Metabolic pathways must be regulated to release energy when required, to store extra energy, and to synthesize molecules when needed. Specific reactions or enzymes in a mechanism serve as key regulatory steps, and usually they are those with largely negative changes in free energy. These are irreversible reactions that cannot be reversed through the manipulation of cellular conditions. If a chemical reaction in one direction is irreversible, then the opposing pathway must use a different chemical reaction and different regulatory enzyme. Different regulatory enzymes for opposing pathways allows for independent regulation based on cellular conditions, also known as fine-tuning. This is essential for regulating pathways. Regulatory enzymes are often as the beginning or end of a pathway, and the step that commits the pathway to a certain response is also highly regulated.
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