Myoglobin proteins like

Hemoglobin Hydrophobic Association of Four Myoglobin-Like Proteins... 

Hemoglobin a biomolecule composed of four myoglobin-like units (proteins plus heme) that can bind and transport four oxygen molecules in the blood. (21.7)... 

Biochemical equihbria can be far more complex than those we have considered so far, but exactly the same principles apply. An example of a complex process is the binding of O2 by hemoglobin in blood, which is described only approximately by reaction B. The protein myoglobin (Mb, Atlas PIO) stores O2 in muscle, and the protein hemoglobin (Hb, Atlas P7) transports O2 in blood. These two proteins are related, for hemoglobin is a tetramer of four myoglobin-like molecules. In each protein, the O2 molecule attaches to ui iron ion in a heme group (Atlas R2) (Fig. 4.8). 

The secondary and tertiary structures of myoglobin and ribonuclease A illustrate the importance of packing in tertiary structures. Secondary structures pack closely to one another and also intercalate with (insert between) extended polypeptide chains. If the sum of the van der Waals volumes of a protein s constituent amino acids is divided by the volume occupied by the protein, packing densities of 0.72 to 0.77 are typically obtained. This means that, even with close packing, approximately 25% of the total volume of a protein is not occupied by protein atoms. Nearly all of this space is in the form of very small cavities. Cavities the size of water molecules or larger do occasionally occur, but they make up only a small fraction of the total protein volume. It is likely that such cavities provide flexibility for proteins and facilitate conformation changes and a wide range of protein dynamics (discussed later). 

Sequence conservation is, in general, much weaker than structural conservation. There are proteins, which are clearly not related in sequence but are closely related in 3D-stmcture and fold, like heamoglobin and myoglobin, which have similar functions. In many proteins, fold elements like 4-helical bundles are repeated. Classifications of known structural folds of proteins are organized in the SCOP or CATH database see e.g., http //scop.mrc-lmb.cam.ac.uk/scop/. 

A.E.F. Nassar, Z. Zhang, N.F. Hu, J.F. Rusling, and T. Kumosinski, Protein-coupled electron transfer from electrodes to myoglobin in ordered biomembrane-like films. J. Phys. Chem. B 101, 2224-2231 (1997). 

Fig. 12. Schematic views of bis-histidyl ferri-, ferro-, and CO-ferro-heme-hemopexin. Unlike myoglobin with one open distal site, heme bound to hemopexin is coordinated to two strong field ligands, either of which a priori may be displaced by CO. This may well produce coupled changes in protein conformation like the Perutz mechanism for 02-binding by hemoglobin (143). The environment of heme bound to hemopexin and to the N-domain may be influenced by changes in the interactions of porphyrin-ring orbitals with those of aromatic residues in the heme binding site upon reduction and subsequent CO binding.

---