Table I.
Comparison of kinetic constants from G. sulphuraria and higher plant pPT homologs
The Michaelis constant (KM) for Pi was determined using various external [32P]Pi concentrations (0.05–10 mm). The competitive inhibition constant (Ki) of [32P]Pi uptake was assayed at two different external Pi concentrations with increasing inhibitor concentrations (0.05–20 mm; Dixon, 1953). All proteoliposomes were preloaded with 30 mm Pi. Data represent the arithmetic mean ± se of at least three independent experiments. Recombinant higher plant pPT data are taken from Flügge (1999). None, No competitive inhibitory constant could be measured under the given experimental conditions.
| Kinetic Constants | TPT Homolog
|
PPT Homolog
|
GPT Homolog
|
|||
|---|---|---|---|---|---|---|
| Plants | G. sulphuraria | Plants | G. sulphuraria | Plants | G. sulphuraria | |
| mm | mm | mm | ||||
| KM (Pi) | 1.0 | 0.33 ± 0.07 | 0.8 | 0.76 ± 0.08 | 1.1 | 5.07 ± 0.4 |
| Ki (DHAP) | 1.0 | 0.5 ± 0.04 | 8.0 | 7.93 ± 0.23 | 0.6 | None |
| Ki (3-PGA) | 1.0 | 7.71 ± 0.47 | 4.6 | 3.95 ± 0.1 | 1.8 | None |
| Ki (PEP) | 3.3 | 8.75 ± 0.40 | 0.3 | 0.36 ± 0.04 | 2.9 | None |
| Ki (Glc-6-P) | None | None | None | None | 1.1 | None |