A good coordination of anabolic and catabolic processes is required for the growth and maintenance of a cell. The working of metabolic machinery is dependent on enzyme catalyzed reactions. Thus a fine control of enzyme catalyzed reactions is essential in the regulation of metabolism. For the study of these reactions, we can make use of both prokaryotic and eukaryotic organisms. There exist four major steps in metabolic regulation. They are compartmentation of enzymes, differentiating pathways for catabolism and anabolism of a key substrate, interaction of substrates with change in kinetic factors and control of enzyme activity and enzyme concentration. The factors which control the actual functioning concentration of the enzymes in a cell are the rates of enzyme synthesis and degradation. A prokaryotic cell is a cell with a plasma membrane. The plasma membrane acts as a substrate for organic enzymes. In a eukaryotic cell, compartmentation of metabolic machineries occurs for very specific purposes. A plasma membrane is associated with this cell too which serves the purpose of selective transportation of important cations, anions and neutral compounds. Metabolic reactions are always reversible which makes use of two types of enzymes. One enzyme is used for forward reaction and the other one for backward reaction. These reactions are called opposing unidirectional reactions or substrate cycles.
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