Complex sentence: The second step (hydroxylamine to nitrite) is carried out step-wise by two different enzymes; hydroxylamine oxidoreductase (HAO), which converts hydroxylamine to nitric oxide, and another as-of-yet unknown enzyme that converts nitric oxide to nitrite.
Split into simpler sentences: The second step (hydroxylamine to nitrite) is carried out step-wise by two different enzymes. Another as-of-yet unknown enzyme that converts nitric oxide to nitrite.