BIOCHEMISTRY: A TEXTBOOK FOR MEDICAL STUDENTS - E. S. Severin - 2004

CHAPTER 8. LIPID METABOLISM

V. Metabolism of Fatty Acids and Ketone Bodies

Fatty acids are obtained from the diet or synthesized within the body (with the exception of polyunsaturated fatty acids). The substrates required for fatty acid synthesis are generated through Glucose Catabolism; thus, a portion of glucose is first converted into Fatty Acids and subsequently into fats. Although the specific pathway of fatty acid catabolism concludes with the Formation of Acetyl-CoA, which serves as the starting substrate for fatty acid synthesis, the processes of synthesis and Oxidation of Fatty acids are irreversible. They take place in different cellular compartments (Biosynthesis occurs in the Cytosol, whereas oxidation occurs in the Cell/35.html">Mitochondria) and are catalyzed by distinct Enzymes. The oxidation of fatty acids as Energy Sources increases during the postabsorptive state, starvation, and physical exertion. Under these conditions, their Blood concentration rises due to mobilization from fat depots, and they are actively oxidized by the Liver, Muscles, and other tissues. During starvation, a fraction of the fatty acids in the liver is converted into alternative "fuel" molecules—Ketone Bodies. Unlike fatty acids, ketone bodies can be utilized by Nervous Tissue as an energy source. During fasting and prolonged physical activity, ketone bodies also serve as an energy source for muscles and certain other tissues.

A. β-Oxidation of Fatty Acids

β-Oxidation is a specific pathway of fatty acid catabolism in which 2-carbon units are successively cleaved off from the carboxyl end of the fatty acid as acetyl-CoA. This metabolic pathway is termed β-oxidation because the oxidation Reactions of the fatty acid occur at the β-carbon atom. The reactions of β-oxidation, along with the subsequent oxidation of acetyl-CoA in The Citric Acid Cycle, constitute one of the primary sources of energy for ATP synthesis via Oxidative Phosphorylation. β-Oxidation of fatty acids takes place exclusively under aerobic conditions.

Activation of Fatty Acids

Prior to participating in various metabolic reactions, fatty acids must be activated—that is, linked to coenzyme A via a high-energy bond:

RСООН + HSКоА + АТФ —> RСО ~ КоА + АМФ + РРi

This reaction is catalyzed by the enzyme acyl-CoA synthetase. The pyrophosphate released during the reaction is hydrolyzed by the enzyme pyrophosphatase: Н₄Р₂O₇ + Н₂O —> 2 Н₃РO₄.

The release of energy during the Hydrolysis of the pyrophosphate high-energy bond shifts the reaction equilibrium to the right, ensuring the completion of the activation process.

Acyl-CoA synthetases are located in both the cytosol and the mitochondrial matrix. These enzymes vary in their Specificity for fatty acids with differing hydrocarbon chain lengths. Short- and medium-chain fatty acids (ranging from 4 to 12 carbon atoms) can cross the mitochondrial matrix membrane via simple diffusion. The activation of these fatty acids occurs directly within the mitochondrial matrix. Long-chain fatty acids, which predominate in The Human Body (from 12 to 20 carbon atoms), are activated by acyl-CoA synthetases situated on the outer mitochondrial membrane.

Transport of Long-Chain Fatty Acids into Mitochondria

Because β-oxidation of fatty acids takes place within the mitochondrial matrix, activated fatty acids must be transported across the mitochondrial membranes. Long-chain fatty acids are translocated across the impermeable inner mitochondrial membrane with the aid of carnitine. Carnitine is either obtained from the diet or synthesized from the Essential Amino Acids Lysine and Methionine. Vitamin C (ascorbic acid) participates in the biosynthetic pathway of carnitine.

The outer mitochondrial membrane houses the enzyme carnitine palmitoyltransferase I (carnitine acyltransferase I), which catalyzes the reaction yielding acylcarnitine.

The resulting acylcarnitine traverses the intermembrane space to the outer face of The inner mitochondrial membrane and is transported across it by carnitine-acylcarnitine translocase to the inner surface of the inner mitochondrial membrane. Here, the enzyme carnitine acyltransferase II catalyzes The transfer of the acyl group to intramitochondrial CoA (Fig. 8-26). Consequently, the acyl-CoA becomes accessible to the enzymes of β-oxidation. Free carnitine is returned to the cytosolic side of the inner mitochondrial membrane by the same translocase.

Class="center">Fig. 8-26. Transport of long-chain fatty acids across mitochondrial membranes. Carnitine acyltransferase I is the regulatory enzyme of β-oxidation; it is inhibited by malonyl-CoA, an intermediate metabolite generated during FATTY ACID BIOSYNTHESIS. * — carnitine-acylcarnitine translocase.

On the inner surface of the inner mitochondrial membrane resides the enzyme carnitine acyltransferase II, which catalyzes the reverse transfer of the acyl group from carnitine to intramitochondrial CoA. Following this, the acyl-CoA enters the β-oxidation pathway.

β-Oxidation of Fatty Acids — a specific pathway of fatty acid catabolism that occurs in the mitochondrial matrix exclusively under aerobic conditions and results in The formation of acetyl-CoA. Hydrogen derived from β-oxidation reactions enters the Electron Transport Chain (ETC), while acetyl-CoA is oxidized in The Citric Acid cycle, which likewise supplies hydrogen for the ETC. Therefore, fatty acid β-oxidation is a crucial metabolic pathway ensuring ATP Synthesis in the Respiratory Chain.

β-Oxidation begins with the dehydrogenation of acyl-CoA by an FAD-dependent acyl-CoA dehydrogenase, creating a double bond between the α- and β-carbon atoms of the reaction product, enoyl-CoA. The FADH₂ coenzyme reduced in this reaction transfers hydrogen atoms to coenzyme Q in the ETC, resulting in the synthesis of 2 ATP molecules (Fig. 8-27). In the subsequent step of β-oxidation, a Water molecule adds across the double bond such that the OH group attaches to the β-carbon atom of the acyl moiety, forming β-hydroxyacyl-CoA. Next, β-hydroxyacyl-CoA is oxidized by an NAD⁺-dependent dehydrogenase. The resulting NADH, upon oxidation in the ETC, provides energy for the synthesis of 3 ATP molecules. The resulting β-ketoacyl-CoA undergoes thiolytic Cleavage by the enzyme thiolase, as a coenzyme A molecule is attached via its sulfur atom at the site of the C–C bond cleavage. Through this sequence of four reactions, a two-carbon unit—acetyl-CoA—is cleaved from the acyl-CoA. The fatty acid, shortened by 2 carbon atoms, once again undergoes the cycle of dehydrogenation, Hydration, dehydrogenation, and cleavage of acetyl-CoA. This sequence of reactions is commonly referred to as the "β-oxidation cycle," implying that the same reactions repeat with the fatty acid radical until the entire molecule is converted into acetyl residues.

Fig. 8-27. β-Oxidation of fatty acids.

The products of each cycle of β-oxidation are FADH₂, NADH, and acetyl-CoA. Although the reactions in each "cycle" are identical, the fatty acid residue entering each successive cycle is shorter by 2 carbon atoms. In the final cycle, a 4-carbon fatty acid is oxidized, yielding 2 molecules of acetyl-CoA rather than 1 as in the preceding cycles. The net equation for the β-oxidation of palmitoyl-CoA, for example, can be expressed as follows: С₁₅Н₃₁СО-КоА + 7 FAD + 7 NAD⁺ + 7 HSKoA —>8 СН₃-СО-КoА + 7 FADH₂ + 7 (NADH + H⁺).

When calculating the ATP yield from the oxidation of palmitic acid (Table 8-7), 2 molecules must be subtracted from the total sum of ATP molecules, as the energy of 2 high-energy bonds is consumed during the initial fatty acid activation step (see the fatty acid activation reaction).

Table 8-7. ATP synthesis during the Complete oxidation of palmitic acid

In many tissues, Fatty acid oxidation serves as a vital energy source, particularly in tissues with high activity of TCA cycle enzymes and the respiratory chain, such as red Skeletal Muscle fibers, cardiac muscle, and Kidneys. Erythrocytes, which lack mitochondria, cannot oxidize fatty acids. Fatty acids also do not serve as an energy source for the Brain and other neural tissues because they cannot cross the blood-brain barrier, much like other hydrophobic substances. Experiments have shown that The rate of fatty acid turnover in neural tissue is significantly lower than in other tissues.

Regulation of the Rate of β-Oxidation

β-Oxidation is a metabolic pathway closely coupled with The electron transport chain (ETC) and the common pathway of catabolism. Consequently, its rate is regulated by the cellular energy demand—specifically, the ATP/ADP and NАDН/NАD⁺ ratios—in the same manner as the reactions of the ETC and the common catabolic pathway (see Section 6). The rate of β-oxidation in tissues depends on substrate availability, i.e., The amount of fatty acids entering the mitochondria. The concentration of free fatty acids in the blood increases during the activation of lipolysis in adipose tissue under METABOLISM/18.html">The Influence of Glucagon during fasting, and under the influence of epinephrine during physical exertion. Under these conditions, fatty acids become the preferred energy source for muscles and the liver, because β-oxidation yields NADH and acetyl-CoA, which inhibit the Pyruvate dehydrogenase complex. The conversion of pyruvate (derived from glucose) into acetyl-CoA slows down, leading to the accumulation of glycolytic intermediates, particularly glucose-6-phosphate. Glucose-6-phosphate inhibits hexokinase and thus prevents the utilization of glucose in Glycolysis. Therefore, the preferential use of fatty acids as the primary energy source in Muscle tissue and the liver spares glucose for neural tissue and erythrocytes.

The rate of β-oxidation also depends on The activity of carnitine acyltransferase I. In the liver, this enzyme is inhibited by malonyl-CoA, an intermediate formed during fatty acid biosynthesis. During the absorptive period, glycolysis is activated in the liver, increasing the formation of acetyl-CoA from pyruvate. The first reaction of fatty acid synthesis is the conversion of acetyl-CoA to malonyl-CoA, which in turn inhibits β-oxidation, ensuring that fatty acids are channeled into fat synthesis instead.

Oxidation of Unsaturated Fatty Acids

About half of the fatty acids in the human body are unsaturated. The β-oxidation of these acids proceeds via the standard pathway until a double bond is located between the third and fourth carbon atoms (Fig. 8-28). Then, the enzyme enoyl-CoA isomerase shifts the double bond from the 3–4 position to the 2–3 position and converts its cis-conformation into the trans-conformation required for β-oxidation. In this specific cycle of β-oxidation, the initial dehydrogenation step is bypassed because a double bond is already present in the fatty acid radical. Subsequent cycles of β-oxidation then proceed normally without further deviations.

Fig. 8-28. Oxidation of a monounsaturated fatty acid.

α-Oxidation of Fatty Acids. Brain Lipids and other regions of neural tissue are predominantly composed of very long-chain fatty acids containing more than 20 carbon atoms. These are oxidized via α-oxidation, a pathway in which single carbon atoms are successively cleaved off the fatty acid and released as CO₂ (Fig. 8-29).

Fig. 8-29. α-Oxidation of fatty acids:

This pathway of fatty acid catabolism is not coupled with ATP synthesis. α-Oxidation also processes Branched-Chain Fatty Acids, such as phytanic acid, which enters the body through plant-based foods (Fig. 8-30). Phytanic acid is derived from phytol, a constituent of chlorophyll. Because this acid features a methyl group at every third carbon atom, standard β-oxidation is sterically blocked. During α-oxidation of phytanic acid, the interfering methyl group is first removed, allowing subsequent rounds of β-oxidation to proceed.

Fig. 8-30. Oxidation of phytanic acid.

B. Disorders of Fatty Acid Oxidation

Impaired Transport of Fatty Acids into Mitochondria. The rate of fatty acid translocation into the mitochondrial matrix—and consequently the rate of β-oxidation—depends on carnitine availability and the activity of carnitine acyltransferase I. β-Oxidation can be disrupted by the following factors:

✵ prolonged hemodialysis, during which the body loses carnitine;

✵ prolonged aciduria, resulting in the renal excretion of carnitine as a base alongside organic acids;

✵ Treatment of Diabetes Mellitus patients with sulfonylurea drugs, which inhibit carnitine acyltransferase I;

✵ reduced activity of enzymes responsible for carnitine biosynthesis;

✵ inherited deficiencies of carnitine acyltransferase I.

In individuals with inherited defects in carnitine acyltransferase I or carnitine biosynthetic enzymes, the rate of fatty acid entry into the mitochondrial matrix is reduced, leading to a correspondingly low rate of β-oxidation. In such cases, long-chain fatty acids cannot be effectively utilized as energy sources, resulting in decreased exercise tolerance and the potential accumulation of fat droplets (vacuoles) within muscle Cells.

Genetic Deficiency of Medium-Chain Acyl-CoA Dehydrogenase

Mitochondria contain Three types of acyl-CoA dehydrogenases that oxidize long-, medium-, and short-chain fatty acids. As the carbon chain is shortened during β-oxidation, fatty acids can be sequentially oxidized by these enzymes. A genetic defect in medium-chain acyl-CoA dehydrogenase is the most common among hereditary Metabolic Disorders, with an incidence of 1:15,000. The carrier frequency of the defective Gene in the European population is 1:40. This autosomal recessive condition results from a T-to-A substitution at position 985 of the gene. The activity of this dehydrogenase is particularly critical for infants, whose primary energy source is milk fat, which is rich in medium-chain fatty acids in triacylglycerols. The inability to utilize fatty acids for energy leads to an accelerated rate of glucose oxidation. Consequently, children develop hypoglycemia—a leading cause of sudden infant death syndrome (accounting for 10% of all neonatal deaths). If such infants survive, fasting for 6 to 8 hours can trigger hypoglycemic episodes marked by weakness, dizziness, vomiting, and loss of consciousness. Administration of glucose rapidly alleviates these symptoms.

Whenever β-oxidation is impaired, fatty acids accumulate within cells and are catabolized via an alternative pathway known as ω-oxidation, which normally proceeds at a very low rate. This oxidation occurs at the terminal methyl carbon (ω-carbon) (Fig. 8-31), yielding dicarboxylic acids that are excreted in the urine. The detection of these acids in urine serves as a diagnostic marker for defective β-oxidation.

Fig. 8-31. ω-Oxidation of fatty acids. ω-Oxidation is upregulated when the rate of fatty acid β-oxidation is reduced. 1 — adipic acid; 2 — suberic acid.

Impaired phytanic acid oxidation. In Refsum disease—a rare inherited disorder caused by a genetic defect in one of the enzymes involved in α-oxidation—dietary phytanic acid cannot be oxidized and accumulates in the body, predominantly within nervous tissue. This leads to structural damage to neural tissue and the manifestation of various neurological symptoms.

B. Ketone Body Metabolism

During fasting, prolonged physical exertion, and in conditions where cells are deprived of an adequate glucose supply, fatty acids serve as the primary energy source for many tissues. Unlike most other tissues, however, the brain and Other components of the Central Nervous system virtually cannot use fatty acids for energy. In the liver, a portion of these fatty acids is converted into ketone bodies, which are then oxidized by the brain, nervous tissue, and skeletal muscles, providing sufficient energy for ATP synthesis while reducing glucose consumption. Ketone bodies include β-hydroxybutyrate, acetoacetate, and acetone. The first two molecules can be oxidized in peripheral tissues to support ATP synthesis. Acetone is formed only when blood ketone body concentrations are exceptionally high; being excreted via the Lungs, urine, and sweat, it allows the body to rid itself of excess ketone bodies.

Synthesis of ketone bodies in the liver. A low Insulin-to-glucagon ratio in the blood stimulates lipolysis in adipose tissue. Fatty acids are delivered to the liver in quantities well above normal, thereby accelerating the rate of β-oxidation (Fig. 8-32). Under these conditions, the rate of TCA cycle reactions is suppressed because oxaloacetate is diverted toward Gluconeogenesis. Consequently, the rate of acetyl-CoA production exceeds the capacity of the TCA cycle to oxidize it. Acetyl-CoA accumulates in hepatic mitochondria and is diverted into ketogenesis. Ketone Body Synthesis occurs exclusively in the mitochondria of liver cells.

Fig. 8-32. Upregulation of ketogenesis during fasting. Dotted lines indicate decreased metabolic pathway rates; solid lines indicate increased rates. During fasting, glucagon action stimulates lipolysis in adipose tissue and β-oxidation in the liver. The concentration of oxaloacetate in the mitochondria decreases because it is reduced to malate, exported to the cytosol, reconverted to oxaloacetate, and used for gluconeogenesis. As a result, TCA cycle flux declines, slowing the oxidation of acetyl-CoA. The intra-mitochondrial concentration of acetyl-CoA rises, which activates ketogenesis. Ketone body synthesis is also elevated in diabetes mellitus (see Chapter 11).

Ketogenesis begins with the Condensation of two molecules of acetyl-CoA, catalyzed by the enzyme thiolase to form acetoacetyl-CoA (Fig. 8-33). Acetoacetyl-CoA then reacts with a third molecule of acetyl-CoA to yield 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). This reaction is catalyzed by HMG-CoA synthase. Subsequently, HMG-CoA lyase catalyzes the cleavage of HMG-CoA into free acetoacetate and acetyl-CoA.

Fig. 8-33. Ketogenesis in hepatocyte mitochondria. The regulatory enzyme of ketogenesis (HMG-CoA synthase) is inhibited by free CoA. * — non-enzymatic reaction occurring at high blood ketone body concentrations.

Acetoacetate can be released into the bloodstream or reduced within the liver into another ketone body, β-hydroxybutyrate.

Active β-oxidation in hepatic cells generates a high NADH concentration. This high NADH/NAD⁺ ratio drives the conversion of most acetoacetate into β-hydroxybutyrate, making β-hydroxybutyrate the predominant ketone body in the blood. During starvation, fatty acids and ketone bodies become the major fuel molecules for many tissues, whereas glucose is preferentially reserved for the nervous tissue and erythrocytes.

At high concentrations of acetoacetate, a fraction of it undergoes non-enzymatic decarboxylation to form acetone. Acetone is not utilized by tissues and is eliminated via expired air and urine. This mechanism allows the body to clear excess ketone bodies that cannot be oxidized quickly enough, thereby preventing them from acting as water-soluble acids that would otherwise cause acidosis.

Regulation of ketogenesis. The key regulatory enzyme of ketone body synthesis is HMG-CoA synthase.

✵ HMG-CoA synthase is an inducible enzyme; its synthesis increases when blood fatty acid levels rise. Circulating fatty acid concentrations increase due to the mobilization of fat from adipose tissue stimulated by Hormones such as glucagon and epinephrine, such as during fasting or Physical Exercise.

✵ HMG-CoA synthase is inhibited by high concentrations of free coenzyme A.

✵ When the influx of fatty acids into hepatocytes increases, CoA is consumed in forming acyl-CoA derivatives, causing the concentration of free CoA to drop, which in turn activates the enzyme.

✵ Conversely, when the influx of fatty acids into liver cells decreases, the concentration of free CoA rises, inhibiting the enzyme. Thus, the rate of hepatic ketogenesis is directly dependent on the supply of fatty acids.

Oxidation of ketone bodies in peripheral tissues

During prolonged fasting, ketone bodies become a major energy source for skeletal muscle, Heart, and kidneys, thereby sparing glucose for oxidation in the brain and red Blood Cells. Within just 2–3 days of fasting, the concentration of ketone bodies in the blood reaches levels sufficient for them to cross the blood-brain barrier, enter brain cells, and undergo oxidation, thereby reducing the brain's demand for glucose.

Upon entering cells, β-hydroxybutyrate (Fig. 8-34) is dehydrogenated by an NAD⁺-dependent dehydrogenase and converted into acetoacetate. Acetoacetate is then activated by reacting with succinyl-CoA, which serves as a CoA donor:

Acetoacetate + Succinyl-CoA -> Acetoacetyl-CoA + Succinate.

Fig. 8-34. Oxidation of ketone bodies in tissues. * — acetoacetate activation reaction.

The reaction is catalyzed by succinyl-CoA-acetoacetate-CoA transferase. Since this enzyme is not synthesized in the liver, the liver does not use ketone bodies as energy sources, but rather produces them "for export." Ketone bodies are excellent fuel molecules; the oxidation of a single molecule of β-hydroxybutyrate to CO₂ and H₂O provides the synthesis of 27 ATP molecules. The equivalent of one high-energy ATP bond (in the succinyl-CoA molecule) is consumed for acetoacetate activation, making the net yield of ATP during the oxidation of one β-hydroxybutyrate molecule equal to 26 molecules.

Ketoacidosis. Under normal conditions, the blood concentration of ketone bodies ranges from 1 — 3 mg/dL (up to 0.2 mM/L), but it increases significantly during starvation. Elevated blood levels of ketone bodies are referred to as ketonemia, and their excretion in the urine as ketonuria. The accumulation of ketone bodies in the body leads to ketoacidosis—a reduction in alkaline reserve (compensated acidosis), and in severe cases, to a shift in pH (uncompensated acidosis), since ketone bodies (with the exception of acetone) are water-soluble organic acids (pK~3.5) capable of dissociation:

СН₃-СО-СН₂-СООН <-> СН₃-СО-СН₂-СОО^- + H⁺.

Acidosis reaches dangerous levels in diabetes mellitus, as the concentration of ketone bodies in this condition can rise to 400 — 500 mg/dL. Severe acidosis is one of the primary causes of death in diabetes mellitus. The accumulation of protons in the blood impairs oxygen binding by Hemoglobin and affects The ionization of protein functional groups, disrupting their conformation and function.

G. Fatty Acid Biosynthesis

A variety of fatty acids, including essential ones, enter the body with food. A significant portion of non-Essential Fatty Acids is synthesized in the liver, and to a lesser extent, in adipose tissue and the Cytology/practical/135.html">Lactating mammary gland. The carbon source for fatty acid synthesis is acetyl-CoA, which is generated during glucose breakdown in the absorptive period. Thus, excess dietary CARBOHYDRATES are transformed into fatty acids and subsequently into fats.

1. Synthesis of Palmitic Acid

Formation of Acetyl-CoA and Its Transport into the Cytosol

Fatty acid synthesis takes place during the absorptive period. Active glycolysis followed by The oxidative decarboxylation of pyruvate leads to an increased concentration of acetyl-CoA in the mitochondrial matrix. Because fatty acid synthesis occurs in The Cell cytosol, acetyl-CoA must be transported across the inner mitochondrial membrane into the cytosol. However, the inner mitochondrial membrane is impermeable to acetyl-CoA; therefore, in the mitochondrial matrix, acetyl-CoA condenses with oxaloacetate to form citrate via citrate synthase: Acetyl-CoA + Oxaloacetate —> Citrate + HS-CoA.

Translocase then transports citrate into the Cytoplasm (Fig. 8-35).

Fig. 8-35. Transport of acetyl groups from mitochondria to the cytosol. Enzymes involved: 1 — citrate synthase; 2 — translocase; 3 — citrate lyase; 4 — malate dehydrogenase; 5 — malic enzyme.

The transport of citrate into the cytoplasm occurs only when mitochondrial citrate levels rise, which happens when isocitrate dehydrogenase and α-ketoglutarate dehydrogenase are inhibited by high concentrations of NADH and ATP. This situation arises during the absorptive period when liver cells receive an adequate supply of energy sources. In the cytoplasm, citrate is cleaved by the action of citrate lyase:

Citrate + HSCoA + ATP —> Acetyl-CoA + ADP + Pi + Oxaloacetate.

Cytoplasmic acetyl-CoA serves as the initial substrate for fatty acid synthesis, while cytosolic oxaloacetate undergoes further transformations (see the scheme below).

Pyruvate is transported back into the mitochondrial matrix. NADPH, generated through the action of the malic enzyme, is utilized as a hydrogen donor for subsequent reactions of fatty acid synthesis. Another source of NADPH is the oxidative Stages of the Pentose Phosphate Pathway of glucose catabolism.

Formation of Malonyl-CoA from Acetyl-CoA — a regulatory reaction in fatty acid biosynthesis.

Scheme

The first reaction of fatty acid synthesis is the conversion of acetyl-CoA to malonyl-CoA. The enzyme catalyzing this reaction (acetyl-CoA carboxylase) belongs to the ligase class. It contains covalently bound biotin (Fig. 8-36). In The First stage of the reaction, CO₂ is covalently bound to biotin at the expense of ATP energy; In the second stage, COO^- is transferred to acetyl-CoA to yield malonyl-CoA. The activity of acetyl-CoA carboxylase determines the rate of all subsequent reactions in fatty acid synthesis.

Fig. 8-36. The Role of biotin in the carboxylation reaction of acetyl-CoA.

Reactions Catalyzed by fatty acid synthase, — the multienzyme complex responsible for catalyzing the synthesis of palmitic acid, are described below.

After the formation of malonyl-CoA, fatty acid synthesis continues on a multienzyme complex known as fatty acid synthase (palmitoyl synthase). This enzyme consists of 2 identical protomers, each having a domain Structure and, accordingly, 7 catalytic centers with distinct activities (Fig. 8-37). The complex sequentially elongates the fatty acid radical by 2 carbon atoms, with malonyl-CoA serving as the donor. The end product of this complex is palmitic acid, which is why the enzyme was formerly called palmitoyl synthase.

Fig. 8-37. STRUCTURE OF THE multienzyme fatty acid synthase complex. The complex is a dimer of two identical polypeptide chains, each containing 7 active centers and an acyl carrier protein (ACP). The SH groups of the protomers belong to different radicals: one SH group belongs to a Cysteine residue, and the other to a phosphopantetheine residue. The cysteine SH group of one monomer is located adjacent to the 4-phosphopantetheine SH group of the other protomer. Thus, the enzyme protomers are arranged in a "HEAD-to-tail" orientation. Although each monomer contains all the catalytic centers, the functional unit is the complex of 2 protomers. As a result, 2 fatty acids are actually synthesized simultaneously. For simplicity, reaction schemes usually depict the synthesis of a single acid molecule.

The first reaction involves the transfer of the acetyl group from acetyl-CoA to the thiol group of cysteine by the acyltransacylase (acetyltransacylase) center (Fig. 8-38). Next, the malonyl residue is transferred from malonyl-CoA to the sulfhydryl group of the acyl carrier protein by the malonyltransacylase center. After this, the complex is ready for the first synthesis cycle.

The acetyl group condenses with the malonyl residue at the site where CO₂ is released. This reaction is catalyzed by the ketoacyl synthase center. The resulting acetoacetyl radical is sequentially reduced by ketoacyl reductase, then dehydrated, and reduced again by enoyl reductase, which are active centers of the complex. As a result of this first reaction cycle, a butyryl radical is formed, which remains bound to the fatty acid synthase subunit.

Fig. 8-38. Synthesis of palmitic acid. Fatty acid synthase: in the first protomer, the SH group belongs to cysteine, and in the second, to phosphopantetheine. Upon completion of the first cycle, the butyryl radical is transferred to the SH group of the first protomer. Then, the same sequence of reactions as in the first cycle is repeated. Palmitoyl-E is the palmitic acid residue bound to fatty acid synthase. In the synthesized fatty acid, only the 2 distal carbon atoms designated by * originate from acetyl-CoA, while the rest come from malonyl-CoA.

Before the second cycle, the butyryl radical is transferred from position 2 to position 1 (where the acetyl group was located at THE START OF the first reaction cycle). Then, the butyryl residue undergoes the same transformations and is elongated by 2 carbon atoms derived from malonyl-CoA.

Similar reaction cycles are repeated until a palmitic acid radical is formed, which is hydrolytically cleaved from the enzyme complex by the thioesterase center, yielding free palmitic acid (palmitate, Figs. 8-38, 8-39).

Fig. 8-39. General reaction Scheme for the synthesis of palmitic acid.

The overall equation for the synthesis of palmitic acid from acetyl-CoA and malonyl-CoA is as follows:

СН₃-СО-SКоА + 7 НООС-СН₂-СО-SКоА + 14 (NАDРН + Н⁺) —> С₁₅Н₃₁СООН + 7 СO₂ + 6 Н₂O + 8 HSКоА + 14 NADP⁺.

Major sources of hydrogen for fatty acid synthesis

Each cycle of palmitic acid biosynthesis includes two reduction reactions in which the coenzyme NADPH serves as the hydrogen donor. The reduction of NADP⁺ occurs in the following reactions:

✵ dehydrogenation during the oxidative stages of The pentose phosphate pathway of glucose catabolism;

✵ dehydrogenation of malate by the malic enzyme;

✵ dehydrogenation of isocitrate by cytosolic NADP-dependent dehydrogenase.

2. Regulation of Fatty acid synthesis

The key regulatory enzyme of fatty acid synthesis is acetyl-CoA carboxylase. This enzyme is regulated through several mechanisms.

Association/dissociation of enzyme subunit complexes. In its inactive form, acetyl-CoA carboxylase exists as separate complexes, each consisting of 4 subunits. The activator of the enzyme is citrate, which promotes the assembly of these complexes, thereby increasing enzyme activity. The inhibitor is palmitoyl-CoA, which causes complex dissociation and a decrease in enzyme activity (Fig. 8-40).

Fig. 8-40. Association and dissociation of acetyl-CoA carboxylase complexes.

Phosphorylation/dephosphorylation of acetyl-CoA carboxylase. In the postabsorptive state or during physical exertion, glucagon or epinephrine activates protein kinase A via the adenylate cyclase system, stimulating the phosphorylation of acetyl-CoA carboxylase subunits. The phosphorylated enzyme is inactive, and fatty acid synthesis halts. In the absorptive period, insulin activates a phosphatase, converting acetyl-CoA carboxylase into its dephosphorylated state (Fig. 8-41). Subsequently, under the influence of citrate, polymerization of the enzyme protomers occurs, rendering it active. In addition to enzyme activation, citrate plays another role in fatty acid synthesis: during the absorptive period, citrate accumulates in the mitochondria of liver cells, serving as the vehicle for transporting the acetyl group into the cytosol.

Fig. 8-41. Regulation of acetyl-CoA carboxylase.

Induction of enzyme synthesis. Prolonged consumption of a carbohydrate-rich and fat-poor diet leads to increased insulin secretion, which stimulates the induction of key lipogenic enzymes: acetyl-CoA carboxylase, fatty acid synthase, citrate lyase, and isocitrate dehydrogenase. Consequently, excessive carbohydrate intake accelerates the conversion of glucose catabolism products into fat. Conversely, fasting or a high-fat diet suppresses the synthesis of these enzymes and, consequently, fat storage.

3. Synthesis of fatty acids from palmitic acid

Elongation of fatty acids. Palmitic acid is elongated in the ER with the participation of malonyl-CoA. The reaction sequence is analogous to that of palmitic acid synthesis; however, in this pathway, the fatty acids are bound to CoA rather than to fatty acid synthase. The enzymes involved in elongation can utilize not only palmitic acid but also other fatty acids as substrates (Fig. 8-42). As a result, the body can synthesize not only stearic acid but also long-chain fatty acids with a higher number of carbon atoms.

Fig. 8-42. Elongation of palmitic acid in the ER. The palmitic acid acyl chain is extended by two carbon atoms donated by malonyl-CoA.

The primary elongation product in the liver is stearic acid (C 18:0); however, brain tissue synthesizes large amounts of very long-chain fatty acids ranging from C₂₀ to C₂₄, which are essential for the formation of Sphingolipids and Glycolipids.

Nervous tissue also synthesizes Other types of fatty acids, namely α-hydroxy acids. Mixed-function oxidases hydroxylate C₂₂ and C₂₄ acids to yield lignoceric and cerebronic acids, which are found exclusively in brain lipids.

Introduction of double bonds into fatty acyl radicals. The introduction of double bonds into fatty acid chains is known as desaturation. The principal monounsaturated fatty acids synthesized in the human body via desaturation (Fig. 8-43) are palmitoleic (C16:1Δ9) and oleic (C18:1Δ9) acids.

Fig. 8-43. Biosynthesis of Unsaturated fatty acids.

The introduction of double bonds into fatty acid chains occurs in the ER through reactions involving molecular oxygen, NADH, and cytochrome b₅. The human fatty acid desaturase enzymes cannot introduce double bonds beyond the ninth carbon atom from the carboxylic carbon, i.e., between the ninth carbon and the terminal methyl group. Therefore, ω-3 and ω-6 fatty acids cannot be synthesized endogenously; they are essential nutrients and must be obtained from the diet, as they perform vital regulatory Functions.

The formation of a double bond in a fatty acid chain requires molecular oxygen, NADH, cytochrome b₅, and a FAD-dependent cytochrome b₅ reductase. The hydrogen atoms removed from the saturated fatty acid are released as water. One atom of molecular oxygen is incorporated into a water molecule, while the other is also reduced to water utilizing electrons from NADH, which are transferred via FADH₂ and cytochrome b₅.