3 research outputs found
Mechanical Transition from α‑Helical Coiled Coils to β‑Sheets in Fibrin(ogen)
We characterized the α-to-β transition in
α-helical
coiled-coil connectors of the human fibrinÂ(ogen) molecule using biomolecular
simulations of their forced elongation and theoretical modeling. The
force (<i>F</i>)–extension (<i>X</i>) profiles
show three distinct regimes: (1) the elastic regime, in which the
coiled coils act as entropic springs (<i>F</i> < 100–125
pN; <i>X</i> < 7–8 nm); (2) the constant-force
plastic regime, characterized by a force-plateau (<i>F</i> ≈ 150 pN; <i>X</i> ≈ 10–35 nm); and
(3) the nonlinear regime (<i>F > </i>175–200 pN; <i>X</i> > 40–50 nm). In the plastic regime, the three-stranded
α-helices undergo a noncooperative phase transition to form
parallel three-stranded β-sheets. The critical extension of
the α-helices is 0.25 nm, and the energy difference between
the α-helices and β-sheets is 4.9 kcal/mol per helical
pitch. The soft α-to-β phase transition in coiled coils
might be a universal mechanism underlying mechanical properties of
filamentous α-helical proteins
Mechanistic Basis for the Binding of RGD- and AGDV-Peptides to the Platelet Integrin αIIbβ3
Binding
of soluble fibrinogen to the activated conformation of
the integrin αIIbβ3 is required for platelet aggregation
and is mediated exclusively by the C-terminal AGDV-containing dodecapeptide
(γC-12) sequence of the fibrinogen γ chain. However, peptides
containing the Arg-Gly-Asp (RGD) sequences located in two places in
the fibrinogen Aα chain inhibit soluble fibrinogen binding to
αIIbβ3 and make substantial contributions to αIIbβ3
binding when fibrinogen is immobilized and when it is converted to
fibrin. Here, we employed optical trap-based nanomechanical measurements
and computational molecular modeling to determine the kinetics, energetics,
and structural details of cyclic RGDFK (cRGDFK) and γC-12 binding
to αIIbβ3. Docking analysis revealed that NMR-determined
solution structures of cRGDFK and γC-12 bind to both the open
and closed αIIbβ3 conformers at the interface between
the αIIb β-propeller domain and the β3 βI
domain. The nanomechanical measurements revealed that cRGDFK binds
to αIIbβ3 at least as tightly as γC-12. A subsequent
analysis of molecular force profiles and the number of peptide−αIIbβ3
binding contacts revealed that both peptides form stable bimolecular
complexes with αIIbβ3 that dissociate in the 60–120
pN range. The Gibbs free energy profiles of the αIIbβ3–peptide
complexes revealed that the overall stability of the αIIbβ3-cRGDFK
complex was comparable with that of the αIIbβ3−γC-12
complex. Thus, these results provide a mechanistic explanation for
previous observations that RGD- and AGDV-containing peptides are both
potent inhibitors of the αIIbβ3–fibrinogen interactions
and are consistent with the observation that RGD motifs, in addition
to AGDV, support interaction of αIIbβ3 with immobilized
fibrinogen and fibrin