Analysis and modeling of substrate uptake and product release by prokaryotic and eukaryotic cells

Translocation of molecules and ions across cell membranes is an important step for a complete description of the metabolic network in terms of kinetics, energetics and control. With a few exceptions, most molecules cross the permeability barrier of the membrane with the aid of membrane-embedded carrier proteins. Uptake of nutrients (carbon, energy and nitrogen sources as well as supplements) and excretion of the majority of products are thus carrier-mediated transport processes. Consequently, they are characterized by particular kinetic properties of the respective carrier systems, they depend on energy sources (driving forces) which must be provided by the cell, and they are subject to regulation both on the level of activity and expression. They are thus fully integrated into the functional and regulatory networks of the cell. Structural (primary structure, conformation and topology) and functional properties (kinetics, energetics and regulation) of the different classes of carrier systems from both prokaryotic and eukaryotic membranes are summarized. The methodical requirements for a quantitative measurement of their function and possible pitfalls in transport studies are described, both for determination using isolated cells and for analysis in a bioreactor. The significance of transport reactions for biotechnological processes in general and for metabolic design in particular is discussed, with respect to nutrient uptake, product excretion and the occurrence of energy wasting combinations of transport reactions (futile cycles). Some examples are given where transport reactions have been incorporated into modeling approaches with respect to metabolic control, to flux analysis, to kinetic properties and to energetic demands.