In vivo & (classical) in vitro
1) "Most measurements in enzyme kinetics are based on initial rate measurements, where only the substrate is present… enzymes in cells operate in the presence of their products" Fell p.54 (Pub)
2) Enzymes & substrates are closer to equimolar than in classical in vitro experiments.
3) Proteins close to crystalline densities so some reactions occur faster while some normally spontaneous reactions become undetectably slow.
e.g. Bouffard, et al., Dependence of lactose metabolism upon mutarotase encoded in the gal operon in E.coli.
J Mol Biol. 1994; 244:269-78. (Pub)