Fig. 6.
Reaction Scheme depicting the see-saw mechanism. In the see-saw mechanism the growing Ub chain is reciprocally transferred between E2 and E3 active sites. After the initial transthiolation step to yield charged E3 ∼ Ub and free E2 (I), a new E2 ∼ Ub conjugate binds to the ligase. A Ub Lys residue on this E2 ∼ Ub conjugate would react with the E3 ∼ Ub thioester to form charged E2 ∼ Ub-Ub (II). The nascent Ub chain is then transferred back to the E3, in a manner analogous to the initial trans-thiolation step (I), to give E3 ∼ Ub-Ub and free E2 (III). Multiple cycles extend the poly-Ub chain and result in the synthesis of charged E2 ∼ Ubn.