Stereoselective synthesis of the head group of archaeal phospholipid PGP-Me to investigate bacteriorhodopsin–lipid interactions

Abstract

Phosphatidylglycerophosphate methyl ester (PGP-Me), a major constituent of the archaeal purple membrane, is essential for the proper proton-pump activity of bacteriorhodopsin (bR). We carried out the first synthesis of the bisphosphate head group of PGP-Me using H-phosphonate chemistry that led to the production of a simplified PGP-Me analogue with straight alkyl chains. To investigate the role of this head group in the structural and functional integrity of bR, the analogue was used to reconstitute bR into liposomes, in which bR retained the original trimeric structure and light-induced photocycle activity. Enhanced ordering of an alkyl chain of the (2)H-labelled analogue was observed in (2)H NMR spectra upon interaction with bR. These results together suggest that the bisphosphate moiety plays a role in the proper functioning of bR through the lipid-protein interaction.

Average Conformation of Branched Chain Lipid PGP-Me That Accounts for the Thermal Stability and High-Salinity Resistance of Archaeal Membranes

“…We envision that 1 could be synthesized by the coupling reaction between archaeol and the chiral diphosphoric bisglycerol moiety, whose synthetic route was already established in our previous study. 47 For archaeol, its methyl-branched side chains could be introduced through an etherification reaction of protected glycerol with phytanyl tosylate. Because the phytanyl moiety is a saturated isoprenoid, it would be efficiently constructed by the iterative carbon elongation via a C sp 3 −C sp 3 cross-coupling reaction of C 5 Grignard reagents and tosylates, which could be prepared from the isoprene unit.…”

“…Its enantiopurity was confirmed to be quite high by the good agreement of the optical rotation value with the reported value. 54 After tosylation of enantiopure phytanol 14, product 15 was connected to a separately prepared glycerol moiety 7 47 followed by debenzylation to afford archaeol 16. Finally, the coupling reaction with the chiral diphosphoric headgroup 6 under the same condition reported previously 47 led to the completion of the first enantioselective total synthesis of PGP-Me (1) in 20 steps from γ-butyrolactone 5.…”

“…Many excellent studies have reported on the synthetic method of archaeal lipids encompassing acyclic and macrocyclic isoterpenoid chains. − Our retrosynthetic analysis for PGP-Me 1 is shown in Figure . We envision that 1 could be synthesized by the coupling reaction between archaeol and the chiral diphosphoric bisglycerol moiety, whose synthetic route was already established in our previous study . For archaeol, its methyl-branched side chains could be introduced through an etherification reaction of protected glycerol with phytanyl tosylate.…”

Enantiomers of phospholipids and cholesterol: A key to decipher lipid‐lipid interplay in membrane

“…86 A previous study revealed that the proton-pump activity of bacteriorhodopsin, which is an archaeal membrane protein, is well preserved in the membrane of a PGP-Me derivative without methyl branching. 87 The acidic head group of PGP-Me may be involved in the stabilization of the bacteriorhodopsin trimer as well as the promotion of photocycling. 88 Commercially available diphytanoyl phosphocholine (DPhPC), bearing two phytanoyl groups (Figure 11) that are a diastereomeric mixture with respect to the methyl branching carbon atoms, was often used as a model lipid to record channel conductance because of the low water permeability and high electric insulation of DPhPC as compared with those of straight acyl-chain lipids.…”

“…Solid‐state NMR analysis and MD simulations of PGP‐Me reported the significant intertwining of the phytanyl chain with methyl branching, which makes the archaeal membranes resistance to extreme environments . A previous study revealed that the proton‐pump activity of bacteriorhodopsin, which is an archaeal membrane protein, is well preserved in the membrane of a PGP‐Me derivative without methyl branching . The acidic head group of PGP‐Me may be involved in the stabilization of the bacteriorhodopsin trimer as well as the promotion of photocycling .…”