<s>[INST] context: The biosynthesis of PUFAs involves the introduction of double bonds (desaturation) in the aliphatic chain of a fatty acid and the extension (elongation) by two carbon units, of the acyl chain. So, the biosynthetic pathway involves the alternate action of enzymes currently named desaturases and elongases. Fatty acid substrates are always in the form of thioesters of acyl carrier proteins (ACP), Coenzyme A (CoA) or esterified to phospholipidsDesaturation is an aerobic process utilizing molecular oxygen and reducing equivalents obtained from an electron transpo chain.The electron donor depends only on the subcellular location of the desaturase, being cytochrome b5 in the endoplasmic reticulum and ferredoxin in plastids and bacteria.Desaturases represent a complex group of enzymes that could be classified in different ways, depending on the kind of esterification of the fatty acid used as substrate; the solubility of the enzyme; the electron donor; subcellular localization and the regioselectivity of the reaction. [/INST] question: Enzymes involved in PUFA synthesis | options: A: Pyruvate carboxylase B: Acyl transferase C: Acetyl transferase D: Desaturase | answer: D </s>