4 research outputs found
Free Energy Calculations for the Peripheral Binding of Proteins/Peptides to an Anionic Membrane. 1. Implicit Membrane Models
The
binding of peptides and proteins to the surface of complex
lipid membranes is important in many biological processes such as
cell signaling and membrane remodeling. Computational studies can
aid experiments by identifying physical interactions and structural
motifs that determine the binding affinity and specificity. However,
previous studies focused on either qualitative behaviors of protein/membrane
interactions or the binding affinity of small peptides. Motivated
by this observation, we set out to develop computational protocols
for bimolecular binding to charged membranes that are applicable to
both peptides and large proteins. In this work, we explore a method
based on an implicit membrane/solvent model (generalized Born with
a simple switching in combination with the Gouy–Chapman–Stern
model for a charged interface), which we expect to lead to useful
results when the binding does not implicate significant membrane deformation
and local demixing of lipids. We show that the binding free energy
can be efficiently computed following a thermodynamic cycle similar
to protein–ligand binding calculations, especially when a Bennett
acceptance ratio based protocol is used to consider both the membrane
bound and solution conformational ensembles. Test calculations on
a series of peptides show that our computational approach leads to
binding affinities in encouraging agreement with experimental data,
including for the challenging example of the bringing of flexible
MARCKS-ED peptides to membranes. The calculations highlight that for
a membrane with a significant fraction of anionic lipids, it is essential
to include the effect of ion adsorption using the Stern model, which
significantly modifies the effective surface charge. This implicit
membrane model based computational protocol helps lay the groundwork
for more systematic analysis of protein/peptide binding to membranes
of complex shape and composition
Molecular Mechanism of Stabilizing the Helical Structure of Huntingtin N17 in a Micellar Environment
Huntington’s
disease is a deadly neurodegenerative disease
caused by the fibrilization of huntingtin (HTT) exon-1 protein mutants.
Despite extensive efforts over the past decade, much remains unknown
about the structures of (mutant) HTT exon-1 and their enigmatic roles
in aggregation. Particularly, whether the first 17 residues in the
N-terminal (HTT-N17) adopt a helical or a coiled structure remains
unclear. Here, with the rigorous study of molecular dynamics simulations,
we explored the most possible structures of HTT-N17 in both dodecylphosphocholine
(DPC) micelles and aqueous solution, using three commonly applied
force fields (OPLS-AA/L, CHARMM36, and AMBER99sb*-ILDNP) to examine
the underlying molecular mechanisms and rule out potential artifacts.
We show that local environments are essential for determining the
secondary structure of HTT-N17. This is evidenced by the insertion
of five hydrophobic residues of HTT-N17 into the DPC micelle, which
promotes the formation of an amphipathic helix, whereas such amphipathic
helices unfold quickly in aqueous solution. A relatively low free-energy
barrier (∼3 kcal/mol) for the secondary structure transformation
was also observed for all three force fields from their respective
folding-free-energy landscapes, which accounts for possible HTT-N17
conformational changes upon environmental shifts such as membrane
binding and protein complex aggregation
Binding Specificity Determines the Cytochrome P450 3A4 Mediated Enantioselective Metabolism of Metconazole
Cytochrome
P450 3A4 (CYP3A4) is a promiscuous enzyme, mediating
the biotransformations of ∼50% of clinically used drugs, many
of which are chiral molecules. Probing the interactions between CYP3A4
and chiral chemicals is thus essential for the elucidation of molecular
mechanisms of enantioselective metabolism. We developed a stepwise-restrained-molecular-dynamics
(MD) method to model human CYP3A4 in a complex with <i>cis</i>-metconazole (MEZ) isomers and performed conventional MD simulations
with a total simulation time of 2.2 μs to probe the molecular
interactions. Our current study, which employs a combined experimental
and theoretical approach, reports for the first time on the distinct
conformational changes of CYP3A4 that are induced by the enantioselective
binding of <i>cis</i>-MEZ enantiomers. CYP3A4 preferably
metabolizes <i>cis</i>-<i>RS</i> MEZ over the <i>cis</i>-<i>SR</i> isomer, with the resultant enantiomer
fraction for <i>cis</i>-MEZ increasing rapidly from 0.5
to 0.82. <i>cis</i>-<i>RS</i> MEZ adopts a more
extended structure in the active pocket with its Cl atom exposed to
the solvent, whereas <i>cis</i>-<i>SR</i> MEZ
sits within the hydrophobic core of the active pocket. Free-energy-perturbation
calculations indicate that unfavorable van der Waals interactions
between the <i>cis</i>-MEZ isomers and the CYP3A4 binding
pocket predominantly contribute to their binding-affinity differences.
These results demonstrate that binding specificity determines the
cytochrome P450 3A4 mediated enantioselective metabolism of <i>cis</i>-MEZ
Lipid Corona Formation from Nanoparticle Interactions with Bilayers and Membrane-Specific Biological Outcomes
<a></a><a>While mixing nanoparticles with certain
biological molecules can result in coronas that afford some control over how engineered
nanomaterials interact with living systems, corona formation mechanisms remain
enigmatic. Here, we report spontaneous lipid
corona formation, i.e. without active mixing, upon attachment to stationary lipid
bilayer model membranes and bacterial cell envelopes, and present ribosome-specific
outcomes for multi-cellular organisms. Experiments show that polycation-wrapped
particles disrupt the tails of zwitterionic lipids, increase bilayer fluidity, and
leave the membrane with reduced ζ-potentials. Computer simulations show contact
ion pairing between the lipid headgroups and the polycations’ ammonium groups leads
to the formation of stable, albeit fragmented, lipid bilayer coronas, while microscopy
shows fragmented bilayers around nanoparticles after interacting with <i>Shewanella oneidensis</i>. Our mechanistic insight
can be used to improve control over nano-bio interactions and to help understand
why some nanomaterial/ligand combinations are detrimental to organisms, like <i>Daphnia magna</i>, while others are not. </a