Myoglobin & Hemoglobin

• Heme proteins• Supply of oxygen – Oxidative metabolism

• Myoglobin– Monomeric – protein of red muscle– Stores oxygen

• O2 storage

• O2 transport

• Hemoglobin– Tetrameric • Cooperative interactions

• 2,3-bisphosphoglycerate (BPG) promotes the– Stabilize the structure of deoxyhemoglobin

• Heme & ferrous iron

Heme

Myoglobin

• Rich in α Helix• 153-aminoacyl residue• MW 17,000 • 75% in eight right-handed– Helices A–H

• Surface of myoglobin is polar• Interior contains only nonpolar– Leu, Val, Phe,

Myoglobin

• Histidines F8 & E7 – Roles in Oxygen binding– Proximal histidine, His F8• The fifth coordination position of the iron

• O2 occupies the sixth coordination position

A model of myoglobin

Hemoglobin

• Tetrameric – α2β2 (HbA)

– α2γ2 (HbF)

– α2S2 (HbS)

– α2δ2 (HbA2)

• the α polypeptide– Seven helical regions

• bind four molecules of O2 per tetramer

• Cooperative binding– A molecule of O2 binds to a hemoglobin tetramer

more readily if other O2 molecules are already bound

• P50 – expresses the relative affinities of different

hemoglobins for oxygen – The partial pressure of O2 that half-saturates Hb

• P50 for HbA and fetal HbF– 26 and 20 mm Hg – HbF,High affinity for O2

• ζ2ε2 fetus Hb

Developmental pattern of the quaternarystructure of fetal and newborn hemoglobins

• Oxygenation of hemoglobin is accompanied by large conformational changes

• binding of the first O2

• Iron motion • rupture of salt bridges • T (taut) state to the R (relaxed) state– Low affinity and high-affinity conformations

The iron atom moves into the plane ofthe heme on oxygenation. Histidine F8 and its associated residues are pulled along with the iron atom.

The transition between the two structures is influenced by protons, carbon dioxide, chloride, and BPG; the higher their concentration, the more oxygen must be bound to trigger the transition.