Analogy between Enzyme and Nanoparticle Catalysis: A Single-Molecule Perspective

Catalysis as a field is classically divided into three areas: homogeneous, heterogeneous, and biological catalysis. For the latter two (i.e., biological catalysis and heterogeneous catalysis), their dissimilarity can be apparent intuitively, from the scenes of fruit fermentation in a rural winery to make wine and oil refining in a process plant to produce gasoline. In addition to the practical differences, technological terms common in both areas show divergences in meaning. A highly relevant example is the term turnover number. It refers to the maximum number of substrate molecules converted to products per enzyme molecule per second, often written as kcat in enzyme catalysis; while the same term means the number of moles of reactants that a mole of catalyst converts before deactivation, usually abbreviated as TON in heterogeneous catalysis. However, the development of characterization tools provides increasingly deeper insights into these catalytic systems, revealing their intrinsic resemblances. One evident yet important similarity is the size of these catalysts. The size of a typical enzyme molecule, the key component of biological catalysts, ranges from a few to ∼10 nm, while the active components of most heterogeneous catalysts are nanoparticles (NPs) with sizes of about a few to hundreds of nanometers. Interestingly, the nanometer sizes of enzymes and NPs (i.e., hundreds or more atoms per enzyme or NP) are often associated with heterogeneity among individual enzymes or NPs, leading to disorder (vide infra). Therefore, some catalytic behaviors of individual enzymes or NPs are often hidden from ensemble-averaged measurements. Yet, because of the small sizes of enzymes and NPs, measurements at the single-enzyme or single-particle level remain difficult, if not impossible, until the recent advent of single-molecule techniques. In this Viewpoint, we discuss the analogies between enzyme and NP catalysis based on selected recent studies of single-molecule enzyme and NP catalysis. Specifically, we focus on comparing reaction kinetics, static disorder, dynamic disorder, parallel reaction pathways, and allosteric effects between enzyme and NP catalysis at the single-molecule level. In the discussions of NPs, we mainly refer to metallic NPs, but we will cite selective references about other types of materials, such as semiconductors, layered double hydroxides, and zeolites, at the appropriate sections.