PGLa belongs to a class of antimicrobial peptides showing strong affinity to anionic bacterial membranes. Using all-atom explicit solvent replica exchange molecular dynamics with solute tempering, we studied binding of PGLa to a model anionic dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol (DMPC/DMPG) bilayer. Due to a strong hydrophobic moment, PGLa upon binding adopts a helical structure and two distinct bound states separated by a significant free energy barrier. In these states, the C-terminus helix is either surface bound or inserted into the bilayer, whereas the N-terminus remains anchored in the bilayer. Analysis of the free energy landscape indicates that the transition between the two states involves a C-terminus helix rotation permitting the peptide to preserve the interactions between cationic Lys amino acids and anionic lipid phosphorus groups. We calculated the free energy of PGLa binding and showed that it is mostly governed by the balance between desolvation of PGLa positive charges and formation of electrostatic PGLa-lipid interactions. PGLa binding induces minor bilayer thinning but causes pronounced lipid redistribution resulting from an influx of DMPG lipids into the binding footprint and efflux of DMPC lipids. Our in silico results rationalize the S-state detected in NMR experiments.