How crowded is the bacterial cell?
The macromolecule concentration within E Coli is estimated to be around 0.3-0.4 g/ml [1]

The concentrations of your substrate in respect to your enzyme are generally fairly analogous to in vitro studies compared to in vivo studies. However, this is heavily based on the assumption that the diffusion constants for both molecules stay consistent. In many cases that is true and errors can be corrected for using PEG to imitate the crowding effect. And yes, people have begun to look at the question [2]

However, for larger macromolecules like DNA and chromosomes that do see effects from subdiffusive transport, the molecules don't obey diffusive random-wal behavior [3]. The classic model system is the lac repressor which exhibits non-diffusive kinetics due to its interactions with DNA.


http://www.ncbi.nlm.nih.gov/pubmed/1748995
http://www.ncbi.nlm.nih.gov/pubmed/21237136
http://prl.aps.org/abstract/PRL/v104/i23/e238102
http://www.ncbi.nlm.nih.gov/pubmed/7317363