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Lecture 13

MCB 354 Lecture Notes - Lecture 13: Cooperative Binding, Intimate Ion Pair, Porphyrin

by mss2

Department
Molecular and Cell Biology
Course Code
MCB 354
Professor
E Procko
Lecture
13

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Lecture 13 - allostery and Hb
Cooperativity delivers more O2 to where its needed
Low affinity for O2 in the tissues so more O2 released
High affinity for O2 in lungs, more O2 pickup
T→ R transition of the Hb tetramer
Monomeric Mb is a close homolog of alpha and beta subunits
Quaternary structure of Hb tetramer restrains it. When restraint is broken, it
binds O2 freely
Ion pairs stabilize T conformer of Hb
T state has low O2 affinity whereas R state resembles Mb and has high O2
affinity
Allosteric interactions are at intersubunit interfaces
T → R conformational switch starts with O2 binding at heme
In T state, porphyrin ring is puckered and the Fe2+ atom displaced towards the
proximal His
In R state, the heme has planar conformation
When O2 is bound in T state, FeO2 interaction pulls Fe into heme plane to
optimize the bond. This strains the proximal His-Fe coordination, shifting the
position of the His and helix F
The movement of the F helix in one subunit disrupts the interactions between it
and its neighbors that stabilize the tetramer’s T state. This predisposes the entire
tetramer to undergo T4 → R4. when it does, O2 can bind with high affinity to any
of the subunits in the R state
Each successive O2 bound to a T subunit indtroduces more strain in the
quaternary structure, increasing the likelihood the tetramer will snap into the R
form, increasing the equilibrium constant in favor of R4.
BPG: an allosteric modulator of Hb
BPG is an intermediate of glycolysis and is a key determinant of the cooperativity
of O2 binding by Hb in vivo
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