No abstract

In the cell, enzymes are almost always spatially confined in crowded and tightly controlled cellular compartments. The entrapment of enzymes in artificial nanoreactors as biomimetic systems can be expected to contribute to the understanding of the activity and the interactions of enzymes in confined spaces. The capsid of the Cowpea Chlorotic Mottle virus (CCMV) represents such an artificial nanoreactor that can be used to encapsulate multiple proteins in its interior. Employing a controlled encapsulation process we are able to load a precise number of proteins (Pseudozyma antarctica lipase B and EGFP) into the CCMV capsid and to study their activity. In the case of the enzyme, our results indicate that the apparent overall reaction rate increases upon encapsulation and is almost independent of the number of enzymes in the capsid. These observation are the result of the extremely high confinement molarity of the enzyme inside the capsid (M conf ¼ $ 1 mM) leading to very rapid formation of the enzyme-substrate complex. These results highlight the importance of small volumes for efficient multi-enzyme cascade catalysis.

The potential of single-enzyme studies to unravel the complex energy landscape of these polymeric catalysts is the next critical step in enzymology. From its inception in Rotman's emulsion experiments in the 1960s, the field of single-molecule enzymology has now advanced into the time-resolved age. Technological advances have enabled individual enzymatic turnover reactions to be observed with a millisecond time resolution. A number of initial studies have revealed the underlying static and dynamic disorder in the catalytic rates originating from conformational fluctuations. Although these experiments are still in their infancy, they may be able to relate the topography of the energy landscape to the biological function and regulation of enzymes. This review summarizes some of the experimental techniques and data-analysis methods that have been used to study individual enzyme molecules in search of a deeper understanding of their kinetics.

We have measured the low-temperature (4.2 K) exciton lifetimes of zinc-blende CdTe nanocrystal quantum dots (NQDs), 2.6-3.8 nm in diameter, in magnetic fields up to 30 T. The exciton photoluminescence decay time decreases with both dot size and magnetic field. We explain the decrease in decay time in magnetic fields by the mixing of bright and dark exciton states due to a small shape asymmetry in the zinc-blende CdTe NQDs. We show that this behavior resembles that of wurtzite CdSe NQDs, and we demonstrate that an asymmetry of NQDs caused by either shape or crystal structure leads to similar exciton decay dynamics.