TABLE 3.
Effective energy costs to the cell of cysteine acquisition from various sulfur sources, in oxic versus anoxic environments
| Sulfur source | ATP equivalenta
|
|
|---|---|---|
| Oxic environment | Anoxic environment | |
| Sulfate | 32 | 3 |
| Cystine | 2 | 0b |
| Sulfide | 16b | 3 |
| Cysteine | 2b | 2 |
a
Calculations are explained in Text S1A. ATP demand is rounded to the nearest whole number.
b
Hydrogen sulfide and cysteine are unlikely to accumulate in fully oxic environments, due to their chemical oxidation by molecular oxygen, while cystine is likely to be reduced to cysteine by ambient sulfide in anoxic environments.