Metabolic regulation is the process by which metabolic pathways, are regulated in a cell. For instance, cellular ATP has a short half-life. Therefore, cells tightly regulate the ATP-producing and utilizing pathways to ensure a continuous supply of ATP for carrying out normal physiological functions. Metabolism is regulated by controlling the activity and concentration of key enzymes involved in a metabolic pathway, using four major mechanisms. Allosteric modulation refers to the attachment of small molecules at a site other than the active site of the enzyme, stimulating or inhibiting the enzyme activity. In covalent modification, the attachment or removal of molecules, such as a phosphate group, activates or inactivates the enzyme. An enzyme's activity is also altered by the attachment of a regulatory protein to its active site, which inhibits substrate binding and subsequent product formation. Regulating the transcription of specific genes coding for metabolic enzymes controls the amount of mRNA produced and the enzyme concentration.