J.B. Herrick

1904: First discovered/observed sickled RBCs

  that characterize sickle cell disease.  (Rush alum)

Linus Pauling

1945 first to ID a mutant product (protein) as cause for disease--recognized biochemical difference btw normal Hb (HbA) and HbS

Hb structure

tetramic (2 α/2 β), "all alpha-globin fold" (globular, exclusively comprised of α-helices[8]), Heme B (protoporphyrin IX and ferrous ion (Fe²⁺⁾

Mb structure

monomeric, "all alpha-globin fold" (globular, exclusively comprised of α-helices[8]), Heme B (protoporphyrin IX and Fe²⁺

Hb location

RBC

Mb location

muscle cells

Mb binds

1 O₂ molecule

Hb binds

4 O₂ molecules (1 per subunit), CO₂, and NO (vasodilation, inhibition of platelet aggregation-> efficient gas exchange)

Ferrous ion

vs

ferric ion

Ferrous: Fe²⁺, 6 valences, therefore binds O₂ and His sidechains of porphyrin ring (Proximal His F8 & Distal His E7)

Ferric: Fe³⁺, 4 valences, therefore binds only porphyrin ring

How do heme and oxygen interact?

O₂ binds Fe²⁺, displacing Distal His E7 of protoporphyrin IX

Methemoglobinemia

Rise in concentration of metHb, oxidized form of Hb (with ferric ion), O₂-Hb dissociation curve shifts left, resulting in increased O₂ affinity (oxygen is not released to tissues)

Human Hb subunit forms

alpha-type (α1, α2, or ζ)

beta-type (β, δ, ε, Gγ, Aγ)

What chromosomes are the Hb subunit types found on?

alpha-type=16

beta type=11

When is zeta-Hb synthesized?

up to 6 wks postconception

When is epsilon- Hb synthesized?

up to 6 wks postconception

When is beta Hb synth?

starts 6 wks postconception and slowly rises, until 12 wks postnatal, when reaches adult levels

When is alpha Hb synthesized?

6 wks postconception = adult levels

When is gamma Hb synthesized?

peaks 6 wks postconception, then begins decline to very low levels around time of birth

When is delta Hb synthesized?

very low levels throughout life

Hb Grower-1

embryonic Hb, zeta2/epsilon2

Hb Portland

embryonic Hb, zeta2/gamma2

HbF

Fetal Hb, alpha2/gamma2, replaced by HbA within ~12 wks of birth, sustained at about 1% of Hb in adults

produced in people with SCD, higher levels associated with milder form of disease, hydroxyurea used to stimulate synthesis in SCD therapy

HbA₁

Adult Hb, alpha2/beta2, 98% of adult Hb

HbA₂

Adult Hb, alpha2/delta2, 2% of adult Hb (function unknown)

physiological jaundice

RBCs with HbF die after birth, accumulates if liver/kidneys not well-developed enough to deal with vol of dying HbF

T state

tense state, deoxy state,  ferrous ion out of plane due to steric hindrance btw prox His and porphyrin ring

R state

relaxed state, oxy form, O₂ binds ferrous ion and pulls porphyrin ring (including F8 helix) into another conformation, ENERGETICALLY FAVORABLE

pO₂ at which Hb releases half of O₂

26 torr, partial pressure in capillaries.  therefore, Hb releases oxygen in capillaries

pO₂ at which Hb is fully saturated

100 torr, pO2 in lungs.  therefore, Hb takes up oxygen in the lungs

Mb saturation at capillary pO₂

100%.  Mb binds oxygen as Hb releases it.

pO₂ at which Mb releases O₂

5 torr, pO₂ of working muscle

Oxygen saturation curve of Mb is described as _________, and that of Hb is _________.

hyperbolic, sigmoidal

Cooperativity

Binding the first oxygen molecule facilitates the binding of oxygen to the other subunits.  Conversely, the release of one oxygen molecule facilitates the release of the rest.

What does BPG do

*crosslinks beta subunits of Hb. 

*Forces beta subunits to release O2 (cooperativity-->alphas follow suit). 

*Facilitates R-->T conformational change (unfavorable)

BPG levels increase in response to:

hypoxia, anemia, high altitudes

"oxygen trap"

storing blood for transfusions in the presence of anticoagulant acid-citrate-dextrose results in lower levels of BPG and therefore poor oxygen release

BPG and pregnancy

BPG does not bind well to gamma subunits of HbF.  As a result, HbF-O2 binding curve is shifted to the left, facilitating release of O2 from mother to baby

Bohr effect

pH affects Hb's oxygen binding affinity

dec in pH yields dec in O2 affinity

acidic envt-->T-->protons taken up

basic envy-->R-->O2 binding breaks H and ionic bonds, releasing protons

hyperventilation

low CO₂ levels--> O₂ affinity curve shifts left--> Hb acts more like Mb, doesn't readily release oxygen to tissues--> pass out

HbS mutation

β6 (Glu-->Val)

HbC mutation

β6 (Glu-->Lys)

mild anemia, crystals in RBCs

Hb M-Boston

β58 (His-->Tyr)

Ferric ion instead of ferrous--> no O2 binding: methemoglobin, cyanosis

Hb M-Saskatoon

β63 (His-->Tyr)

methemoglobin, cyanosis

"hot spots" for Hb mutations (where change is most likely to be pathological)

Heme-binding pocket, Fe²⁺-binding amino acids (His E7 or His F8), subunit-to-subunit interactions, break in alpha helix (proline)