Lipid-Based Catalysis Demonstrated by Bilayer-Enabled Ester Hydrolysis

TL;DR

This paper shows that lipid bilayers can catalyze ester hydrolysis, challenging the traditional view of lipids as non-catalytic in biology.

Contribution

Demonstrates lipid aggregates can act as catalysts by enabling ester hydrolysis in a bilayer environment.

Findings

Lipid bilayers catalyzed ester hydrolysis of calcein-AM, producing a fluorescent product.

Catalytic turnover numbers were measured at 10−7 to 10−8 s−1, slower than enzymatic reactions.

1H-NMR confirmed the reaction product was consistent with ester hydrolysis.

Abstract

Lipids have not traditionally been considered likely candidates for catalyzing reactions in biological systems. However, there is significant evidence that aggregates of amphiphilic compounds are capable of catalyzing reactions in synthetic organic chemistry. Here, we demonstrate the potential for the hydrophobic region of a lipid bilayer to provide an environment suitable for catalysis by means of a lipid aggregate capable of speeding up a chemical reaction. By bringing organic molecules into the nonpolar or hydrophobic region of a lipid bilayer, reactions can be catalyzed by individual or collections of small, nonpolar, or amphiphilic molecules. We demonstrate this concept by the ester hydrolysis of calcein-AM to produce a fluorescent product, which is a widely used assay for esterase activity in cells. The reaction was first carried out in a two-phase octanol–water system, with the organic phase containing the cationic amphiphiles cetyltrimethylammonium bromide (CTAB) or octadecylamine. The octanol phase was then replaced with phospholipid vesicles in water, where the reaction was also found to be carried out. The reaction was monitored using quantitative fluorescence, which revealed catalytic turnover numbers on a scale of 10−7 to 10−8 s−1 for each system, which is much slower than enzymatic catalysis. The reaction product was characterized by 1H-NMR measurements, which were consistent with ester hydrolysis. The implications of thinking about lipids and lipid aggregates as catalytic entities are discussed in the context of biochemistry, pharmacology, and synthetic biology.

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