C5a anaphylatoxin chemotactic receptor
1 (C5aR1) is an important
target in anti-inflammatory therapeutics. The cyclic peptide antagonist
PMX53 binds to the orthosteric site located in the extracellular vestibule
of C5aR1, and the non-peptide antagonist NDT9513727 binds to the allosteric
site formed by the middle region of TM3 (trans-membrane helix), TM4,
and TM5. We catch a sight of the variational binding mode of PMX53
during the Gaussian accelerated molecular dynamic (GaMD) simulations.
In the binary complex of C5aR1 and PMX53, the PMX53 takes a dynamic
binding mechanism during the simulation. Namely, the side chain of
Arg6 of PMX53 extends to TM6-TM7 (pose 1) or swings to
TM5 (pose 2), forming a salt bridge with Glu199. Meanwhile, in the
ternary complex of C5aR1 with PMX53 and NDT9513727, the side chain
of Arg6 of PMX53 swings to TM5 (pose 2) from extending
to TM6–TM7 (pose 1) at the beginning of the GaMD simulation.
In subsequent simulation, PMX53 stabilizes in the pose 2 binding mode
by forming a stable salt bridge with Glu199. The free-energy perturbation
(FEP) calculations demonstrate that pose 1 (ΔG
binding = −10.94 kcal/mol) is more stable in the
binary complex and pose 2 (ΔG
binding = −7.91 kcal/mol) is unstable because of highly dynamic TM5.
NDT9513727 interacts directly with TM4 and TM5 and stabilizes the
hydrophobic stack between the extracellular sides of the two helices.
Therefore, pose 2 (ΔG
binding = −16.27
kcal/mol) is notably stable than pose 1 (ΔG
binding = −9.78 kcal/mol) in the ternary complex.
The identification of a novel binding mode of PMX53 and the detailed
structural information of PMX53 interacting with a receptor obtained
by GaMD simulations will be helpful in designing potent antagonists
of C5aR1.