Viruses as model nanoreactors to study enzyme kinetics

In nature, the interior of a cell hosts a remarkable network of organelles and subcellular compartments in which many biochemical reactions take place. From membrane-based assemblies (such as the mitochondrion, Golgi apparatus, vesicles, and peroxisomes) to non-membrane-enclosed ribosomes, the compartmentalization of enzymes, substrates, and cofactors within a confined environment allows the cell to gain excellent spatial and temporal control over biochemical pathways [1-3]. This organization leads to a significant enhancement in substrate turnover and catalytic efficiency. As a consequence, there is much interest in the field of nanotechnology to develop enzyme nanoreactors that mimic the natural organization of cells and organelles. This involves, however, two key challenges, (1) the precise arrangement of enzymes in space and time and (2) mimicking of the molecular crowding effects imposed by the confined environment. While recent advances using top-down nanolithography approaches provide significant spatial and temporal control, it is increasingly evident that the confined environment plays a crucial role in optimizing catalytic efficiency [4]. For these reasons, much effort focused toward the development of enzyme nanoreactors, in which the enzymes are either tethered to the exterior of a nanosized assembly or encapsulated inside. While there are several examples of enzyme nanoreactors based on biosynthetic assemblies such as nanoparticles, nanopores, polymersomes, and vesicles [5, 6], such assemblies are often highly concentration dependent, display limited substrate selectivity, and have difficulties to precisely control the positioning of enzymes [7]. In recent years, much attention has turned toward protein-based assemblies, particularly viruses, as alternative building blocks for the assembly of model nanoreactors [7-12].