Allosteric Models

Question 1

Allostery

Answer 1

Binding of one molecule to a protein on one site can communicate conformational changes to a distant site on another protein

Question 2

Describe hemoglobin subunit coupling

Answer 2

• Oxygen binding to one subunit of hemoglobin induces a conformational change in the neighboring subunit
• This increases the likelihood that the the other subunits will bind to O2

Question 3

What can the Hill Coefficient (nH) tell us about the cooperatively of an enzyme?

Answer 3

nH = 1: No cooperativity
nH > 1: Positive cooperativity
nH < 1: Negative cooperativity

Question 4

What are two allosteric models?

Answer 4

• MWC (all or nothing model)
• KNF (sequential model)

Question 5

MWC Model

Answer 5

• All or nothing
• Only two confirmations of the tetramer can exist (only T or R state no mixed states)
• All subunits are in either the T or R state, NOT BOTH
• Subunits undergo transitions simultaneously
• Ligand (O2) can bind to both T and R state but has a much higher affinity for R state
• Each successive ligand binding increases the likelihood of a T to R transition

Question 6

KNF Model

Answer 6

• Sequential
• Ligand (O2) can bind to both T and R state
• Subunits can be in different transition states
• Each successive ligand binding increases the likelihood of a T to R transition
• Existence of many mixed state tetramer conformations
• Different number of equilibriums are possible

Question 7

Which model ( concerted or sequential) is correct?

Answer 7

• Both are equally valid
• Concerted model: simple, can be described by only two parameters (affinity of T and R state for O2 and position of equilibrium in absence of ligand)
• Sequential model: compatible with negative cooperativity (which there is evidence for)

Question 8

How does hemoglobin transport H+ and CO2?

Answer 8

Bohr Effect

Question 9

Describe the Bohr Effect

Answer 9

• Hemoglobin has different affinities for O2 at different PH’s
• Low PH = low affinity for O2
• High PH = high affinity for O2
• PH of tissues is more acidic than PH of lungs (because of carbonic anhydrase)
  Hemoglobin is more likely to release O2 in tissues and pick up O2 in lungs

Question 10

Describe histidine in in both he oxy and deoxy state

Answer 10

Deoxy state: His HC3 forms ionic interactions with ASP FG1
Oxy state: His must be protonated to promote formation of ionic bond which promotes T state
PH change from lungs to tissue favors the deprotonation of His HC3 (ionic bonds break) and R state to T state transition occurs

Question 11

What allows hemoglobin to bind to O2 reversibly?

Answer 11

Histidine

Question 12

How does hemoglobin transport CO2 from our tissues to our lungs?

Answer 12

• Amino terminal residue of hemoglobin reacts with CO2
• This forms a new ion on the terminus of hemoglobin (stabilizing T state) and releasing O2 in the tissues
• Hemoglobin goes back to the lungs and transitions to the R state which causes CO2 to be released

Question 13

Describe BPG

Answer 13

• Stabilizes T state
• Prevents transition to R state
• BPG allows us to adapt to different elevations where O2 concentrations are different
• BPG binds to cavity present in T state hemoglobin
• Transition to R state removes cavity and blocks the ability of BPG binding

Question 14

Describe the fractional binding curve of BPG

Answer 14

At sea level:
- Concentrations O2 = high in lungs and low in tissue (hemoglobin is almost entirely in R state)

High elevations:
- Concentration of O2 = low (hemoglobin is not fully bound to O2 in lungs)
- O2 transport capacity is reduced (less O2 is delivered in tissues)
- Concentration of BPG increases
- BPG binds to and stabilizes T state resisting transition to R state
- Less O2 is picked up in the lungs / more O2 is being released in tissues