The regulation of enzyme activity may involve reversible covalent modifications. This allows specific pathways to be regulated by external signals, such as hormones in mammals, that are unrelated to the metabolic intermediates. Regulatory enzymes modulated by covalent modification can greatly amplify a chemical signal. An example of this amplification phenomenon can be seen in the glycogenolysis cascade.
The covalent modification of an enzyme is often accomplished by the addition of a phosphate group to a specific amino acid residue. Residues which are the usual sites of phosphorylation are serine, threonine and tyrosine hydroxyl groups. Transfer of the phosphoryl group which is derived from ATP is catalysed by a protein kinase. The reaction is a reversible one with phosphatases catalyzing the dephosphorylation reaction.
We will now examine the role of phosphorylation in the regulation of bacterial expression.