Chapter 7 - Hemoglobin

Question 1

Pertuz Mechanism

Answer 1

2 stable conformations of hemoglobin

R (oxy) and T (deoxy)

Question 2

Positive allosteric effector (activators)

Answer 2

Shift equilibrium to the R state

Oxygen; high pH (low [H+])

Question 3

Negative Allosteric Effectors

(inhibitors)

Answer 3

stabilize the T state and shift equilibrium to the deoxy form

Low pH (Bohr effect)

2, 3-Bisphophoglycerate

Carbamates

Question 4

Binding of oxygen to hemoglobin affects mostly what sites

Answer 4

A1-B2

and A2-B1

between the protomers

Question 5

Oxygen binding induces conformational changes around heme Fe, which are transmitted through subunit interactions

Describe changes

Answer 5

O2 binds to Fe2+ -> reduces Fe electron density

Fe slips into poryphyrin ring cavity (porhyrin bent without O2 bound)

Fe-His ligand shifts position and pulls the F8 helix up

Helix shift transmits structural changes to subunit interface contacts

Question 6

T state

Answer 6

low affinity

Question 7

R

Answer 7

high affinity

Question 8

Greatest changes from T->R

Answer 8

hydrophobic and ion pairs at the a1-b2 and a2-b1 interfaces and in the central cavity -> up to 6 A shift

Fe-O2 bond formation energizes breaking H-bond interactions stabilizing the T state

Question 9

Allosteric effectors

Answer 9

(modulators) bind to a protein at a site separate from the functional binding site (activators or inhibitors)

Question 10

Carbamates

Answer 10

bind to Hb N-terminus at high CO2

Reversible CO2 binding to Hb N-terminus in T-state (tissues)

Question 11

Bohr effect

Answer 11

pH effect on oxygen transport

O2 binding and shift to R-state disuprts ion pairs in H-bond networks

Result: 0.6 protons release per O2 bound -> pH decrease reduces O2 affinity

Lower pH protonates groups leading to ion pair formation -> stabilizes T-state

Question 12

Tissues:

Answer 12

CO2 + H2O -> H+ + HCO3-

correlates with blood pH decrease and HCO3- increase (#1 CO2 transport in blood)

Higher [H+] favors ion-pair formation in T-state & efficient O2 release in tissues

Question 13

Tissues: Muscles

Answer 13

lactic acid lowers pH causing more efficient O2 release with heavy respiration

Question 14

Lungs:

Answer 14

High [O2] favors O2 binding and H+ release

H+ + HCO3- -> CO2 (g) + H2O

Question 15

Regulation of Hb O2 binding by 2,3 BPG

Answer 15

negative allosteric effector - stabilizes T form of Hb

Lowers affinity for oxygen (increases P50 from ~12 to ~26 torr)

Negatively charged and bound to six (+) charged groups in the central cavit between subunits

Question 16

Binding of 2,3 BPG to deoxyhemoglobin

Answer 16

Anionic 2.3 BPG groups pair with (+) charges lining the central cavity and stabilize the deoxy Hb form - shifts PO2 higer (lys, His, B-subunit N-termini)

Question 17

Fetal Hb

Answer 17

has low BPG binding due to His143->Ser in B-subunits

Question 18

Effects of CO2 and BPG on Hb O2 dissociation curve

Answer 18

They both stabilize deoxy Hb

Question 19

Symmetry Model of Allosterism

Answer 19

accounts for cooperativity

requires that all subunits shift at the same time

Monod Wyman and Pierce Model

Question 20

Sequential Model of Allosterism

Answer 20

Daniel Koshland

Each subunit changes conformation upon ligand binding and affects neighboring subunits