De-adenylation

The adenylation and phosphorolysis reactions are catalyzed by the same enzyme, adenylyl transferase. Sequence analysis indicates that this adenylyl transferase comprises two homologous halves, suggesting that one half catalyzes the adenylation reaction and the other half the phospholytic de-adenylation reaction. What determines whether an AMP unit is added or removed The specificity of adenylyl transferase is controlled by a regulatory protein (designated P or Pjj), a trimeric protein that can exist in two forms, P and Pq (Figure 24.27). The complex of P and adenylyl transferase catalyzes the attachment of an AMP unit to glutamine synthetase, which reduces its activity. Conversely, the complex of Pj) and adenylyl transferase removes AMP from the adenylylated enzyme. 

Figure 24.22 Regulation by adenylation. (A) A specific tyrosine residue in each subunit in glutamine synthetase is modified by adenylation. (B) Adenylation of tyrosine is catalyzed by a complex of adenylyl transferase (AT) and one form of a regulatory protein (Pa). The same enzyme catalyzes de-adenylation when it is complexed with the other form (Pq) of the regulatory protein.

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