Hemoglobin and Myoglobin

Question 1

Ligand

Answer 1

Any molecule that can bind reversibly to a protein

Question 2

Binding site

Answer 2

Location on a protein that interacts with a ligand

Question 3

Cooperativity

Answer 3

The binding of one ligand one subunit affects the binding of other ligands to that subunit

Question 4

Positive cooperativity

Answer 4

Binding positively impacts (promotes) the binding of other ligands to subunit

Question 5

Negative cooperativity

Answer 5

Binding negatively impacts (prohibits) the binding of other ligands to subunit

Question 6

Kd

Answer 6

Describes the binding affinity of ligands and proteins

Kd = [P] * [L] / [PL]
At equilibrium

Question 7

What percentage of ligands are bound to their proteins at the Kd?

Answer 7

50%

Question 8

Heme group

Answer 8

• Not a protein
• Iron in center
• 4 portoporphyrin rings

Question 9

How many coordination sites does iron have and what are they bound to?

Answer 9

• 6 coordination sites
• 4 = portoporphyrin rings
• Proximal coordination site = for histidine
• Distal coordination site = O2

Question 10

Myoglobin structure

Answer 10

• Monomeric
• Composed of 8 alpha helices (A-H)
• Helices are connected by small loops
• 153 residues

Question 11

How does heme generate specificity for O2?

Answer 11

• Heme has a higher affinity for CO than for O2
• O2 bind to iron in bent conformation (CO binds to iron in straight conformation)
• H-bond between O2 and distal histidine on myoglobin allows it to generate specificity for O2

Question 12

Describe myoglobin binding to O2 (think of whales)

Answer 12

• At surface: O2 concentration is high and myoglobin is fully bound to O2
• When dive begins: O2 concentration is lower, myoglobin remains fully bound
• Deep diving: O2 concentration is very low, myoglobin releases O2

Question 13

Hemoglobin structure

Answer 13

• Oligomeric protein (tetrameric)
• 4 different subunits
• 2 alpha , 2 beta
• Each subunit can bind to a heme group (therefore an O2 molecule)
• each subunit has a large interface to allow communication between subunits (positive cooperativity)

Question 14

What are the 2 states of hemoglobin?

Answer 14

1. Oxy (R state): O2 = bound
2. Deoxy (T state): O2 = not bound

Question 15

T state

Answer 15

• Tense state
• Low affinity for O2
• Stabilized by multiple ionic interactions

Question 16

R state

Answer 16

• Relaxed state
• High affinity for O2

Question 17

What drives the transition from the T to the R sate?

Answer 17

The rearrangement of heme

Question 18

What is the significance of the transition of the T state to the R state?

Answer 18

Transition allows hemoglobin to be an effective transport molecule

Question 19

Describe the rearrangement of heme

Answer 19

• T state: heme = puckered
• R state: Iron bound to O2 moves downward into the plane of the polyporphryrin ring causing ionic interactions to break

Question 20

What would happen if you just had R state hemoglobin?

Answer 20

Hemoglobin would pick up O2 from lungs but would NOT release O2 and transport it to tissues

Question 21

What would happen if you just had T state hemoglobin?

Answer 21

Hemoglobin would release O2 in tissues but would NOT pick up O2 from lungs