Bacteria such as Bacillus subtilis use the enzyme tryptophan synthase to produce L-tryptophan from indole and L-serine. B. subtilis can also import external L-tryptophan as a direct nutrient source from its environment. When external L-tryptophan is absent, the bacterium synthesizes it using tryptophan synthase.
A scientist investigated how increasing concentrations of L-tryptophan affected tryptophan synthase activity in B. subtilis. He prepared several liquid culture media of equal volume, each containing a different initial concentration of L-tryptophan.
For each culture:
Table 1 shows the results of this investigation.
| Concentration of L-tryptophan / μg cm−3\mu\text{g cm}^{-3}μg cm−3 | Tryptophan synthase activity / arbitrary units | Concentration of L-tryptophan remaining in liquid medium / μg cm−3\mu\text{g cm}^{-3}μg cm−3 |
|---|---|---|
| 0 | 120 | 0 |
| 15 | 85 | 0 |
| 30 | 50 | 0 |
| 45 | 15 | 0 |
| 60 | 0 | 12 |
| 75 | 0 | 27 |
| 90 | 0 | 42 |
Apart from temperature and pH, give two variables the scientist would have controlled when preparing the liquid medium cultures.
A student concluded from these results that L-tryptophan inhibits tryptophan synthase activity in all bacteria. Use all the information to evaluate the student's conclusion.
Tryptophan synthase catalyses the synthesis of L-tryptophan, requiring the metabolic equivalent of 2 molecules of ATP for each molecule of tryptophan produced. Explain how the bacterium benefits when L-tryptophan inhibits tryptophan synthase activity.
