Enzyme Activity within the Living Cell

Enzyme processes do not display any apparent “finality” when considered isolated, however they do within the cell. In fact the global behavior of the cell relies upon a co-ordination of many enzyme reactions. The aim of the so-called “metabolic control theory” is to show how many enzyme reactions are connected as to form a coherent network that possesses properties that are quantitatively and qualitatively different from those of individual enzyme reactions. The concept of “cascade” is discussed in this Chapter and their properties are shown to be different from those of the same enzymes considered in isolation. Many enzymes within the cell are associated with cell structures that behave as polyanions. Electrostatic repulsion of negatively charged substrates by the negative charges of plant cell wall generates an apparent negative co-operativity of enzyme reactions that take place in the cell wall. These effects can be modulated by external ionic strength. It follows that the behavior of enzymes in the cell have properties quite different from the same enzymes in free solution.

Keywords: Metabolic control theory, Parameters of a metabolic process, Summation theorems, Homogeneous functions, Elasticity of an enzyme reaction, Enzyme cascades, Electrostatic partitioning of ions, Enzymes and electrostatic partitioning, Donnan equation, Bound-enzyme activity and heterogeneously charged matrices, Enzyme activity and heterogeneously charged matrices, Enzyme co-operativity and complexity of charged matrices, Enzymes and plant cell wall extension.