Organization of metabolic pathways in vivo

A common characteristic of metabolic pathways is that the product of one enzyme in sequence is the substrate for the next enzyme and so forth. In vivo, biocatalysis takes place in compartmentalized cellular structure as highly organized particle and membrane systems. This allows control of enzyme-catalyzed reactions. Several multienzyme systems have been studied by many researchers. They consist essentially of membrane- [104] and matrix- [105,106] bound enzymes or coupled enzymes in low water media [107]. 

Traditionally, drug metabolism studies have relied on the use of model systems to predict metabolic pathways in human. For this purpose, either in vivo whole animal system (utilizing small laboratory animal models) or in vitro enzyme systems (microsomal preparations, tissue cultures or perfused organ systems) have been widely employed [14, 15]. In biomedical research, animal systems continue to serve an important role as models for biological responses in man. 

This system displays a two-enzyme kinetic model in which bioconversion is controlled by the interaction between the two reactions and the mass transfer. This situation offers a more realistic model for the conditions occurring in vivo, in which some pathways of intermediary metabolism consist of linear sequences of reactions. These pathways take place in highly organized compartments. 

---