Cooperativity myoglobin

As noted, hemoglobin is an tetramer. Each of the four subunits has a conformation virtually identical to that of myoglobin. Two different types of subunits, a and /3, are necessary to achieve cooperative Oa-binding by Hb. The /3-chain at 146 amino acid residues is shorter than the myoglobin chain (153 residues), mainly because its final helical segment (the H helix) is shorter. The a-chain (141 residues) also has a shortened H helix and lacks the D helix as well (Figure 15.28). Max Perutz, who has devoted his life to elucidating the atomic structure of Hb, noted very early in his studies that the molecule was... 

The 02-binding curve for myoglobin is hyperbolic, but for hemoglobin it is sigmoidal, a consequence of cooperative interactions in the tetramer. Cooperativ-ity maximizes the ability of hemoglobin both to load O2 at the PO2 of the lungs and to deliver O2 at the PO2 of the tissues. 

Rice-Evans, C., Green, E., Paganga, G., Cooper, C. and Wri esworth, J. (1993). Oxidised low density lipoproteins induce iron release from activated myoglobin. FEBS Lett. 326, 177-182. 

Figure 6.2 The single chain of myoglobin (a) and the four chains (two a and two P) of hemoglobin (b). The cooperative behavior of hemoglobin (sigmoid curve), as indicated in the left panel (c), can be seen as an emergent property, resulting from the interaction of the four chains.

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