Myoglobin molecule

Kendrew, J.C., et al. A three-dimensional model of the myoglobin molecule obtained by x-ray analysis. Nature 181 662-666, 1958. 

If Y is defined as the fractional saturation of myoglobin with Og, that is, the fraction of myoglobin molecules having an oxygen molecule bound, then... 

The value of Franges from 0 (no myoglobin molecules carry an Og) to 1.0 (all myoglobin molecules have an Og molecule bound). Substituting from Equation (A15.1), ([Mb] [OgD/Alfor [MbOg] gives... 

In this form, Alhas the units of torr.) The relationship defined by Equation (A15.4) plots as a hyperbola. That is, the MbOg saturation curve resembles an enzyme substrate saturation curve. For myoglobin, a partial pressure of 1 torr for jbOg is sufficient for half-saturation (Figure A15.1). We can define as the partial pressure of Og at which 50% of the myoglobin molecules have a molecule of Og bound (that is, F= 0.5), then... 

Figure 6-1. Heme. The pyrrole rings and methylene bridge carbons are coplanar, and the iron atom (F62 ) resides in almost the same plane. The fifth and sixth coordination positions of Fej are directed perpendicular to—and directly above and below—the plane of the heme ring. Observe the nature of the substituent groups on the (3 carbons of the pyrrole rings, the central iron atom, and the location of the polar side of the heme ring (at about 7 o clock) that faces the surface of the myoglobin molecule.

Fig. 8.14 TEM images showing (A) self-assembled layers of individual myoglobin molecules wrapped by condensed oligomers of AMP (B) top view of single sheet showing ordered superstructure of the hydrophobic organoclay wrapped myoglobin molecules.

Each of the subunits, a and P (as well as the closely related myoglobin molecule), has a prosthetic heme group to which the oxygen molecule binds. There are no covalent bonds between the subunits of Hb. The aggregate is maintained by a combination of weak direct subunit-subunit interactions as well as by indirect interactions mediated by the solvent. 

Cellular oxygen is bound by myoglobin molecules that store it until it is required for metabolic action, where upon they release it to other acceptors. Hemoglobin has a additional function, however, and that is to carry CO2 back to the lungs this is done by certain amino acid side chains, and the heme groups are not directly involved. Because the circumstances under which Hb and Mb are required to bind and release O2 are very different, the two substances have quite different binding constants as a function of O2 partial pressure (Fig.). 

If/represents the fraction of myoglobin molecules bearing oxygen and P represents the equilibrium partial pressure of oxygen then... 

Location of polar and nonpolar amino acid residues The interior of the myoglobin molecule is composed almost entirely of nonpolar amino acids. They are packed closely together, forming a structure stabilized by hydrophobic interactions between these clustered residues (see p. 19). In contrast, charged amino acids are located almost exclusively on the surface of the molecule, where they can form hydrogen bonds, with each other and with water. 

Hg. 19j The myoglobin molecule (a) the folding of Ihe polypeptide chain about the heme group (represented by the disk) (b) close-up view of the heme environment. (Modified from Kendrew. J. C. Dickerson. R. E.. Strand berg, B. E.. Hart. R. C. Davies. D. R. Phillips D. C.. Shore. V. C. Nature 1960.185,422-423. Reproduced with permission.)... 

A representation of the myoglobin molecule. The Fe2 ion is coordinated to four nitrogen aloms in the porphyrin of the heme (represented by the disk in the figure) and one nitrogen in the protein chain. This leaves a sixth coordination position (indicated by the W) available for an oxygen molecule. 

The bulky protein around the heme group in myoglobin prevents two myoglobin molecules from getting close enough to form the oxygen bridge therefore, oxidation of the Fe-+ is prevented. 

Figures 4.4 and 4.5 show examples of two common types of representations of protein structures. In Figure 4.4 some of the global structural features of the myoglobin molecule are shown from the most commonly used perspective [341]. In this view the helical segments as well as the heme group are clearly recognizable. In Figure 4.5 a ribbon model of the same molecule is shown.

Myoglobin is different. The globin portion of myoglobin embeds only one heme group, and each myoglobin molecule can bind only one O2. 

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