Chapter 15

Question 1

What is allosteric regulation of an enzyme?

Answer 1

non-covalent interaction with an enzyme/protein and a small molecule (not a substrate)

Question 2

At what site does allosteric regulation occur in an enzyme?

Answer 2

At some place other than the active site

Question 3

What structural feature is common to many allosteric enzymes?

Answer 3

quaternary structure

Question 4

Describe the MWC (symmetry) model of allosteric regulation

Answer 4

equilibrium between T and R state, ligand binding shifts the equilibrium toward R but does not change conformation

Question 5

Describe the KNF (sequential) model of allosteric regulation

Answer 5

ligand binding triggers a conformational change, different conformations have different affinities for ligand

Question 6

What is the structure of a heme?

Answer 6

Fe ion surrounded by a porphyrin ring

Question 7

what are the 6 preferred ligands of the Fe ion?

Answer 7

4 nitrogens from porphyrin ring, 1 nitrogen from HisF8, 1 oxygen molecule

Question 8

What is the function of myoglobin?

Answer 8

oxygen-binding protein, stores oxygen in muscles

Question 9

what is the structure of myoglobin?

Answer 9

a globular protein made of a single polypeptide chain with 8 helices

Question 10

what type of O2 binding does it exhibit?

Answer 10

classic saturation binding, similar to an enzyme that obeys Michaelis menten

Question 11

what is the function of hemoglobin?

Answer 11

oxygen binding protein, binds O2 in lungs and transports it to muscles

Question 12

what is the structure of hemoglobin?

Answer 12

globular protein, tetramer (4 polypeptide chains) 2 alpha chains, 2 beta chains

Question 13

what type of O2 binding does it exhibit, why is this important to its biological role?

Answer 13

sigmoidal binding of O2, important because at low O2 content, has a weaker affinity than Mb allowing for transfer

Question 14

How does the Fe ion in the heme change upon O2 binding? What conformational changes does this trigger?

Answer 14

Fe moves from being above the plane to almost within the plane, this pulls HIsF8 thus pulling the alpha helix which is transmitted to the interface causing a 15 degree rotation and a transition from the T state to the R state

Question 15

WHat is the difference in the T and R states in Hb?

Answer 15

T state favored in deoxyHb, R state is favored in OxyHb

Question 16

how does Hb show characteristics of the MWC model?

Answer 16

has T & R states in equilibrium, R state has a higher O2 affinity, binding O2 shifts equilibrium from T to R state

Question 17

How does Hb show characteristics of the KNF model?

Answer 17

O2 binding triggers conformational change in the subunit it is bound to triggering change in the adjacent subunits, increasing O2 binding affinity

Question 18

How does pH affect O2 binding in Hb? What s=is the name of this phenomenon?

Answer 18

as pH decreases, O2 disassociation from Hb increases, Bohr effect

Question 19

What is the chemical/structural explanation for the pH effect in Hb?

Answer 19

at decreased pH, HisB148 is in its protonated form which participates in an ionic bond with AspB94 stabilizing Hb in the T state, lowering O2 affinity

Question 20

Why is this pH effect in Hb important from a metabolic perspective?

Answer 20

metabolically active tissues (muscles) have a slightly lower pH, by Hb being sensitive to minor pH changes, it easily releases its O2 to Mb when Hb reaches the muscles, allowing Hb to perform its biological role

Question 21

How does CO2 affect O2 binding in Hb?

Answer 21

CO2 can directly lower blood pH through the bicarbonate equilibrium, thus decreasing Hb’s affinity for O2

Question 22

WHat is the chemical/structural explanation for the CO2 effect on Hb?

Answer 22

binding of CO2 to Hb turns the N-terminal from a positive/neutral charge to a negatively charged carbamate, which can then form ionic bonds that stabilize the T state and lower Hb’s affinity for O2

Question 23

How does 2,3-bisphosphoglycerate affect O2 binding in Hb?

Answer 23

binds to tetrameric Hb in the cavity created by the 4 subunits, decreasing Hbs affinity for O2

Question 24

What is the chemical/ structural explanation for the effect of 2,3-BPG on Hb?

Answer 24

BPG being a highly negative molecule is attracted to the binding pocket in Hb which is lined with positive AA. BPG crosslinks the B-chain stabilizing the T state, lowering Hbs affinity for O2

Question 25

How does the affinity for O2 differ between HbF and HbA/

Answer 25

Hbf has a higher affinity for O2 than HbA

Question 26

What is the chemical/structural explanation for the increased O2 affinity of HbF over HBA

Answer 26

Serine is substituted in place of His143 making the B chain into a gamma chain. This substitution removes 2 positive charges from the BPG binding pocket making Hb more likely yo be in the R state with a higher O2 affinity

Question 27

Why is the affinity difference between HbF and HbA important?

Answer 27

important because fetus gets all O2 from mother so HbF having a higher affinity for O2 makes it easier for fetus to get necessary O2

Question 28

what is sickle cell anemia?

Answer 28

a recessive disease that causes blood cells to be abnormal and crescent shaped

Question 29

What is the molecular explanation for sickle cell anemia?

Answer 29

valine is substituted for GluB6 (ionic –> nonpolar). valine gets buried into the hydrophobic pocket through the hydrophobic effect, leading to the polymerization of Hb to HbS making it insoluble until it precipitates and exists as a solid clump in the cell giving it an abnormal shape

Question 30

what is the affinity of CO for Hb compared to O2

Answer 30

Cos affinity is 200x stronger than O2

Question 31

What is HbA1C, why is it important?

Answer 31

hemoglobin that has reacted with glucose in the blood, can be used to give a 3 month snapshot of blood glucose levels, better than the fehlings test and much more effective for detecting diabetes