Two possible Bi Ter Ping Pong Uni-Uni Uni-Bi kinetic mechanisms for DNA ligase. E represents the enzyme with no ligands. EA represents the enzyme with NAD+(A) bound. EB represents the substrate-inhibited enzyme with nicked DNA (B) bound. F represents the adenylylated enzyme. FP represents the adenylylated enzyme with NMN (P) bound. FB represents the adenylyated enzyme with nicked DNA bound. EQR represents the enzyme with sealed DNA (Q) and AMP (R) bound. EQ represents the enzyme with sealed DNA bound. ER represents enzyme with AMP bound noncovalently. The mechanism in which the release of sealed DNA and AMP is ordered is shown at the top. The mechanism in which the order of release of sealed DNA and AMP is random is shown at the bottom. Kinetic constants relevant to each binding, dissociation, or catalytic step are shown in italics. Since no substrates bind and no products dissociate after the 2nd step and before the 3rd step of the reaction, the two steps are combined into FB↔EQR.