BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 17. FATTY ACID METABOLISM

17.17. Fatty Acid Synthesis Intermediates Are Attached to an Acyl Carrier Protein

P. Roy Vagelos discovered that the intermediates in fatty acid synthesis are bound to an acyl carrier protein (ACP), specifically via the sulfhydryl terminus of its phosphopantetheine group (Fig. 17.11). In fatty acid degradation, this component is part of CoA, whereas in their synthesis, it is linked to a Serine residue in ACP. This single polypeptide chain of 77 residues can be viewed as a giant prosthetic group, a "macro-CoA."

Class="center">Fig. 17.11. Phosphopantetheine is the reactive moiety of ACP and CoA

17.18. The Elongation Cycle in Fatty Acid Synthesis

The enzyme system catalyzing the synthesis of saturated long-chain Fatty acids from acetyl-CoA, malonyl-CoA, and NADPH is called fatty acid synthase. In higher organisms, it exists as a multienzyme complex. In contrast, the enzyme components of fatty acid synthases in Bacteria dissociate upon Cell lysis. The availability of these isolated Enzymes has facilitated The Study of the stages of fatty acid synthesis (Table 17.3). In fact, the reactions responsible for fatty acid synthesis in higher organisms are very similar to those in bacteria.

Table 17.3. Main reactions in fatty acid synthesis

The elongation phase in fatty acid synthesis begins with the Formation of Acetyl-ACP and malonyl-ACP. These reactions are catalyzed by acetyl transacylase and malonyl transacylase.

Acetyl-CoA + ACP ⇄ Acetyl-ACP + CoA,

Malonyl-CoA + ACP ⇄ Malonyl-ACP + CoA.

Malonyl transacylase is highly specific, whereas acetyl transacylase can also transfer acyl groups other than the acetyl moiety, albeit at a significantly lower rate. Fatty acids with an odd number of carbon atoms are synthesized starting from propionyl-ACP, which in turn is formed from propionyl-CoA by the action of acetyl transacylase.

Acetyl-ACP and malonyl-ACP interact to form acetoacetyl-ACP. This Condensation reaction is catalyzed by acyl-malonyl-ACP condensing enzyme.

Acetyl-ACP + Malonyl-ACP → Acetoacetyl-ACP + ACP + C1O₂.

In the aforementioned condensation reaction, a four-carbon component is formed from a two-carbon and a three-carbon component, with the release of C1O₂. Why is the four-carbon component not formed from two two-carbon fragments? In other words, why do acetyl-ACP

and malonyl-ACP act as reactants rather than two molecules of acetyl-ACP? The answer lies in the fact that the reaction equilibrium is extremely unfavorable for the synthesis of acetoacetyl-ACP from two molecules of acetyl-ACP. Conversely, the equilibrium favors the involvement of malonyl-ACP in this synthesis, because the decarboxylation of the latter leads to a substantial decrease in Free energy. In reality, the condensation reaction is driven by ATP, although ATP does not directly participate in it, but is instead used to form an energy-rich substrate through the carboxylation of acetyl-CoA to malonyl-CoA. The free energy stored in malonyl-CoA As a result of the carboxylation reaction is released during the decarboxylation that accompanies The formation of acetoacetyl-ACP. Although HCO₃^- is required for fatty acid synthesis, its carbon atom does not appear in the resulting product. All carbon atoms of even-chain fatty acids originate from acetyl-CoA.

The next three stages of fatty acid synthesis consist of the reduction of the oxo group at C-3 to a methylene group (Fig. 17.12). First, acetoacetyl-ACP is reduced to D-3-hydroxybutyryl-ACP. This reaction differs from the corresponding degradation reaction in two respects: (1) the D-epimer is formed predominantly rather than the L-epimer, and (2) NADPH serves as the reducing agent, whereas NAD⁺ is used as the oxidizing agent in β-oxidation. This distinction illustrates the general principle that biosynthetic reactions consume NADPH, whereas energy-yielding reactions generate NADH. Next, D-3-hydroxybutyryl-ACP is dehydrated to form crotonyl-ACP, which is trans-Δ²-enoyl-ACP. The final step of the cycle is the reduction of crotonyl-ACP to butyryl-ACP. NADPH again acts as the reducing agent, whereas FAD⁺ serves as the oxidant in the corresponding β-oxidation reaction. Through these last three reactions—reduction, dehydration, and a second reduction—acetoacetyl-ACP is converted into butyryl-ACP, completing the first elongation cycle.

Fig. 17.12. Sequence of reactions in fatty acid synthesis: condensation, reduction, dehydration, and reduction. The intermediates shown here are formed in the first cycle of synthesis

In the second cycle of fatty acid synthesis, butyryl-ACP condenses with malonyl-ACP to yield C₆-β-oxoacyl-ACP. This reaction is analogous to the condensation of acetyl-ACP with malonyl-ACP in the first cycle, which leads to the formation of C₄-β-oxoacyl-ACP. Through reduction, dehydration, and a second reduction, C₆-β-oxoacyl-ACP is converted into C₆-acyl-ACP, which can then enter the third elongation cycle. The described elongation cycles continue until C₁₆-acyl-ACP is formed. This intermediate cannot serve as a substrate for the condensing enzyme and is hydrolyzed to yield palmitate and ACP.

17.19. Stoichiometry of Fatty Acid Synthesis

The stoichiometry of palmitate synthesis is as follows:

Ацетил-СоА + 7 Малонил-СоА + NADРН + 7Н⁺ → Пальмитат + 7СO₂ + 14NADР + 8СоА + 6Н₂O.

The following equation is used for the synthesis of malonyl-CoA:

7 Ацетил-СоА + 7СО2 + 7АТР → 7 Малонил-СоА + 7АDР + 7 Рi + 7Н⁺.

From this, we derive the overall stoichiometry of palmitate synthesis:

8 Ацетил-СоА + 7АТР + 14NADPH → Пальмитат + 14NADP⁺+ 8СoА + 6Н₂O + 7ADP + 7Рi.