In the D. vulgaris structure, the residues making contact between
heterodimers at the central interface are two proline residues
(P368B1/B2) that stack on each other (Table 1, Figure
6A),
whereas the interacting residues in the MV2-Eury homology model based on
the D. vulgaris structure are two arginine residues (R337B1/B2)
detected in the EVCoupling analysis as possible monomeric FPECs (Table
1, Figure 6B). This central interfacial residue is found to be an
asparagine in 33.5% of the sequences and a serine in 7% of the
sequences. In 42% of the sequences in the Dsr multiple sequence
alignment (MSA) used by the EV Couplings server, the C-terminus is
truncated prior to this residue at the central interface, which may
correspond to sequencing errors or inclusion of partial sequences in
this database. Note that, in the D. vulgaris structure, the B1
and B2 helices at the central interface swap positions with respect to
the A. fulgidus structure (Figure 1). The aromatic residues,
F365B1 and Y412A2, in the D. vulgaris heme road are switched with
respect to Y348B1 and F394A2 in A. fulgidus . Like the MV2-Eury
heme road, the third aromatic residue in A. fulgidus, F317A1 is
replaced by a residue from the B1 subunit, I190B1.
Examination of the heme road residues in the structural alignment of the
MV2-Eury homology and AF2 models reveals that they are slightly shifted
between the two models, with S167 of the B1/2 subunits in the AF2 model
exhibiting a steric clash between models with two of the Fe-S centers in
the homology model. The two residues constituting the central interface
between heterodimers in MV2-Eury B1/2 subunits, R337B1-R337B2, are
superimposed at the level of the backbone between the homology and AF2
models, but the side chains point away from each other in the AF2 model,
while they interact closely with each other in the homology model
(Figure S3). The positioning of the A-subunit C-terminal heme road
residue, N369A1/2, in MV2-Eury is quite similar between the homology and
AF2 models, despite the very different positioning of the C-terminal
helix (Figure 4). The three residues in each heterodimer comprising the
heme road on either side of the central interface are not in direct
contact, supporting the notion that the co-variance is not uniquely
structural in nature. Results of the evolutionary coupling using
different subunits from different homologues as bait supports the notion
that at least one pair of these residues, T351B1-N393A2 and vice versa
in A. fulgidus , is truly co-varying. The positioning of N180B1/2
in interaction with the siroheme and relatively close to N393A2/1 is
suggestive that this constitutes a second co-varying pair. Based on
these observations, we hypothesize that the heme road represents a
pathway of cooperative communication between active sites within the
DsrAB heterotetramer. The aromatic residues packed between them (Figure
5D) could help modulate information transfer in this putative allosteric
route. Alternatively, given the relatively short distances between the
heme road residues and the intervening aromatic residues, we cannot rule
out that this represents a pathway for electron transfer between
heterodimer active sites.