All-Atom Molecular Dynamics Simulations of Polyethylene Glycol (PEG) and LIMP-2 Reveal That PEG Penetrates Deep into the Proposed CD36 Cholesterol-Transport Tunnel

Polyethylene glycol (PEG) is the most prominent clinically administered synthetic polymer. For example, over 300 million people have been administered PEGylated liposome vaccines for SARS-CoV-2. PEG is used in mammals because it has low affinity for most proteins and vice versa. However, this makes it difficult to study the few interactions with proteins that PEG has. On the atomistic level, there are two PEG -protein structures: (1) PEG-LIMP-2 and (2) PEG-alpha PEG. In the first structure, two monomers of a 1.5 kDa PEG polymer (PEG2) had electron density deep in the postulated cholesterol transport tunnel of LIMP-2, a lysosomal cholesterol transport protein and member of the CD36 super family of proteins. It is unclear how PEG entered this tunnel. In the second structure, PEG wrapped around a surface-exposed tryptophan on its antibody. Since tryptophan is a rare residue, it is unclear if this PEG-Trp interaction is ubiquitous. To gain deeper mechanistic insight into PEG-protein interactions, we surrounded the LIMP-2 apo structure with 13 PEG chains of 10 monomers each (PEG10), water, and KCl and simulated the system using NAMD. One of the 13 chains penetrated LIMP-2 and came within 3 angstrom of PEG2. This was possible because of the strong hydrogen bonding between multiple oxygens along PEG10 and Arg192 but, most importantly, the clamping of the tertiary structure on PEG10. Clamping stabilized the movements of PEG10, and the leading oxygen of PEG10 was able to penetrate LIMP-2 and head toward to the position occupied by PEG2. Phe383 appears to act as a gate for objects to move through this cavity, which continues to the basal/membrane side of LIMP-2. Of all residues, PEG10 molecules had the most sustained interactions with lysine and arginine because of their strong hydrogen-bonding capabilities. These results show that the oxygens of PEG bind residues with high hydrogen bonding capabilities. However, the PEG-protein interaction is likely to be transient unless groups of resides can clamp down on PEG or a cavity that at least part of the PEG chain can enter is in close proximity to lower PEG's entropy.

Automated summary

Polyethylene glycol (PEG) is widely used in clinical settings and has low affinity for most proteins. However, its interactions with proteins are difficult to study. In this research, two structures of PEG-protein interactions were identified. It was found that PEG can penetrate proteins through hydrogen bonding and clamping of tertiary structure. The study also revealed the importance of residues with high hydrogen bonding capabilities in PEG-protein interactions.