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Figure 3.
Fig. 3. Models of the carboxysome shell based on pentamer and
hexamer components. (A) A flat layer of hexagons can be folded
to give pentagonal vertices by removing one sector at each
vertex. Twelve such vertices are present in an icosahedral
shell. (B) Taken in combination, alternate choices for the
curvature of the hexagonal layer and the orientation of the
pentamer lead to four possible constructions, numbered 1 to 4
according to the quality of fit. Combination 4 led to impossible
steric collisions. The structures are colored according to
calculated electrostatic potential, from negative (red) to
positive (blue). (C) Illustration of the best packing solutions
for constructions 1 to 3. EN, calculated packing energies (27)
(with more negative values being favorable); SC, surface
complementarity (26); and SA, buried surface area between a
pentamer and a single neighboring hexamer (with higher values of
these parameters being favorable). (D) Two alternate models for
the complete carboxysome shell, based on the two constructions,
1 and 2, judged to be most plausible. There are 740 hexamers and
12 pentamers in a T = 75 arrangement. The packing of hexamers is
derived from multiple consistent crystal structures. The two
models differ with respect to the orientation of the hexameric
layer. The hexagonal layer is colored according to
hydrophobicity, with increases showing as blue to orange. The
CcmL pentamers are shown in magenta. The diameter from vertex to
vertex is 1150 Å.
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