This page describes the application of some control theory concepts to metabolic regulation. It covers allosteric enzymes, covalent modification of proteins, futile cycles, regulation of gene expression and the sequestration of enzymes and substrates within compartments. Biochemical feedback loops are discussed in terms of reference values, error signals, closed loop gain, internal phase shifts and stability criteria. Input and output impedances are derived for metabolic processes, and it is shown how biologically advantageous control systems are often constrained by conventional engineering criteria. Many applications of these concepts can be seen in the separate page on the overall integration of liver metabolism. Click here to skip over the detailed contents section.