BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 17. FATTY ACID METABOLISM

17.14. Animals Cannot Convert Fatty Acids into Glucose

It is important to note that animals are unable to convert Fatty acids into glucose. Acetyl-CoA cannot be turned into Pyruvate or oxaloacetate in the animal body. The two carbon atoms of the acetyl group in acetyl-CoA enter The Tricarboxylic Acid Cycle, but two carbon atoms leave this cycle during the decarboxylation Reactions Catalyzed by isocitrate dehydrogenase and α-oxoglutarate dehydrogenase. Consequently, oxaloacetate is regenerated rather than synthesized anew when the acetyl component of acetyl-CoA is oxidized in the tricarboxylic acid cycle. Unlike animals, plants possess two additional Enzymes that grant them The ability to convert the carbon atoms of acetyl-CoA into glucose.

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17.15. The Synthesis and Degradation of Fatty Acids Proceed by Different Pathways

The pathway of fatty acid synthesis is by no means simply the reverse of their degradation. It represents a distinct sequence of reactions, serving as yet another example of the differences between biosynthetic and degradative pathways in biological systems. Let us examine some of the KEY FEATURES OF FATTY ACID Biosynthesis.

1. Synthesis takes place in the Cytosol, in contrast to degradation, which occurs within the mitochondrial matrix.

2. The intermediates of fatty acid synthesis are covalently linked to The sulfhydryl groups of acyl carrier protein (ACP), whereas the intermediates of fatty acid degradation are bound to coenzyme A.

3. Many fatty acid synthesis enzymes in higher organisms are organized into a multienzyme complex called fatty acid synthetase. In contrast, the enzymes catalyzing fatty acid degradation show no such tendency to associate.

4. The growing fatty acid chain is elongated by the sequential addition of two-carbon units derived from acetyl-CoA. Malonyl-ACP serves as the activated donor of two-carbon units during the elongation stage. The elongation reaction is driven by the release of CO₂.

5. NADPH acts as the reducing agent in fatty acid synthesis.

6. Elongation by the fatty acid synthetase complex halts at the palmitate (C₁₆) stage. Further elongation and the Introduction of double bonds are carried out by other enzyme systems.

17.16. The Formation of Malonyl-Coenzyme A Is the Committed Step in Fatty Acid Synthesis

Salih Wakil's discovery that bicarbonate is required for fatty acid biosynthesis proved to be the key to elucidating The Mechanism of this process. Indeed, fatty acid synthesis begins with the carboxylation of acetyl-CoA to form malonyl-CoA. This irreversible reaction represents the committed step in fatty acid synthesis.

The synthesis of malonyl-CoA is catalyzed by acetyl-CoA carboxylase, which contains biotin as a prosthetic group. The carboxyl group of biotin is covalently attached to the ε-amino group of a Lysine residue, much like what occurs in pyruvate carboxylase (Section 15.15). Another shared property of acetyl-CoA carboxylase and pyruvate carboxylase is that the carboxylation of acetyl-CoA occurs in two stages. First, carboxybiotin is formed as an intermediate at the expense of ATP. The activated CO₂ group within this intermediate is then transferred to acetyl-CoA to yield malonyl-CoA.

Biotin - enzyme + ATP + HCO3 -⇄ CO₂ ~ Biotin-enzyme + ADP + Pi.

CO₂ ~ Biotin - enzyme + Acetyl-CoA ⇄ Malonyl-CoA + Biotin-enzyme.

Substrate binding to the enzyme and product release follow a specific ordered sequence (Fig. 17.8). Acetyl-CoA carboxylase exemplifies a ping-pong reaction mechanism, where one or more products are released before all substrates have bound.

Fig. 17.8. Reaction sequence catalyzed by acetyl-CoA carboxylase

Acetyl-CoA carboxylase from E. coli has been dissociated into subunits that catalyze partial reactions. Biotin is covalently attached to a small protein (22 kDa) known as the carboxybiotin-carrier protein. The carboxylation of the biotin component in this complex is catalyzed by a second subunit, biotin carboxylase. The third component of the system is transcarboxylase, which catalyzes The transfer of the activated CO₂ from carboxybiotin to acetyl-CoA. The length and flexibility of the tether between biotin and its carrier protein allow the activated carboxyl group to swing from one Active Site of the enzyme complex to another (Fig. 17.9), analogous to the mechanism in pyruvate carboxylase (Section 15.16).

In eukaryotes, acetyl-CoA carboxylase exists either as an enzymatically inactive protomer (450 kDa) or as an active filamentous polymer (Fig. 17.10). Their interconversion is regulated allosterically, as one would expect since acetyl-CoA carboxylase catalyzes the first committed step in fatty acid synthesis. The key allosteric activator is citrate, which shifts the equilibrium toward the active fibrous form of the enzyme. Optimal orientation of biotin relative to the substrates is achieved in this fibrous state. Conversely, palmitoyl-CoA shifts the equilibrium toward the inactive protomeric form. Thus, palmitoyl-CoA, the end product, inhibits the very first committed step of fatty acid biosynthesis. The regulation of acetyl-CoA carboxylase in E. coli differs markedly from that in eukaryotes. In Bacteria, fatty acids serve primarily as phospholipid precursors rather than as fuel reserves, and consequently, the control of their synthesis follows a different pattern. Citrate has no effect on E. coli acetyl-CoA carboxylase. Instead, The activity of the transcarboxylase component of the system is regulated by guanine NUCLEOTIDES, which coordinate fatty acid synthesis with bacterial growth and Cell Division.

Fig. 17.9. Diagram showing the proposed translocation of the biotin prosthetic group from the site where it attaches the carboxyl group from HCO₃^- to the site where it transfers this group to acetyl-CoA

Fig. 17.10. Electron micrograph of the active filamentous form of acetyl-CoA carboxylase from chicken Liver