Lecture 15 Ion pumps and other transport proteins

Question 1

What is active transport and give an example.

Answer 1

• When ions must be transported against a concentration gradient, energy is required
• This energy is often obtained from the hydrolysis of
  adenosine triphosphate (ATP)
• Such ATP requiring transport processes are
  examples of:
• ACTIVE TRANSPORT
• An example of such a process is the transport of
  Sodium (Na+) and potassium (K+) ions by:
• The sodium/potassium (Na+/K+) ATPase

Question 2

Describe the sodium/potassium ion pump.

Answer 2

• The sodium/potassium (Na+/K+) ATPase
• It is located in the plasma membrane
• It consists of two copies each of two types of subunit, α and β
• It is glycosylated (i.e. has covalently bound sugars)
• It transports 3 Na+ and 2 K+ in opposite directions = Net ONE POSITIVE CHARGE across the membrane
• It belongs to a family of related pumps in different species
• The P-type ATPases
• They are named because they form a key
  phosphorylation intermediate

Question 3

What is the general structure of the P-type ATPase family example, sarcoplasmic reticulum Ca2+ATPase (SERCA)?

Answer 3

• An example in this family is the sarcoplasmic reticulum Ca2+ ATPase (SERCA)
• It takes Ca2+ from the cytoplasm to the sarcoplasmic
  reticulum of muscle cells
• The overall structure of SERCA is:
• A large cytoplasmic headpiece which has in it
• The N domain = Binds ATP
• The P domain = Accepts a phosphate onto
  a key aspartate residue
• The A domain = The “actuator” region linking changes in the N and P domains to the transmembrane part of the enzyme

Question 4

Elaborate on the structure and function of SERCA.

Answer 4

• The transmembrane domain part of the enzyme, consists of 10 alpha helices
• Structures exist for FIVE different states of this pump which provide us with knowledge of the mechanism
• There are major structural changes combined with the
  mechanism of SERCA
• The transmembrane domain can bind Ca2+ ions
• Structurally well organised when ONLY Ca2+ bound
• The binding regions are OPEN to the cytoplasm
• Structurally poorly organised when the aspartate is phosphorylated and NO Ca2+ bound
• The binding regions are now OPEN to the other side of the membrane (the non-cytoplasmic side)
• Additionally = Major rearrangements of N, P and A domains are now present
• N and P are now wrapped around the phosphorylated Asp site
• A has undergone a significant rotation
• The mechanism has complexity but the overall process moves the Ca2+ ions across the membrane

Question 5

What are carrier proteins?

Answer 5

• Biological transport processes must obviously involve more than just ions
• Proteins that perform such processes are often referred to as carrier proteins
• These do NOT need ATP to function
• They pass a limited number of molecules across the membrane in one cycle of function

Question 6

What is secondary active transport?

Answer 6

• Carrier proteins can couple a thermodynamically
  unfavourable process with a thermodynamically favourable processes
• Unfavourable = UP a concentration gradient
• Favourable = DOWN a concentration gradient
• This is called SECONDARY ACTIVE TRANSPORT

Question 7

What types of secondary active transport exist?

Answer 7

• Two (main) types:
• Antiporters = Coupled DOWN of one component with UP of another in OPPOSITE DIRECTIONS
• Symporters = Coupled DOWN of one component with UP of another in the SAME DIRECTION
• Also have:
• Uniporters = The transport of ONE component, the DIRECTION being governed by its own concentration gradient (few cases)

Question 8

What is lactose permease?

Answer 8

• Many bacteria can take up metabolites from the surrounding medium and transport them into their cells
• The Lactose Permease of E.coli transports the sugar lactose across the bacterial cell membrane
• Note that this process relies on the gradient of protons (H+) across the membrane (the DOWN component)
• The mechanism of Lactose Permease can be considered as a SIX STEP process

Question 9

Describe the six step process of lactose permease

Answer 9

STEP ONE:
- Initially: The binding pocket faces the OUTSIDE of the cell
- A proton binds from OUTSIDE the cell
STEP TWO:
- When protonated, Lactose is then bound from OUTSIDE the cell
STEP THREE:
- The structure EVERTS to allow the movement of the Lactose across the membrane together with the proton
STEP FOUR:
- Lactose is released INSIDE the cell
STEP FIVE:
- The proton is released INSIDE the cell
STEP SIX:
- The structure everts to complete the cycle which makes the binding pocket face the OUTSIDE of the cell again

Question 10

Describe another way of ion transport and give an example.

Answer 10

• Transport of ions can even be driven by light
• Bacteriorhodopsin is a protein found in specialised membranes of the bacterium Halobacterium halobium
• It uses light induced changes in the isomerization of a non-protein pigment molecule, RETINAL, to transport protons (H+)
• Bacteriorhodopsin consists of seven transmembrane protein helices, connected principally by unstructured loops of polypeptide:
• The retinal cofactor is attached via LYSINE 216
• The protons are transported via a series of
  ASPARTIC ACID RESIDUES (sequence numbers
  82, 85 and 96) and the nitrogen of Lysine 216
• The absorption spectrum changes with the isomerization and protonation of the retinal and allows the progress of the reaction to be followed