Enhancement in enzymatic activity after attachment to nanoparticle surfaces has been observed in numerous enzyme systems, although the underlying mechanism for these enhancements remains largely unknown. This work explores the utility of a model based on a reaction scheme that takes into account some of the many interactions between substrate, product, and nanoparticle that can occur. This model was utilized to make predictions about the type of behavior that should manifest itself with quantum dots peripherally displayed around beta-galactosidase (&beta-gal) and confirmed empirically. &beta-gal is a homotetrameric enzyme which at ~465 kDa is significantly larger than the 4.2 nm diameter green emitting quantum dots utilized to decorate its periphery. Because &beta-gal operates near the diffusion limit, this provides an opportunity to selectively investigate certain aspects of enzyme enhancement when attached to a nanoparticle with minimal perturbation to the native enzyme structure. Enzymatic assays were performed with both free enzyme and quantum dot-decorated enzymes in a side-by-side format where kinetic processes were challenged by increasing viscosity with glycerol and competitive inhibitors such as lactose. The results from this model suggest it is possible to achieve significant enhancements in a diffusion limited enzyme’s catalytic rate (< i>k< sub>cat< sub>< i>) after NP attachment without substantial changes to the enzyme’s structure or function. Because cell free synthetic biology is gaining importance, this approach will yield insights on how enzymes can be utilized ex vivo and how being attached to NP scaffolds yields kinetic enhancement, possibly through enhanced product dissociation.
Semiconductor quantum dots (QDs) serve as a valuable platform for understating the intricacies of nanoparticle cellular uptake and fate for the development of theranostics. Developing novel internalization peptides that maximize cellular uptake while minimizing the amount of peptide is important to allow space on the nanoparticle for other cargo (e.g. drugs). We have designed a range of branched, dendritic internalization peptides composed of polyarginine (Arg<sub>9</sub>) branches (1 to 16 repeats) attached a dendritic wedge based on the sequence WP9G2H6. By attaching these branched dendritic peptides to QD’s, we can study the influence of branching on cellular uptake as a function of time, ratio, and degree of branching.
Enzymes are important players in multiple applications, be it bioremediation, biosynthesis, or as reporters. The business of catalysis and inhibition of enzymes is a multibillion dollar industry and understanding the kinetics of commercial enzymes can have a large impact on how these systems are optimized. Recent advances in nanotechnology have opened up the field of nanoparticle (NP) and enzyme conjugates and two principal architectures for NP conjugate systems have been developed. In the first example the enzyme is bound to the NP in a persistent manner, here we find that key factors such as directed enzyme conjugation allow for enhanced kinetics. Through controlled comparative experiments we begin to tease out specific mechanisms that may account for the enhancement. The second system is based on dynamic interactions of the enzymes with the NP. The enzyme substrate is bound to the NP and the enzyme is free in solution. Here again we find that there are many variables , such as substrate positioning and NP selection, that modify the kinetics.
Nanosensors employing quantum dots (QDs) with appended biofunctional moieties offer tremendous promise for disease surveillance/diagnostics and chemical/biological threat activity. Their small size permits cell penetration and their inherent photochemical properties are well-suited for rapid, optical measurement. The effectiveness of enzymes immobilized on QDs, however, are not completely understood, hindering development of chemical/biological sensors and remediation materials. Here, we analyze enzyme effectiveness for the neutralization of a simulant nerve agent when attached to two distinctly-sized QDs. Two sizes of QDs, 525 or 625 nm, were appended with DHLA ligands to improve aqueous stability and prevent aggregation. Various molar ratios of de novo phosphotriesterase trimer (PTE<sub>3</sub>) were rapidly self-assembled via spontaneous metal coordination of the PTE oligohistidine tag onto the Zn<sup>2+</sup>-rich QD surface. PTE catalyzes the detoxification of organophosphate pesticides (e.g, paraoxon, an analog of sarin) to p-nitrophenol whose absorbance can be measured at 405 nm. The optimal ratio of PTE<sub>3</sub> to 525 nm and 625 nm QD’s was determined to be 12 and 24, respectively. The enhanced enzyme performance in both cases is most likely due to increased enzyme-substrate interactions from improvements in enzyme orientation, enzyme density, and substrate diffusion on or near the QD. Development of these nansosensors as optical-based biosensors (e.g., within compact microfluidic devices) may greatly improve the sensitivity of conventional biological/chemical detection schemes.
The controlled delivery of nanomaterials to the plasma membrane is critical for the development of nanoscale probes that can eventually enable cellular imaging and analysis of membrane processes. Chief among the requisite criteria are delivery/targeting modalities that result in the long-term residence (e.g., days) of the nanoparticles on the plasma membrane while simultaneously not interfering with regular cellular physiology and homeostasis. Our laboratory has developed a suite of peptidyl motifs that target semiconductor nanocrystals (quantum dots (QDs) to the plasma membrane where they remain resident for up to three days. Notably, only small a percentage of the QDs are endocytosed over this time course and cellular viability is maintained. This talk will highlight the utility of these peptide-QD constructs for cellular imaging and analysis.