To efficiently catalyze multi-step biochemical reaction pathways, cells have
optimized the synergistic action of a multitude of enzymes. They not only
control the concentrations and activities of enzymes, but often also
coordinate enzymes from the same biochemical reaction pathway by arranging
them in self-assembled multi-enzyme complexes. Such complexes are the basis
of `channeling' mechanisms, where intermediate products in multi-step 
reactions are effectively passed from one enzyme to the next. These
same principles can be applied in artificial nano-scale systems, 
with the ultimate goal to control and optimize biochemical reactions 
at will, e.g. for the production of medical substances or renewable 
energy sources. While enzymatic activity has been studied for over 
a century, quantitative experiments were limited to the bulk level 
until the recent advent of single-molecule enzymology techniques, 
which permit the quantitative characterization of the stochastic 
reaction dynamics of enzymes. Single-molecule experiments with 
enzymes have also spurred the theoretical analysis of stochastic 
enzyme dynamics, but the theoretical description and analysis of 
enzyme complexes is only beginning to develop. I will describe 
some recent progress in this direction.