Table 3.
Steady-state kinetic parameters of S. pyogenes NadD and NadC enzymes and comparison with human counterparts
| Enzymea | Variable substrate | Fixed substrateb | Km (μM)c | kcat (s−1)c | kcat/Km (s−1 mM−1) | Source or reference |
|---|---|---|---|---|---|---|
| sp.NadDd | NaMN | ATP | 103 ± 18 | 17.4 ± 0.7 | 1.7 × 102 | This study |
| NMN | ATP | ND | ND | This study | ||
| sp.NadC | Qa | PRPP | 10 ± 3 | 0.1 ± 0.01 | 10 | This study |
| hsPNAT-1e | NaMN | ATP | 68 ± 2 | 43 ± 1 | 6.3 × 103 | 46 |
| NMN | ATP | 22 ± 3 | 54 ± 3 | 2.4 × 103 | 46 | |
| hsQaPRTg | Qa | PRPP | 22 ± 3 | 1.19 ± 0.05f | 38 |
Initial rates were measured by coupled (sp.NadD) or direct (sp.NadC) spectrophotometrical assays.
For fixed substrates, concentrations were 0.25 mM PRPP and 2 mM ATP.
The kinetic parameters Km and kcat are apparent values (±standard deviations) determined at fixed (saturating) concentrations of cosubstrates. ND, not detectable.
The NMN adenylyltransferase activity of sp.NadD was not detected under the assay conditions that ensured a maximal rate for NaMN.
Note that for hsPNAT, data were reported only for human PNAT isoform 1 (hsPNAT-1), the most abundant and ubiquitous of the three human PNAT isoforms. The kinetic properties of hsPNAT-2 and hsPNAT-3 are similar to those of hsPNAT-1 (46, 47).
Expressed in μM min−1.
hsQaPRT, human quinolinate phosphoribosyltransferase.