Nanoparticle Size Influences Localized Enzymatic Enhancement—A Case Study with Phosphotriesterase

Enhancements in enzymatic catalytic activity are frequently observed when an enzyme is displayed on a nanoparticle (NP) surface. The exact mechanisms of how this unique interfacial environment gives rise to this phenomenon are still not understood, although evidence suggests that it can help alleviate some of the enzyme's rate-limiting mechanistic steps. The physicochemical limitations that govern when this process arises are also not known including, in particular, the range of NP size and curvature that are associated with it. To investigate the latter, we undertook a case study using the enzyme phosphotriesterase (PTE) and a series of differentially sized gold NPs (AuNPs). PTE, expressed with a terminal hexahistidine sequence, was ratiometrically coordinated to a series of increasing size AuNPs (diameter ≃ 1.5, 5, 10, 20, 55, 100 nm) surface-functionalized with Ni²⁺-nitrilotriacetic acid ligands and its activity assayed in a comparative format versus that of equivalent amounts of free enzyme controls. PTE-AuNP samples were prepared where the total PTE concentration and NP surface density were kept fixed by varying AuNP concentration along with the converse format. Assembly to the AuNPs increased PTE k_cat ca. 3-10-fold depending upon NP size, with the smaller-sized particles showing the highest increase, while enzyme efficiency only increased 2-fold. Further kinetic testing suggested that the PTE enhancement again arose from alleviating its rate limiting step of enzyme-product release and not from a change in the activation energy. Comparison of k_cat and enzyme specificity with AuNP diameter revealed that enhancement was directly correlated to AuNP size and curvature with the smaller NPs showing the largest kinetic enhancements. Kinetic simulations showed that almost all of the PTE enhancement variation across AuNP sizes could be reproduced by adjusting only the rate of enzyme-product dissociation. Understanding how NP size directly affects the enhancement of an attached enzyme can provide a rational basis for designing hybrid enzyme-NP materials that specifically exploit this emergent property.