BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 17. FATTY ACID METABOLISM

17.20. Fatty Acids Are Synthesized in Eukaryotes by a Multienzyme Complex

Unlike their bacterial counterparts, eukaryotic fatty acid synthetases are highly organized multienzyme complexes. The complex isolated from Yeast has a mass of 2300 kDa and appears on electron micrographs as an ellipsoid with a length of 250 Å and a cross-sectional diameter of 210 Å (Fig. 17.13). It consists of only Two Types of polypeptide chains, each encoded by a single Gene. Subunit A (185 kDa) contains the acyl carrier protein, the condensing enzyme, and ß-oxoacyl reductase, whereas subunit B (175 kDa) contains acetyl transacylase, malonyl transacylase, ß-hydroxyacyl dehydratase, and enoyl reductase. The mammalian fatty acid synthetase (400 kDa) is likewise composed of two types of subunits similar to those of yeast. In fact, many eukaryotic multienzyme complexes consist of multifunctional Proteins in which different enzymatic activities are covalently linked into a single polypeptide chain. The advantage of this Organization is The ability to coordinate the synthesis of different Enzymes. Furthermore, a multienzyme complex composed of covalently linked enzymes is more stable than a complex formed by noncovalent interactions.

Class="center">Fig. 17.13. Electron micrograph of the yeast fatty acid synthetase complex

Lynen proposed that the elongating fatty acid chain is shuttled back and forth between the ACP and the condensing enzyme during each elongation cycle. The first translocation frees a site for the incoming malonyl component, while the second occurs at the Condensation step. Interestingly, similar translocations take place during Protein Synthesis.

The flexibility and maximum length of 20 Å of the phosphopantetheine moiety appear critical for the function of the multienzyme complex, as they ensure close contact between the growing fatty acid chain and the Active Site of each enzyme within the complex. Extensive structural rearrangements of the enzyme subunits are not required for substrate interaction, since the substrate itself, attached to a long flexible arm, can reach every active site. Recall that biotin and Lipoic Acid in their respective multienzyme complexes are likewise equipped with long flexible arms. The organized Structure of fatty acid synthetases in yeast and higher organisms enhances overall process efficiency through the direct transfer of intermediates from one active site to the next. Reacting molecules are not diluted in the Cytosol, nor do they need to "find" each other via random diffusion. Another advantage of such a multienzyme complex is that the covalently bound intermediates are sequestered and protected from competing reactions.

Lyases are enzymes that catalyze the Cleavage of C—C, C—O, or C—N bonds by elimination, generating a double bond in the process.

17.21. Citrate Transports Acetyl Groups from Mitochondria to the Cytosol for Fatty Acid Synthesis

The synthesis of palmitate requires 8 molecules of acetyl-CoA, 14 NADPH, and 7 ATP. Fatty acids are synthesized in the cytosol, whereas acetyl-CoA is generated from Pyruvate in the Cell/35.html">Mitochondria. Consequently, fatty acid synthesis requires The transport of acetyl-CoA from the mitochondria into the cytosol. However, the mitochondrial inner membrane is impermeable to acetyl-CoA, and carnitine transports only long-chain fatty acids. This barrier for acetyl-CoA is bypassed by means of citrate, which carries acetyl groups across The inner mitochondrial membrane. Citrate is formed in the mitochondrial matrix by the condensation of acetyl-CoA and oxaloacetate. It then diffuses into the cytosol, where it is cleaved by citrate lyase:

Citrate + ATP + CoA → Acetyl-CoA + ADP + Pi + Oxaloacetate.

Thus, acetyl-CoA and oxaloacetate are transported from the mitochondria to the cytosol at the expense of one ATP molecule.

17.22. Sources of NADPH for Fatty Acid Synthesis

The oxaloacetate generated during the transport of the acetyl group into the cytosol must then be returned to the mitochondria. Because the inner mitochondrial membrane is impermeable to oxaloacetate, bypass reactions are required. Importantly, these reactions generate a significant portion of the NADPH needed for fatty acid synthesis. The first reaction involves the reduction of oxaloacetate to malate by NADH, which takes place in the cytosol and is catalyzed by malate dehydrogenase.

Oxaloacetate + NADH + H⁺ ⇄ Malate + NAD⁺.

The second reaction is The oxidative decarboxylation of malate by an NADP⁺-dependent malate dehydrogenase (decarboxylating), commonly known as the malic enzyme. We encounter this reaction here for the first time.

Malate + NADP⁺ → Pyruvate + CO₂ + NADPH.

The resulting pyruvate readily diffuses into the mitochondria, where it is carboxylated to oxaloacetate by pyruvate carboxylase.

Pyruvate + CO₂ + ATP + H₂O → Oxaloacetate + ADP + Pi + 2H⁺.

Summing these three reactions yields

NADP⁺ + NADH + ATP + H₂O → NADPH + NAD⁺ + ADP + Pi + H⁺.

Thus, for every molecule of acetyl-CoA transported from the mitochondria to the cytosol, one molecule of NADPH is produced. Consequently, The transfer of eight molecules of acetyl-CoA into the cytosol during palmitate synthesis generates eight molecules of NADPH. The remaining six molecules of NADPH required for the process are generated by the Pentose Phosphate Pathway.

Fig. 17.14. Acetyl-CoA is transported from the mitochondria into the cytosol, accompanied by The conversion of NADH to NADPH via the indicated pathway