Gold nanoparticles partition to and increase the activity of glucose-6-phosphatase in a synthetic phospholipid membrane system

Engineered nanomaterials can alter the structure and/or function of biological membranes and membrane proteins but the underlying mechanisms remain unclear. We addressed this using a Langmuir phospholipid monolayer containing an active transmembrane protein, glucose-6-phosphatase (G6Pase). Gold nanoparticles (nAu) with varying ligand shell composition and hydrophobicity were synthesized, and their partitioning in the membrane and effects on protein activity characterized. nAu incorporation did not alter the macroscopic properties of the membrane. Atomic force microscopy showed that when co-spread with other components prior to membrane compression, nAu preferentially interacted with G6Pase and each other in a functional group-dependent manner. Under these conditions, all nAu formulations reduced G6Pase aggregation in the membrane, enhancing catalytic activity 5–6 fold. When injected into the subphase beneath pre-compressed monolayers, nAu did not affect G6Pase activity over 60 minutes, implying they were unable to interact with the protein under these conditions. A small but significant quenching of tryptophan fluorescence showed that nAu interacted with G6Pase in aqueous suspension. nAu also significantly reduced the hydrodynamic diameter of G6Pase in aqueous suspension and promoted catalytic activity, likely via a similar mechanism to that observed in co-spread monolayers. Overall, our results show that nAu can incorporate into membranes and associate preferentially with membrane proteins under certain conditions and that partitioning is dependent upon ligand shell chemistry and composition. Once incorporated, nAu can alter the distribution of membrane proteins and indirectly affect their function by improving active site accessibility, or potentially by changing their native structure and distribution in the membrane.