EVCoupling analysis also revealed another FPEC pair involving residues
(D120/K316) near the structural siroheme moiety (in A. fulgidus )
(bold in Table 2). This pair was identified as an FPEC pair when
compared against a contact map of the D. vulgaris DsrAB X-ray
crystal structure.
However,
when highlighted on the structure of A. fulgidus DsrAB, the
residues are in contact. Hence it is a true contact for some members of
the protein family, and a false predicted contact for others. Aligning
the three structures of DsrAB from A. fulgidus , D.
vulgaris, and the model of MV2-Eury (Figure 8) reveals an antiparallel
β-hairpin/loop insert in the A. fulgidus DsrB subunit (Figure 8,
light pink) which brings aspartate 120B within 7.4 Å of lysine 316B and
also makes contact with the end of a helix in the DsrA subunit. This
insert includes a tryptophan residue (in CPK color) that blocks access
to the structural heme in the A. fulgidus structure (see also
Figure 3). The MV2-Eury and D. vulgaris DsrB subunits (Figure 8,
cyan, deep teal) lack this extension. The phenylalanine residue in these
sequences, that corresponds to the aspartate in the FPEC pair residue
D120B/K316B in A. fulgidus, is far (~27 Å) from
its co-evolving partner residue (Figure 8, red, raspberry sticks on the
right side), thus providing a false positive signal. Interestingly, this
insert in the DsrB subunit of A. fulgidus (Figure 3, pink) is
absent from all other DsrB sequences (Fig. 9), posing the question of
why the co-evolutionary coupling probability was high (99%) for this
pair.