A,/3 Dimers

The MoFe proteins are all a2 2 tetramers of 220-240 kDa, the a and (3 subunits being encoded by the nifD and K genes, respectively. The proteins can be described as dimers of a(3 dimers. They contain two unique metallosulfur clusters the MoFeTSg homocitrate, FeMo-cofactors (FeMoco), and the FesSy, P clusters. Neither of these two types of cluster has been observed elsewhere in biology, nor have they been synthesized chemically. Each molecule of fully active MoFe protein contains two of each type of cluster 2-7). 

The quaternary structure of hemoglobin features strong interactions between unlike subunits. The i/3i interface (and its a2 2 counterpart) involves more than 30 residues, and its interaction is sufficiently strong that although mild treatment of hemoglobin with urea tends to cause the tetramer to disassemble into a/3 dimers, these dimers remain intact. The a (and a2/3i) interface involves 19 residues (Fig. 5-8). Hydrophobic interactions predominate at the interfaces, but there are also many hydrogen bonds and a few ion pairs (sometimes referred to as salt bridges), whose importance is discussed below. 

Weak ionic and hydrogen bonds occur between a 3 dimer pairs In the deoxygenated state. 

The structure of spectrin and the location of spectrin in the cytoskeleton. (a) An a/3 dimer of spectrin. Both a and f3 subunits are extended structures consisting of end-to-end domains of 106 amino-acyl residues folded into three a helices the subunits twist about one another loosely as shown. (b) The erythrocyte membrane skeleton. Spectrin tetramers ((X2P2), shown in yellow, are linked to the cytoplasmic domain of the anion channel (blue) by the protein ankyrin (red), and to glycophorin and actin filaments by protein 4.1. This structure lends stability to the red cell membrane while maintaining sufficient flexibility to allow erythrocytes to withstand substantial shear forces in the peripheral circulation. 

Transition from the T to the R structure does not involve significant changes in the contact between the components of the a/3 dimer. The change in quaternary structure results rather from large changes in the association of the al/3l dimer with the a2(32 dimer. 

The potential of the method of using mutant and chemically modified Hbs to assign proton resonances of Hb is greatly enhanced when correlated with both intensity measurements and calculations from the crystal structure of Hb as determined by X-ray diffraction. A representative example of this approach is El 1 Val in HbCO A. Based on intensity measurements on a mixture of HbCO A and ferrocy-tochrome c, Lindstrom etal. (1972b) have shown for the ring-current-shifted proton resonances at —5.86, -6.48, and -6.58 ppm from HDO that each arises from one CH3 per a(3 dimer. In the spectrum of a mixture of HbCO A and HbCO Sydney [(367(E1 l)Val — Ala],... 

Fig. 8. Ribbon diagram (S/) of a hypothetical model for the docking of Fe protein to an a 3 dimer of the MoFe protein. The Fe protein subunits, a subunit and 3 subunit, are colored light blue, red, and dark blue, respectively, while white atomic models are provided for the metal centers and ADP. 

Alternatively, free a(3 dimers may pass through the glomerular filter into the urine. The dimers are normally reabsorbed and catabolized by the proximal tubules and catabolized, with the iron incorporated into cellular ferritin and hemosiderin. Iron may reach toxic levels in the tubular... 

Figure 7.5 Quaternary structure of deoxyhemoglobin. Hemoglobin, which is composed of two a chains and two p chains, functions as a pair of a(3 dimers. (A) A ribbon diagram. (B) A space-tng model. [Drawn from lA3N.pdb.]...

As protons flow through F0, the rotation of c12 (the proton channel) is transferred to the y subunit that projects into the core of the Fj component. The rotation of the ysubunit puts it in three possible positions relative to each a/3-dimer. The binding affinity of the catalytic [3 subunit changes with the alternating position of the y subunit. Conformation A binds ADP and P weakly, conformation... 

The MoFe proteins from Clostridiumpasteurianum and from Azotobacter vinelandii have been crystallized. For the former protein, crystals of space group P2i are obtained, with two molecules per unit cell of dimensions 70 X 151 X 122 A. There is good evidence for a molecular two-fold axis, which presumably relates equivalent sites in the two a/3 dimers that make up the protein molecule.Preliminary refinement reveals that the two FeMoco units per protein are about 70 A apart and the four P clusters are grouped in two pairs. 

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