Bacteria such as Salmonella enterica use the enzyme threonine deaminase to produce L-isoleucine from L-threonine in the environment. S. enterica can also use external L-isoleucine as a direct nutrient source. When an external source of L-isoleucine is not available, this bacterium synthesizes it using threonine deaminase.
A scientist investigated the effect of an increase in the concentration of L-isoleucine on the activity of threonine deaminase in this bacterium. He prepared several liquid medium cultures. Each liquid culture had the same volume. He grew each culture in a different concentration of L-isoleucine.
In each culture:
Table 1 shows the scientist's results.
| Concentration of L-isoleucine / μg cm−3\mu\text{g cm}^{-3}μg cm−3 | Threonine deaminase activity / arbitrary units | Concentration of L-isoleucine remaining in liquid medium / μg cm−3\mu\text{g cm}^{-3}μg cm−3 |
|---|---|---|
| 0 | 150 | 0 |
| 5 | 110 | 0 |
| 10 | 70 | 0 |
| 15 | 30 | 0 |
| 20 | 0 | 4 |
| 25 | 0 | 9 |
| 30 | 0 | 14 |
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-isoleucine inhibits threonine deaminase activity in all bacteria. Use all the information to evaluate the student's conclusion.
Threonine deaminase catalyses the first step in the biosynthesis of L-isoleucine, which requires significant amounts of metabolic energy (ATP). When L-isoleucine reduces threonine deaminase activity, the bacteria may benefit. Explain how.
