel protein

Channel allows passive diffusion of ions and can open and close to allow the passage of specific ions

What are the 3 types of transport pumps

Uniport, symport, antiport

What is a uniport, why is it not a pump

Transports one type of solute across the membrane at a time

Not a pump as it is passive transport down a concentration gradient

What is a symport

Pump which moves a pair of solutes in the same direction across a membrane

What is an antiport

Pump which moves solutes in opposite directions

What mechanism of transport do both symports and antiports use

Active transport (electrochemical gradient of one solute can be used to drive the transport of the other up its concentration gradient)

Why is saying passive transport requires no energy wrong

Because it uses energy from thermal motion to drive conformational changes of membrane proteins BUT no additional energy is required

What is the process of active transport

Transport protein has to do work to drive the solute against the concentration radiant or electrochemical gradient

What two components make up the electrochemical gradient

The concentration gradient and the membrane potential make the electrochemical gradient

What is the driving force of an electrochemical gradient

Concentration difference between the inner/outer membrane, as well as attraction to membrane charge/potential

What are the three states of a transporter protein in passive transport

Outward-open (solute binds to binding site)

Occluded (solute enclosed by protein)

Inward-open (solute released into inner membrane)

What are the 3 types of active transporters

Coupled transporter

ATP-driven pump

Light-driven pump

What is a coupled transporter

Gradient driven transporter, which pumps one solute uphill (against) and another downhill (following)

What is an ATP driven pump

Uses energy released by ATP hydrolysis to drive uphill transport

What is a light-driven pump

Uses energy derived from sunlight to transport solutes (usually in bacterial cells)

What are coupled transporters most commonly seen in

Transport of Na+ not cytosolic and glucose into plasma membrane

What is the process of the transport of Na+ and glucose in animal cells

Pump oscillates between alternative states (open either side or occluded)

Because Na+ concentration is high in extracellular space, the binding site is readily occupied when facing outward

The pump has to wait for a rare glucose molecule to bind before closing, and then transport both to the open-inward state to release them into the cytosol where the Na+ readily dissociates and the glucose eventually leaves

What is the type of binding called in these transporters

Co-operative binding (binding of 1 enhances binding of the other)

What supplies the energy for a Na+/K+ pump

ATP

What is the main action of this pump

Expels Na+ out of the cell, and brings K+ in - both DOWN concentration gradients

Through multiple conformational changes, Na+/K+ binding AND hosphate group binding

Where does the phosphate from the ATP hydrolysis go

Attached directly to the pump

What is another examples of an ATPase pump that works similarly to the Na+ only one

Ca2+ in the sarcoplasmic reticulum, pumped in order to keep the cytosolic concentration of Ca+ low (makes cells more sensitive to a sudden increase)

What 2 types of glucose transporters enable gut epithelial cells to transfer glucose across the epithelial lining of the gut

Na+ entering the cell via the Na+ driven glucose symport is pumped out by Na+ pumps in the basal and lateral plasma membranes

What are the 3 types of gated ion channels

Voltage gated

Ligand gated

Mechanically gated

When does a voltage gated channel open

When there is a change in membrane potential/ voltage across membrane

Ligand gated channels can be either

Intracellular or extracellular

What is their opening controlled by

Binding of a molecule (ligand) to the channel

How does a mechanically gated channel open

Channel is pulled open by application of physical force

How do ion channels in nerve cells work

Nerve cell receives, conducts and transmits signals via changing electrical signals across plasma membrane (controlled by ion channels)

What causes the voltage-gated Na+ channels to open

Stimulus causes sufficient depolarisation of the membrane

What does the influx of Na+ cause

Further depolarisation, more Na+ channels open

What happens to the channels after

Become inactivated (to prevent membrane being permanently depolarised)

How does the channel change conformation

Voltage sensors have positive site which face negative inside of membrane = closed

depolarised membrane flips which side is more negative, so voltage sensors flip

This causes channel to open

Why does the cell become inactivated

The inactivated conformation is more stable than the open one

What is the Nernst equation

V = 62 log (Co/Ci)

V = membrane potential in millivolts

Co = Outside concentration of ion

Ci = inside concentration of ion

Assumes that ion carrie single positive charge and temp is 37oC

Why does action potential only move in one direction

The channels behind it have been inactivated

What happens to membrane patches after they have been depolarised

Return to resting state

What happens with Ca2+ channels at nerve terminals

Action potential reaches nerve terminal voltage gated Ca2+ channels open

Ca2+ enters cell

Causes fusion of vesicles with plasma membrane and release of neurotransmitters

What can defects in ion channels cause

Inherited diseases like cystic fibrosis