Biochem Ch 5: Hemoglobin/Myoglobin

principle oxygen-containing protein in muscle, stores excess O₂ used during times when O₂ concentration is low

myoglobin

protein cofactor that holds oxygen

heme

6 coordination sites- 4 to heme group, 1 for oxygen, 1 for histidine side chain in protein

Iron in heme coordination sites

4 subunits whose interactions are crucial for regulating O₂ binding

hemoglobin subunits

binds O₂ less tightly —> more difficult to bind O₂ and easier to unbind

T-state hemoglobin

binds O₂ more tightly —> easier to bind O₂ and harder to unbind

R-state hemoglobin

cooperative binding

what makes hemoglobin so good at carrying O₂?

binding of one oxygen molecule to one of its four heme groups increases the affinity of the remaining heme groups for O₂ making subsequent binding easier

cooperative binding

T-state

hemoglobin state at low P_O2

R-state

hemoglobin state at high P_O2

singmoidal curve

hemoglobin O₂ graph type

Ability of hemoglobin to release O₂ at low P_O2 levels and pick it up at high P_O2 levels

what does sigmoidal curve demonstrate?

• picks up oxygen in the lungs at high P_O2

• release oxygen in tissues at low P_O2

Why is cooperativity a good thing?

0.4 Angstom (0.4 × 10^-10 m)

T-form to R-form movement length

heme goes from non-planar in T-form to planar in R-form and is pulled back by O₂ due to steric hindrance

How heme changes in T-form vs R-form

ionic interactions between groups on different subunits stabilize T-state

How is T-state stabilized

when subunit binds O₂, ionic interactions between subunits and change allowing easy change to R-state

How does hemoglobin change from T-state to R-state

homotropic

both the substrate and modulator bind to the same site

heterotropic

modulator binds different site from substrate

binding of a chemical on one site of a multimeric protein affects the strength of binding on another site of the protein

Allosteric regulation

binding of 1 molecule increases binding of another molecule

positive cooperativity

• n_H = 1 —> no cooperativity

• n_H > 1 —> positive cooperativity

• n_H < 1 —> negative cooperativity

Hill coefficient meanings

3

hemoglobin hill coefficient

1

myoglobin hill coefficient

point mutation, autosomal recessive

Sickle cell anemia genetics

hypoxia, sudden pain crisis in limbs, chest, etc.

Sickle cell anemia symptoms

mutation at position 6 causes change in folding of β-subunit

Where is the point mutation located in hemoglobin for sickle cell anemia?

Mutation in folding of β-subunit —> exposes more hydrophobic regions —> sickle cell hemoglobins aggregate with 2 other hemoglobins —> compounds to form large aggregates of deoxygenated HbS

Why does sickle cell hemoglobin polymerize

rigid and form helical structures

HbS aggregate shape

rightward

which way is hemoglobin curve shifted to decrease affinity?

R-form of hemoglobin better able to hide hydrophobic residues

Why is T-form more likely to form larger aggregates

CO₂, decreasing pH ([H⁺]), BPG

Things that stabilize T-form

chemical in blood that increases in high altitude, binds to gap present in T-form

BPG

fetal hemoglobin (HbF)

things that shift curve left

Oxygen, NO

Things that stabilize R-form

pH affects the binding of oxygen, as pH decreases, saturation of O₂ decreases

the Bohr Effect

How does increases H⁺ concentration changing ability to form ionic bonds holding R-groups together —> T-form requires less ionic bonds than R-form —> disrupts ionic bonds and stabilizes T-form —> extra oxygen released

decreasing pH affect hemoglobin?

CO₂ binds to amino terminal residue —> forms ionic bridges between subunits —> stabilizes T-form

How does CO₂ affect T-form?

no β chain

Fetal hemoglobin difference vs adult hemoglobin