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COI PROTEIN
Structure
The COI protein, which
ranges in length from 510-530 amino acids among different
animal species, is divided into 25 structural regions.
These regions consist of the amino and carboxyl terminals,
6 external loops that extend into the cellular cytoplasm,
5 internal loops and 12 alpha-helix segments which traverse
the inner membrane of the mitochondrion. To simplify topographic
discussions, we divide the protein into two sections (COI-5',
COI-3'). The COI-5' section is 215 amino acids long in
Drosophila melanogaster and extends from the
amino terminal to the begining of M6, the sixth membrane
segment. The COI-3' section which extends from the point
to the carboxyl terminal of the protein is 233 amino acids
long in this species.
COI
Amino Acid Variation
Although the COI protein has
a critical metabolic function that is conserved across
all life that employs oxidative metabolism, there is substantial
variation in its amino acid composition. For example,
Lunt et al (1996) found variation in 125 of 522 amino
acid positions in their comparison of COI proteins from
9 insect species. Levels of amino acid variation varied
across the molecule with the carboxyl terminal showing
the greatest variation. Among the 5 internal loops, I1
was least variable, while I2 and I4 were most variable.
M1, M3, M4, and M12 were the most variable of the membrane
segments, while M6 was least diverse. All of the external
loops showed similar levels of variation except E4 that
was invariant.
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