Hemoglobin Flashcards
Heme
4 porphyrin rings come together
Iron in ferrous oxidation state (Fe2+) = crucial; when oxidized to Fe3+ so oxygen cannot bind
O2 vs. CO2 Binding
Want the heme to bind oxygen, and when binds, because of oribitals and electron structure, the first two O2 bind in bent fashion
CO can bind – don’t want this, and prefers to bind up and down; bind 25,000x better binding compared to O2; not an issue because heme group has hydrophobic pockets with steric hinderance to force molecules to bind at an angle, which is what O2 does; CO also has a higher affinity than O2, but the binding only has 200x better than O2, so not as likely to bind for the same reasons
Myoglobin vs. Hb Sequences
Primary sequence: AA sequence
Very few regions are similar, so AA sequences are very different
Even though primary sequence is different, secondary alpha and beta globins are similar
O2 Binding Curves of Myoglobin vs. Hb
Myoglobin curve: hyperbolic and reflects the function of binding O2 and hold onto it tightly until muscle needs O2 (at very low partial pressures of O2 it is released); single polypeptide, holds onto O2 (high affinity for O2)
Hb curve: sigmoidal curve which reflects cooperativity between 4 subunits that influences how well each heme group binds to O2; once one O2 binds, it facilitates that more bind
Two States of O2 Binding Curve for Hb
There are two primary states of Hb:
Low affinity or T state (deoxyHb) where O2 released to tissues
High affinity or R state (relaxed state) and looks like myogloblin state where O2 bound in lungs
Transitions from low affinity to high affinitiy O2 binding state, and as transitioning we get sigmoidal curve
Salt Bridges and O2
Salt bridges are interactions between + and – and stabilizes deoxy state and each polypeptide and doesn’t favor O2 binding and O2 binds poorly
As O2 molecule binds to polypeptides, especially in lungs when high concentration, a molecule binds and breaks salt bridges, and then conformational change causing more salt bridges to break and slowly transitions to O2 binding state
As it transitions to relaxed state and becomes high affinity state
Fully bind Hb and reverses in tissues when O2 falls off and salt bridges reform and changes confirmation to deoxy state
Bohr Effect
Acidity also influences O2 binding = Bohr effect
Reciprocal binding of CO2 and O2
Metabolism generates H20 and CO2, which is acidic, so in periphery have lower pH and get unloading of O2 in tissues, but opposite in lungs where the O2 forces off the protons
In periphery have CO2 and water from metabolism and forms carbonic acid and bicarb travels through blood and more O2 unloaded
Protons and O2 play opposite roles; when one is bound it kicks off the other
Binding of CO2 stabilizes deoxy state and salt bridges
BPG
Product of glycolysis and plays a role mainly when at high altitudes
BPG decreases Hb affinity for O2, therefore stabilizing the deoxy state (T)
Resembles myoglobin and helps with cooperativity
No BPG, no hyperbolic curve
If you raise BPG concentration (like in high altitudes) it shifts curve to the right; normal at sea level we release 38% of O2, but if higher altitudes and have higher O2 concentrations O2 released around 30% unless you increase BPG which would get 37%
Shifts in Hb Curve
Right shift: decreased affinity for O2/easier release; increased acidity, CO2, BPG, and temperature
Left Shift: increased affinity for O2/harder to release; decreased acidity, CO2, BPG, and temperature
Globin Gene Arrangement
Globin genes are arranged on two chromosomes
Alpha gene on 16, and non-alpha is on 11
Must have a balance between the two, so genes are regulated so that we have even generation/production of alpha and non-alpha to be able to form 2:2 and form Hb
Synthesis occurs from left to right so left is embryonic and right is developmentally expressed later
Alpha 1 and 2 Duplication
α2 and α1 duplication:
differ only in promoters
α2 promoter is stronger
α2:α1 expression is 2:1
Gγ and Aγ Duplication
Gγ and Aγ duplication:
Gly to Ala substitution
functionality, performance, expression level are identical
no biological or disease significance
Developmental Expression
As epsilon and zeta drops off (both embyonically), alpha and gamma pick up respectively
Gamma drops off (fetal) to become beta (adult)
HbA, HbA2, HbA1c, HbF, HbH, Hb Bart’s
Four betas = HbH - increased in alpha thalassemia
4 gammas = Bart’s Hb - increased in alpha thalassemia
2 alpha and 2 beta = HbA (normal)
HB A1C is used to study glucose control in diabetes
2 alpha and 2 delta = HbA2
2 alpha and 2 gamma = HbF - increased in beta thalassemia
HbF (α2γ2) vs HbA (α2β2)
γ chain vs. β chain – 72% identical
one important difference is γ chain has a Ser instead of His
binds BPG with reduced affinity
affinity for O2 is increased and HbF can take O2 from HbA
BPG stabilizes the deoxy state (T state), so low BPG binding would cause a left shift and favor the R state and raises O2 affinity up to take O2 away from mother’s Hb to make sure fetus is oxygenated
