oxygen binding proteins

week 2 i cant believe i went to this workshop twice

myoglobin (Mb)

• single polypeptide chain of globular proteins, tertiary structure

• O₂ storage (primarily muscle tissue)

• Contains one haeme prosthetic group, binds one O₂ molecule - high affinity

two redox states of iron

• Fe²⁺ (ferrous)

  • can bind to O2

  • toxic to cells

• Fe³⁺ (ferric)

  • cannot bind to O2

  • Fe³⁺-bound Hb compromises the body’s ability for O₂ transport and storage

O2 dissociation curve

• graph showing the relationship between the amount of O₂ (saturation) bound to Mb or Hb vs pO₂

O2 dissociation curve for myoglobin

• Mb is almost completely saturated at high and low pO₂

• Very similar in the lungs and in the tissue

• Hyperbolic curve

• This is great for a STORAGE protein!

myoglobin KD

• KD = dissociation constant

• measure of affinity of a protein to its ligand → high affinity = low KD

• Mg has a low KD (loves to bind hates to dissociate)

• pO2 when 50% of binding sites are bound to O2 = KD

Haemoglobin (Hb)

• Has 2 identical alpha subunits and 2 identical beta subunits (a2ß2) globins

  • “alpha globin chain”: each globin is one individual protein making up the quaternary complex

• 1 haeme and 1 Fe2+ subunit

• Hb binds up to 4 O2 molecules

• better at transporting O2

• sensitive to small changes in pO2

structure of haeme

• porphyrin ring covalently bound in a deep pocket in each Hb subunit (protect the Fe²⁺!)

• Fe²⁺ ion in centre

• Fe²⁺ makes 6 covalent bonds

  • 4 to the planar haeme ring

  • one to a Histidine amino acid of Hb

  • one to O₂

• deoxygenated (Tense) state:

  • concave shape

  • high affinity for oxygen

• oxygenated (Relaxed) state

  • flat, planar shape

  • low affinity for oxygen

haemoglobin colour (deoxygenated vs oxygenated)

• deoxygenated (Tense) state: darker red

• oxygenated (Relaxed) state: lighter red (scarlet)

• Pulse oximeter uses colour to detect pressure of oxygen by UV light

cooperative binding

• Hb can change its affinity for O₂ from high affinity (in the lungs) to low affinity (at the tissues)

• affinity dependent on partial pressure of O2

• Once 2 subunits, in the T state, bind O₂ a T → R transition occurs

  • α and β subunits slide past each other, breaking ionic bonds that stabilize the T-state

  • makes it easier for the remaining subunits to bind O₂

O2 dissociation curve for haemoglobin

• S shaped curve due to changing affinity

• dissociation high in tissues low in lungs

allosteric effectors

• Haemoglobin can bind other small molecules at sites away from the O2-binding site

  • H+ (increased acid load)

  • CO2 (increased acid load) (forms carbaminoHb)

  • 2,3-bisphosphoglycerate (2,3-BPG) (First step of glycolysis: form 2,3 BPG)

• binding to allosteric site

  → conformational change in Hb structure

  → decreased affinity for O2 and altered functionality

  → T-state (deoxy) stabilises (is favoured)

  → off-loads more O2

factors allowing Hb to remain relaxed in lungs despite when pO2 levels drop

• Other factors aid in Hb's changing affinity

  • pH (H+, CO2)

  • temp

  • 2,3-BPG

• Therefore Hb can remain highly saturated even with large decreases in pO2

  • Lungs have low temp, high pH, high 2,3-BPG

the BOHR effect

affinity of hemoglobin for oxygen decreases when the pH of the blood decreases

↓ H⁺ (increasing pH): BOHR effect

• occurs in the lungs

• T→R transition → Hb releases H⁺ 

• R state stabilises → picks up O₂

• Causes a shift to the left of the ODC

↑ H⁺ (lower pH): BOHR Effect

• occurs at the periphery

• Hb binds H⁺ (acts as buffer)

• T-state stabilises → off-loads O₂ to periphery.

• Causes a shift to the right of the ODC

(e.g. exercise)

2,3-BPG regulation of O2 off-loading

• Binding of 2,3-BPG dramatically decreases Hb affinity for O₂

• 2,3-BPG ↑ at high pressures where levels of O2 ↓ (high altitudes (during acclimatisation))

• shifts ODC curve right

foetal haemoglobin (HbF)

• the foetus depends on mother for oxygen, so their Hb has higher affinity (can bind oxygen faster)

• HbF contains 2 γ subunits instead of ß subunits (α₂γ₂)

• 2,3-BPG cannot bind as well, increasing O2 affinity

  • (due to lack of positive amino acid to bind to)

• lose HbF across foetal development

carbon monoxide poisoning

• Binds to Fe2+ in haeme with 200x affinity than O2

• Bound CO prevents O2 from binding

sickle cell anaemia (HbS)

• Sickle shaped RBCs caused by a Hb mutation

• Mutation of glutamate (a charged, polar amino acid) for a non-polar valine

• HbS is insoluble

• RBCs cannot go through blood vessels and get stuck → pain

• Cannot carry oxygen efficiently → hypoxia

• killed by spleen bc low haematocrit → anaemia

Hb blood pressure regulation

• Nitric oxide (NO) is produced in the tissues

  • when unbound: vasodilates the walls of cardiac blood vessels (BP↓)

• Hb binds to NO

  • NO binds to specific cysteine residues in Hb and to the Fe2+ in haeme

• Excess haemoglobin/RBCs:

  • vasoconstriction → hypertension