-!- This enzyme is responsible for the dehydration step of the dissociated (type II) fatty-acid biosynthesis system that occurs in plants and bacteria. -!- The enzyme uses fatty acyl thioesters of ACP in vivo. -!- Different forms of the enzyme may have preferences for substrates with different chain length. -!- For example, the activity of FabZ, the ubiquitous enzyme in bacteria, decreases with increasing chain length. -!- Gram-negative bacteria that produce unsaturated fatty acids, such as Escherichia coli, have another form (FabA) that prefers intermediate chain length, and also catalyzes EC 5.3.3.14. -!- Despite the differences both forms can catalyze all steps leading to the synthesis of palmitate (C16:0). -!- FabZ, but not FabA, can also accept unsaturated substrates. -!- Formerly EC 4.2.1.58, EC 4.2.1.60 and EC 4.2.1.61.