Myoglobin & Hemoglobin

Heme proteins Supply of oxygen – Oxidative metabolism Myoglobin – Monomeric – protein of red muscle – Stores oxygen

O 2 storage O 2 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 O 2 occupies the sixth coordination position

A model of myoglobin

Hemoglobin Tetrameric – α 2 β 2 (HbA) – α 2 γ 2 (HbF) – α 2 S 2 (HbS) – α 2 δ 2 (HbA 2 ) the α polypeptide – Seven helical regions bind four molecules of O 2 per tetramer

Cooperative binding – A molecule of O 2 binds to a hemoglobin tetramer more readily if other O 2 molecules are already bound

P 50 – expresses the relative affinities of different hemoglobins for oxygen – The partial pressure of O 2 that half-saturates Hb P 50 for HbA and fetal HbF – 26 and 20 mm Hg – HbF,High affinity for O 2

ζ 2 ε 2 fetus Hb

Developmental pattern of the quaternary structure of fetal and newborn hemoglobins

Oxygenation of hemoglobin is accompanied by large conformational changes binding of the first O 2 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 of the 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.