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Figure 1.
Figure 1. Two cadmium (Cd^2+) binding sites on the p-side of
the M. laminosus b[6]f complex. (a) Position of higher occupancy
site (Cd1) is close to the inter-monomer interface, and that of
lower occupancy (Cd2) site is near the small subunits and the
exterior of the complex. View is parallel to the plane of the
membrane. Distances: (i) from Cd1 site, and (ii) from Cd2, to
the [2Fe-2S] cluster on the same and opposite side monomer, (i)
38.9 Å and 40.1 Å, and (ii) 57.1 and 28.0 Å.
Color code: cytochrome b[6] (cyan), subunit IV (purple),
cytochrome f (red), ISP (yellow), PetG, PetL, PetM, and PetN
(green). (b, stereo) Environment of Cd1 and Cd2 sites shown in
more detail. Lower occupancy of the Cd2 site is shown by the
smaller cage of electron density. A lipid molecule (possibly
galactolipid) described in the coordinates of the C. reinhardtii
b[6]f complex (pdb; 1Q90), but not previously discussed, is
closer to Cd2. Distances: higher (Cd1) to lower occupancy (Cd2)
Cd^2+ site, 23.2 Å; higher occupancy Cd1 site to heme
b[p], 24.0 Å; Cd2 to residues Glu78, Trp79 and Y80 of the
PEWY loop on the same monomer, 13, 12, and 16 Å. The seven
peaks of largest amplitude in the anomalous difference map are:
Cd1 site (18.5 σ), heme b[p] (18.1 σ), heme c[n] (17.6 σ),
heme b[n] (17.0 σ), heme f (13.3 σ), [2Fe-2S] site (11.1 σ),
Cd2 site (6.3 σ). The anomalous scattering factors (f  letter
double prime ) for Fe and Cd at 0.98 Å are 1.50 and 2.13,
respectively. The B factors of the Cd1 and Cd2 sites are 80
Å^2 and 178 Å^2.
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