Hb Allostery (3)

changes in ionic interactions of T state

formation of ion pairs increases the pK values of the groups

• Increased pK- less acidic, will

keep protons (higher pH)

changes in ionic interactions of R state

interaction ion pairs are absent, causing a decrease in the pK values of these groups

• Decreased pK- more acidic, will

give up protons (lower pH)

how many protons (H+) does Hb release for each O2 it binds

0.6

other effectors for Hb’s affinity for O2

1. [H+] Bohr Effect

2. CO2

3. 2,3-Bisphosphoglycerate ([BPG])

The higher the [H+] (lower the pH), the further the O2 binding curve of Hb moves to the

right – signaling decreased affinity for O2, higher p50 value

Bohr effect

the decrease in O2 binding affinity of hemoglobin in

response to a decrease in pH

The Bohr effect has important physiological functions in

transporting O2 from the lungs to respiring tissue

blood buffering is important because

a constant biological pH must be maintained for physiological reactions

Decrease in pH (increase in H+) _______ Hb’s affinity for O2

diminishes

Increase in pH (decrease in H+) __________ Hb’s affinity for O2

enhances

What is in the blood buffering system

that maintains physiological pH?

Carbonic acid (H2CO3)/bicarbonate (HCO3

-)

sleeping burns fuel so we get how much CO2 and byproduct?

less

being active creates how much CO2 and byproduct?

more which increases lactic acid, protons, and lactate in the liver

pH decreases (6.8) so what happens

– Increase in [H+] from metabolic reactions

– Equilibrium shifts towards more H2CO3

– H2CO3 loses water to produce CO2, that is expired from the lungs

pH increased (7.4) so what happens?

– Decrease in [H+]

– Breathing is adjusted so more CO2 in the lungs can be introduced to the blood for conversion to H2CO3

carbonic anhydrase

enzyme of the blood buffering system in the RBCs

CO2 is expelled in…

lungs

in tissues HCO3- will travel in…

blood to lungs

Diffusion of CO2 from respiring tissues for transportation back to the lungs is slow and in red blood cells (where Hb is located), the enzyme carbonic anhydrase greatly _______ this reaction

accelerates

Hb in T- state (bind H+)

↓ pO2 and H+ from H2CO3 is taken by Hb to produce HCO3-

Hb in R-state (release H+)

↑ pO2 and H+ from Hb is taken by HCO3

- to produce H2CO3 → H2O and CO2

carbamation

Hb also acts as a CO2 transporter

some CO2 transported as HCO3– is carried in the

blood, some CO2 binds to

the N-terminal amino groups of the Hb subunits to form carbamates

Because the T form binds more CO2 than the R form, and because H+ is released upon binding,

the R form is shifted to T form and O2 release is favored

This reaction aids in the transport of CO2 to the lungs and…

it also increases the Bohr effect by releasing H+

bound carbamates form what to stabilize deoxy Hb (T state)

ion paris

Hb is responsible for 50% of…

CO2 transported by the blood to be expelled by the lungs

BPG binds tightly to what state and does what?

deoxyHb (T-state) and decreases Hb’s affinity for O2

Presence of BPG that helps Hb “unload”

its bound O2 to tissues

BPG binds deoxy Hb (T-state) because ?

the central

cavity forms BPG’s binding site

The quaternary shift (T → R transition) narrows

the central cavity causing ?

Hb to release BPG

shifting to R state causes what?

the central cavity to collapse

Fetal Hb has low or high BPG affinity?

low

fetal Hb is alpha 2 gamma 2

gamma chain is a variant of the beta chain

why does Fetal Hb have a Ser in place of His143

so BPG binding isn’t stabilized

allosteric effects

binding of a ligand at one site affects the binding of another ligand at another site

Hb is what type of protein?

an allosteric protein

allosteric effectors of Hb

1. CO2

2. BPG

• affects Hb’s ability to bind to O2

sequential model

– Ligand binding induces a conformational change

in the subunit it binds and cooperative

interactions arise as a result

– These cooperative interactions induce

conformation changes in neighboring subunits

that influence that subunits affinity for ligan

HbA

adult hemoglobin; wild type normal sequence that leads to normal fxn

HbF

fetal hemoglobin

Hemolytic anemia

deficiency in red blood cells, compromises O2 delivery to tissues

Cyanosis

O2 binding site mutation in alpha or beta subunit (methemoglobin subunit) leads to oxidation of Fe2+ (Fe3+) and reduces O2 binding cooperativity, bluish skin color results due to increase amount of methemoglobin in blood

polycythemia

mutations increase Hb’s O2 affinity, results in increased red blood cells to compensate for less O2 being delivered leading to a ruddy complexion (red skin)

Single amino acid substitutions cause

sickle cell anemia HbS

Heterozygous individuals

can suffer from mild sickle-cell anemia; carry 1 copy of mutation

Homozygous recessive individuals

suffer from sickle-cell anemia; carry 2 copies of mutation

• Mis-shaped red blood cells that cannot pass through capillaries properly

• Can be fatal

polymerization of Hb S molecules arises because ?

Val replaces Glu on the surface of β- chains

polymerization

many proteins connect and associate tg which leads to changed RBC shape

Val side chains can insert into what?

hydrophobic pockets in neighboring HbS molecules which changes the shape

HbD heterozygotes are more resistant to ?

malaria