Table 2.
Consistency of experimental evidence with the proposed mechanisms for CPSase
| Experimental evidence | Consistent with proposed mechanisms
|
||
|---|---|---|---|
| Sequential model (Fig. 1) | Coupled model (Fig. 2) | ||
| 1. | Functional equivalence of domains B and C (30) | No | Yes |
| 2. | Catalysis of bicarbonate-dependent cleavage of ATP to ADP + Pi (47) | Yes | Yes |
| 3. | Domain B localization of bicarbonate-dependent ATPase activity (22, 23) | Yes | Yes |
| 4. | Catalysis of ATP formation from CP + ADP (47) | Yes | Yes |
| 5. | Domain C localization of ATP formation from CP + ADP (22, 23) | Yes | Yes |
| 6. | Significant increases in the Km for ATPB observed in ATPC site mutants but not in the Km for ATPC in response to corresponding mutations at the ATPB site (22, 32, 46) | No | Yes |
| 7. | Structural equivalence of domains B and C to each other and to the N-ligases (29) | No | Yes |
| 8. | Absence of significant sequence identity for domain B or C with carbamate kinase | No | Yes |
| 9. | Expected lability of carbamate during movement from domain B to domain C occurring in the sequential mechanism | No | Yes |
| 10. | Chemical and kinetic competency of carboxy-phosphate (14–21) | Yes | Yes |
| 11. | Absence of ATP/ADP or ATP/Pi exchange, indicating ADP remains bound as a component of the enzyme–carboxy–phosphate complex (18–20) | Yes | Yes |
| 12. | ATP positional isotope exchange evidence for an alternative pathway forming carboxy-phosphate in addition to the main CP-forming pathway (18, 21) | No | Yes |
| 13. | Binding of both molecules of ATP before ammonia is bound (16, 52–54) | No | Yes |