Figure 3.
FIGURE 3. The loss of active site pairing in carboxysomal
-carbonic anhydrases. In
P. sativum (A, upper panel), active site pairing is accomplished
through homodimerization. Molecule A of the dimer is shown in
yellow; molecule B is shown in red. Green spheres mark the
location of the two identical active site zinc ions. An ellipse
marks the location of a two-fold symmetry axis. Superimposed
molecules A and B are shown in C, lower panel. Only the very
C-terminal (C-term) tails are not superimposable (colored gray).
In P. purpureum (B, upper panel), the same pairing is
accomplished by a single polypeptide, not a dimer. The
N-terminal (N-term) domain is shown in yellow; the C-terminal
domain is shown in red. Pseudo-two-fold symmetry is still
evident but is not exact (B, lower panel). Non-superimposable
regions are shown in gray. Presumably, the P. purpureum carbonic
anhydrase arose from gene duplication and fusion. Divergence
appears minimal (70% sequence identity between domains). In A,
upper panel, CsoSCA, like the P. purpureum enzyme appears to be
the result of gene duplication and fusion. However, divergence
between the two internal domains has progressed to the extent
that the C-terminal domain has lost all active site residues
that it presumably once contained. Superimposed catalytic and
C-terminal domains (C, lower panel) show much larger areas of
structural nonequivalence (gray).
The above figure is reprinted
by permission from the ASBMB:
J Biol Chem
(2006,
281,
7546-7555)
copyright 2006.
-carbonic anhydrases. In
P. sativum (A, upper panel), active site pairing is accomplished
through homodimerization. Molecule A of the dimer is shown in
yellow; molecule B is shown in red. Green spheres mark the
location of the two identical active site zinc ions. An ellipse
marks the location of a two-fold symmetry axis. Superimposed
molecules A and B are shown in C, lower panel. Only the very
C-terminal (C-term) tails are not superimposable (colored gray).
In P. purpureum (B, upper panel), the same pairing is
accomplished by a single polypeptide, not a dimer. The
N-terminal (N-term) domain is shown in yellow; the C-terminal
domain is shown in red. Pseudo-two-fold symmetry is still
evident but is not exact (B, lower panel). Non-superimposable
regions are shown in gray. Presumably, the P. purpureum carbonic
anhydrase arose from gene duplication and fusion. Divergence
appears minimal (70% sequence identity between domains). In A,
upper panel, CsoSCA, like the P. purpureum enzyme appears to be
the result of gene duplication and fusion. However, divergence
between the two internal domains has progressed to the extent
that the C-terminal domain has lost all active site residues
that it presumably once contained. Superimposed catalytic and
C-terminal domains (C, lower panel) show much larger areas of
structural nonequivalence (gray).