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The structure of the reduced form of cytochrome c(6) from the mesophilic
cyanobacterium Synechococcus sp. PCC 7002 has been determined at 1.2 A and
refined to an R-factor of 0.107. This protein is unique among all known
cytochromes c(6), owing to the presence of an unusual seven-residue insertion,
KDGSKSL(44-50), which differs from the insertion found in the recently
discovered plant cytochromes c(6A). Furthermore, the present protein is unusual
because of its very high content (36%) of the smallest residues (glycine and
alanine). The structure reveals that the overall fold of the protein is similar
to that of other class I c-type cytochromes, despite the presence of the
specific insertion. The insertion is located within the most variable region of
the cytochrome c(6) sequence, i.e. between helices II and III. The first six
residues [KDGSKS(44-49)] form a loop, whereas the last residue, Leu50, extends
the N-terminal beginning of helix III. Several specific noncovalent interactions
are found inside the insertion, as well as between the insertion and the rest of
the protein. The crystal structure contains three copies of the cytochrome c(6)
molecule per asymmetric unit, and is characterized by an unusually high packing
density, with solvent occupying barely 17.58% of the crystal volume.
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