Chapter 7 - Hemoglobin Flashcards
Pertuz Mechanism
2 stable conformations of hemoglobin
R (oxy) and T (deoxy)
Positive allosteric effector (activators)
Shift equilibrium to the R state
Oxygen; high pH (low [H+])
Negative Allosteric Effectors
(inhibitors)
stabilize the T state and shift equilibrium to the deoxy form
Low pH (Bohr effect)
2, 3-Bisphophoglycerate
Carbamates
Binding of oxygen to hemoglobin affects mostly what sites
A1-B2
and A2-B1
between the protomers
Oxygen binding induces conformational changes around heme Fe, which are transmitted through subunit interactions
Describe changes
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
T state
low affinity
R
high affinity
Greatest changes from T->R
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
Allosteric effectors
(modulators) bind to a protein at a site separate from the functional binding site (activators or inhibitors)
Carbamates
bind to Hb N-terminus at high CO2
Reversible CO2 binding to Hb N-terminus in T-state (tissues)
Bohr effect
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
Tissues:
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
Tissues: Muscles
lactic acid lowers pH causing more efficient O2 release with heavy respiration
Lungs:
High [O2] favors O2 binding and H+ release
H+ + HCO3- -> CO2 (g) + H2O
Regulation of Hb O2 binding by 2,3 BPG
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
Binding of 2,3 BPG to deoxyhemoglobin
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)
Fetal Hb
has low BPG binding due to His143->Ser in B-subunits
Effects of CO2 and BPG on Hb O2 dissociation curve
They both stabilize deoxy Hb
Symmetry Model of Allosterism
accounts for cooperativity
requires that all subunits shift at the same time
Monod Wyman and Pierce Model
Sequential Model of Allosterism
Daniel Koshland
Each subunit changes conformation upon ligand binding and affects neighboring subunits
