The Hydrophilic Subunits

Wolinella succinogenes QFR subunit A, of 73 kDa (Lauterbach et al., 1990), is composed of four domains (Fig. 3a, see color insert), the bipartite FAD binding domain (blue, residues Al-260 and A366-436, with A  

This mechanistic interpretation of the structure is supported by the results from site-directed mutagenesis, where Arg A301 was replaced relatively conservatively by a Lys (Lancaster etal., 2001). Strain FrdA-R301K contained a variant enzyme, very similar to the wild-type enzyme in terms of cofactor and subunit composition, in particular a fluorescence typical 

General Comparison of Membrane-Integral Diheme Cytochrome b Proteins 

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The L and the M subunits are firmly anchored in the membrane, each by five hydrophobic transmembrane a helices (yellow and red, respectively, in Figure 12.14). The structures of the L and M subunits are quite similar as expected from their sequence similarity they differ only in some of the loop regions. These loops, which connect the membrane-spanning helices, form rather flat hydrophilic regions on either side of the membrane to provide interaction areas with the H subunit (green in Figure 12.14) on the cytoplasmic side and with the cytochrome (blue in Figure 12.14) on the periplasmic side. The H subunit, in addition, has one transmembrane a helix at the car-boxy terminus of its polypeptide chain. The carboxy end of this chain is therefore on the same side of the membrane as the cytochrome. In total, eleven transmembrane a helices attach the L, M, and H subunits to the membrane. 

The bulk of the hydrophilic residues of the p subunit are exposed on the extra-... 

The ai polypeptide contains the receptors for the Ca " channel effectors as it can be photolabelled by DHPs and PAAs [30,39-41,56,57,63] and various other Ca channel effectors. The other subunits do not appear to be required for proper binding of Ca channel effectors [64], although there is some evidence from biochemical studies that the other subunits may modulate binding [65]. The oti and y subunits are very hydrophobic, while the a.2jd and jS subunits are very hydrophilic [57]. The a [30], 2, 6 and y [57] peptides are glycosylated, but oc is much less so than 2 [30]. The of both a peptides varies slightly on different types of SDS gels... 

The 4TM receptors are pentameric complexes composed of subunits of 420 to 550 amino acids. The subunits exhibit sequence identities from 25 to 75%, with a similar distribution of hydrophobic and hydrophilic domains (Table 3.1). The hydrophilic 210 to 230 amino-acid N-terminal domain is followed by three closely spaced hydrophobic and putative transmembrane domains, then a variable-length intracellular loop, and finally a fourth putative transmembrane region shortly before the C-terminus (Figure 3.1). Of the four candidate transmembrane regions, evidence suggests that TM2 forms an a-helix, while the other hydrophobic regions more likely are folded as (3-sheets. 

Fe(II) penetrates inside the spherical shell by the hydrophilic channels. After an oxidation on ferroxidase sites, located on H subunits, Fe(III) iron ions migrate to a nucleation site, situated on L subunits, where a crystal of hydrated iron oxide grows. Up to 4500 Fe(III) can be stored inside this mineral phase (31). The number of iron atoms contained in the ferritin molecule is called the loading factor (LF). 

Fig. 18.7. Schematic representation of the immobilization of the biotinylated cholera toxin B subunit using the biotin-avidin bridging system. The hydrophilic co-polymer on the transducer surface improves the permeation of the enzymatically generated quinone.

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