Figure 4: 2D free energy landscape diagram of Tau_plane ,
Tau_glyc , Tau_phos , Tau_p356 and Tau_p352 along with the structure
corresponding to the local energy minima. The figure shows the wide
distribution of structural ensembles in Tau_plane with a large energy
barrier, while in Tau_glyc and Tau_phos, we have comparatively narrow
distribution and fewer local energy minima.
Figure S4 shows Tau_plane having four funnels each
corresponding to a local minimum with an energy barrier separating them
suggesting the folding process is rugged and not stable. The 2D contour
plot, Figure 5 , indicates the wide distribution of ensembles
suggesting many of the confirmations are trapped in high energy states.
Tau_glyc shows two local minima, without much energy barrier separating
them and the conformational ensembles are not widely distributed as in
Tau_plane. This suggests a comparatively stable folding process in
Tau_glyc as compared to Tau_plane. Tau_phos system have a single
energy minimum, suggesting the protein folds into a single native
structure, but we can see that the funnel is broad and have many
confirmations trapped in high energy states. Five local energy minima
observed in the FEL of Tau_p356 indicate the rugged nature of the
folding process. These local minima are separated by a large energy
barrier, suggesting the possibility of the protein getting trapped in
one local energy minima. Tau_p352 FEL indicates two local minima, one
at low energy and another at high energy, separated by a large energy
barrier, suggesting the entrapment of protein in high energy states.
Tau_plane having four local energy minima metastable states have an
energy barrier of about 3-4 KJ/mol separating them, while in tau_glyc
we have two low energy metastable states with ~‘2KJ/mol
energy barrier. In Tau_phos the we have a single prominent lower energy
state separated by the next higher energy state by a transition barrier
of ~3KJ/mol. In the case of tau_p356 and tau_p352 the
lowest energy metastable states are separated by an energy barrier of
~ 6-7 KJ/mol.
Structural analysis of the lowest energy metastable conformations from
each of the systems obtained from FEL were performed. The four local
energy minima conformations of Tau_plane (I-IV) was analyzed for
secondary structure preference, especially beta sheet formation, and
none of the four local energy minimal structures had beta sheet
confirmations, while only III had a 5 percent isolated beta bridge. Both
local minima confirmations (V & VI) of Tau_glyc had beta sheets with
10 percent beta sheets in each structure. Both V and VI were analyzed
for salt bridges and we found that V had two salt bridges Asp345-Lys343
& Glu372- Lys375, while VI also had two salt bridges, Glu380-Lys347 &
Glu380-Lys353. The lowest energy conformation of Tau_phos VII had zero
percent beta sheets but had around 10 percent residues with beta bridges
and no salt bridges. The energy minimal metastable structures of
Tau_p356 and Tau_p352 did not have any beta sheets but largely exist
in random coils and helix. This suggests that the N-glycosylation
induced folding of tau is able to form beta sheet rich regions in the
protein, while the unmodified tau is having an unstable folding process
with none of the lowest energy metastable structures having beta sheet
propensity.