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تسجيل

Biosynthesis of Fatty Acids and Eicosanoids:- Malonyl-CoA Is Formed from Acetyl-CoA and Bicarbonate

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  p787-789

2026-06-28

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Biosynthesis of Fatty Acids and Eicosanoids:- Malonyl-CoA Is Formed from Acetyl-CoA and Bicarbonate

The formation of malonyl-CoA from acetyl-CoA is an irreversible process, catalyzed by acetyl-CoA carboxylase. The bacterial enzyme has three separate poly peptide subunits (Fig. 21–1); in animal cells, all three activities are part of a single multifunctional polypep tide. Plant cells contain both types of acetyl-CoA carboxylase. In all cases, the enzyme contains a biotin prosthetic group covalently bound in amide linkage to the -amino group of a Lys residue in one of the three polypeptides or domains of the enzyme molecule. The two-step reaction catalyzed by this enzyme is very similar to other biotin-dependent carboxylation reactions, such as those catalyzed by pyruvate carboxylase (see Fig. 16–16) and propionyl-CoA carboxylase (see Fig. 17–11). The carboxyl group, derived from bicarbonate (HCO₃), is first transferred to biotin in an ATP dependent reaction. The biotinyl group serves as a temporary carrier of CO₂, transferring it to acetyl-CoA in the second step to yield malonyl-CoA.

FIGURE 21–1 The acetyl-CoA carboxylase reaction. Acetyl-CoA carboxylase has three functional regions: biotin carrier protein (gray); biotin carboxylase, which activates CO₂ by attaching it to a nitrogen in the biotin ring in an ATP-dependent reaction (see Fig. 16–16); and transcarboxylase, which transfers activated CO₂ (shaded green) from biotin to acetyl-CoA, producing malonyl-CoA. The long, flexible biotin arm carries the activated CO₂ from the biotin carboxylase region to the transcarboxylase active site, as shown in the diagrams below the reaction arrows. The active enzyme in each step is shaded blue.