L2- Hemoglobin and Myoglobin

Question 1

Make up of Heme?

Answer 1

Fe2+ form bonds with:

• 4 nitrogen atoms of protophorphyrin
• histidine residue
• oxygen

Question 2

Myoglobin Function ______?

Answer 2

found in heart and skeletal muscle

• reservoir for oxygen
• nitric oxide metabolism

Question 3

Myoglobin Structure______?

Answer 3

• compact folded structure
• 8 stretches of a-helices (A-H)
• interior is hydrophobic
• charged amino acids on surface of molecule (which can form hydrogen bonds)

Question 4

How does Heme fit into myoglobin?

Answer 4

F helix (8th residue), binds directly to the Fe2+ of the heme

• E helix (7th residue), stabilizes the binding of oxygen to the fe2+
• O2, CO, NO and H2S can all bind to heme

Question 5

Hemoglobin Structure?

Answer 5

• tetramer of four subunits

- each subunit is similar to myoglobin

Question 6

Different dimers of Hb?

Answer 6

• most common is HbA (a2b2)
• one a globin + one b globin form a dimer
  - > strong hydrophobic interactions
• two identical dimers form the tetramer
  - > via ionic and hydrogen bonds

Question 7

Hb dissociation curve is what shape?

Answer 7

• sigmoidal
• at the lungs, saturation is 95%
• in the tissues, saturation is 50%

Question 8

Mb dissociation curve?

Answer 8

• hyperbolic
• at the lungs, saturation is 95%
• at the tissues, saturation is 90%
• only when there is a low pO2, does Mb release significant O2

Question 9

What is cooperative binding?

Answer 9

Subunits of Hb increase their affinity for oxygen for every oxygen that is bound to it
-more difficult for the first oxygen to bind

Question 10

What does the T-form of Hb look like?

Answer 10

• Deoxygenated form of hemoglobin
• ionic/hydrogen bonds constrain the movement of the polypeptide chains
• LOW OXYGEN AFFINITY

Question 11

What does the R-form of Hb look like?

Answer 11

• binding of oxygen breaks some of the ionic/hydrogen bonds
• subunits have more freedom
• HIGH OXYGEN AFFINITY

Question 12

What is allosterism?

Answer 12

the regulation of protein activity through the binding of effectors

• ONLY proteins with multiple subunits (quaternary) can be allosterically regulated

Question 13

What type of proteins express sigmoidal ligand-binding curves?

Answer 13

Allosteric proteins, ie hemoglobin

Question 14

What type of binding curve does Mb express?

Answer 14

Hyperbolic, as it only has one subunit

Question 15

Negative inhibitors shift the binding curve in which direction?

Answer 15

To the right, as they decrease protein activity

Question 16

Positive inhibitors shift the binding curve in which direction?

Answer 16

to the left, as they increase protein activity

Question 17

What is the Bohr Effect? What direction does it shift the curve?

Answer 17

-protons are heterotrophic, negative allosteric effectors of Hb
-decreased oxygen affinity -> shift to the right
- T form is stabilized
( a higher pH has the opposite effect)

Question 18

What is the effect of the 2,3-BPG of the Hb oxygen affinity curve?

Answer 18

• heterotrophic negative effector
• by product of glycolysis
• significant decrease in )2 affinity -> shift to the right
• reduction in affinity allows for more O2 unloading at tissues (beneficial in hypoxic situations)
• 2,3-BPG binds to T-form ONLY and stabilizes it

Question 19

What is the role of CO2 in hemoglobin transport?

Answer 19

(majority transported to lungs at bicarbonate)

• 25% bind to N-terminal of Hb as carbamate
  
  • > this is carbaminohemoglobin
• the negative charge creates an ionic bond which stabilizes the T state
  
  • > affinity for O2 is lowered by CO2 binding

Question 20

How does carbon monoxide effect Hb?

Answer 20

• binds tightly to Hb iron -> carbon monody hemoglobin
• 220x greater affinity for CO than O2
• When CO binds, Hb shifts to the R form
  
  • > unable to release oxygen at the tissues

Question 21

How does hemoglobin become glycated?

Answer 21

• through covalent attachment of glucose
• usually on the NH2 group of valine of B-globin
• Most glycated is HbA1c (measure of diabetes)

Question 22

What does the Michaelis-Menton measure?

Answer 22

• the response of an enzyme to changes in the substrate concentration
• a hyperbolic curve
• a higher Km have a low affinity for the substrate
• a lower Km have a high affinity for the substrate

Question 23

Enzymes following Michaelis-Mentin kinetics show what kind of curve?

Answer 23

Hyperbolic

Question 24

Allosteric enzymes show what kind of curve?

Answer 24

Sigmoidal

Question 25

Allosteric enzymes are regulated by what?

Answer 25

Effectors. They bind at regulatory site that alter the shape of the catalytic site
- can be activators or inhibitors