Fatty acid synthase reactions catalyzed

Fig. 2. Various reactions leading to acyl-CoA. The pyruvate dehydrogenase reaction is in fact a sequence of five reactions catalyzed by a three enzyrmes complex. It is the major pathway producing acetyl-CoA, a key molecule in many important metabolic pathways such as the oxidative degradation of glucides (citric acid cycle, also known as Krebs cycle ). Acetyl-CoA is also the starting point for the symthesis of fatty acids, through the fatty acid synthase reaction, which increases the length of the starting fatty acid (R) by two carbon atoms. Most of the other acyl-CoAs are obtained through the acyl-CoA ligase reaction.

Rittenberg and Bloch showed in the late 1940s that acetate units are the building blocks of fatty acids. Their work, together with the discovery by Salih Wakil that bicarbonate is required for fatty acid biosynthesis, eventually made clear that this pathway involves synthesis of malonyl-CoA. The carboxylation of acetyl-CoA to form malonyl-CoA is essentially irreversible and is the committed step in the synthesis of fatty acids (Figure 25.2). The reaction is catalyzed by acetyl-CoA carboxylase, which contains a biotin prosthetic group. This carboxylase is the only enzyme of fatty acid synthesis in animals that is not part of the multienzyme complex called fatty acid synthase. 

The enzymes that catalyze formation of acetyl-ACP and malonyl-ACP and the subsequent reactions of fatty acid synthesis are organized quite differently in different organisms. We first discuss fatty acid biosynthesis in bacteria and plants, where the various reactions are catalyzed by separate, independent proteins. Then we discuss the animal version of fatty acid biosynthesis, which involves a single multienzyme complex called fatty acid synthase. 

From here it s just the reaction catalyzed by fatty acid synthase. 

The first step is carboxylation of acetyl CoA to malonyl CoA. This reaction is catalyzed by acetyl-CoA carboxylase [5], which is the key enzyme in fatty acid biosynthesis. Synthesis into fatty acids is carried out by fatty acid synthase [6]. This multifunctional enzyme (see p. 168) starts with one molecule of ace-tyl-CoA and elongates it by adding malonyl groups in seven reaction cycles until palmi-tate is reached. One CO2 molecule is released in each reaction cycle. The fatty acid therefore grows by two carbon units each time. NADPH+H is used as the reducing agent and is derived either from the pentose phosphate pathway (see p. 152) or from isocitrate dehydrogenase and malic enzyme reactions. 

Fatty acid synthase in vertebrates consists of two identical peptide chains—i. e., it is a homodimer. Each of the two peptide chains, which are shown here as hemispheres, catalyzes all seven of the partial reactions required to synthesize palmitate. The spatial compression of several successive reactions into a single multifunctional enzyme has advantages in comparison with separate enzymes. Competing reactions are prevented, the individual reactions proceed in a coordinated way as if on a production line, and due to low diffusion losses they are particularly ef dent. 

All the reactions in the synthetic process are catalyzed by a multienzyme complex, fatty acid synthase. Although the details of enzyme structure differ in prokaryotes such as Escherichia coli and in eukaryotes, the four-step process of fatty acid synthesis is the same in all organisms. We first describe the process as it occurs in A1, coli, then consider differences in enzyme structure in other organisms. 

The remaining series of reactions of fatty acid synthesis in eukary-l otes is catalyzed by the multifunctional, dimeric enzyme, fatty acid synthase. Each fatty acid synthase monomer is a multicatalytic polypeptide with seven different enzymic activities plus a domain that covalently binds a molecule of 4 -phosphopantetheine. [Note 4-Phosphopantetheine, a derivative of the vitamin pantothenic add (see p. 379), carries acetyl and acyl units on its terminal thiol (-SH)j group during fatty acid synthesis. It also is a component of 00-enzyme A.] In prokaryotes, fatty acid synthase is a multienzyme complex, and the 4 -phosphopantetheine domain is a separate protein, referred to as the acyl carrier protein (ACP). ACP is used below to refer to the phosphopantetheine-binding domain of the eukaryotic fatty acid synthase molecule. The reaction numbers in1 brackets below refer to Figure 16.9. [Note The enzyme activities listed are actually separate catalytic domains present in each mulf-1 catalytic fatty acid synthase monomer.]... 

The First Step in Fatty Acid Synthesis Is Catalyzed by Acetyl-CoA Carboxylase Seven Reactions Are Catalyzed by the Fatty Acid Synthase... 

Seven Reactions Are Catalyzed by the Fatty Acid Synthase... 

Fatty acid synthesis begins when the substrates, acetyl-CoA and malonyl-CoA, are transferred onto the protein by malonyl-CoA acetyl-CoA-ACP transacylase (MAT, steps 1 and 2 in fig. 18.12a). The numbers in parentheses below the abbreviation of the enzyme in this figure refer to the reactions shown in fig. 18.12. (Whereas E. coli has separate enzymes that catalyze the transfer of acetyl- and malonyl-CoA to ACP, both reactions are catalyzed by the same enzymatic activity (MAT) on the animal fatty acid synthase.) Subsequently, /3-ketobutyryl-ACP and CO2 are formed in a condensation reaction catalyzed by /3-ketoacyl-ACP synthase (KS, step 3 in fig. 18.12a). 

Except for malonyl-CoA formation, all the individual reactions for palmitate synthesis reside on a single multifunctional protein (fatty acid synthase) in animal cells. It has been shown that a dimer of the multifunctional protein is required to catalyze palmitate synthesis. Explain the molecular basis of this observation. 

In prokaryotes, each of the reactions of fatty acid synthesis is catalyzed by a separate enzyme. However, in eukaryotes, the enzymes of the fatty acid synthesis elongation cycle are present in a single polypeptide chain, multifunctional enzyme complex, called fatty acid synthase. The fatty acid synthase complex exists as a dimer, with the ACP moiety shuttling the fatty acyl chain between successive catalytic sites, and from one subunit of the dimer to the other. It is, in effect, a highly efficient production line for fatty acid biosynthesis. 

Fatty acids are made in the body by a series of Claisen-type reactions catalyzed by an enzyme called a fatty acid synthase. The enzyme uses the thioesters of malonate and acetate as building blocks (see Figure 22-3 on page 1081). 

Fig. 4. X-ray determined protein crystal structures of multienzyme ensembly lines, (a) Mammalian fatty acid synthase at 4.5 A resolution (PDB 2cf2). Domain organization A starter substrate, acetyl-CoA or malonyl-CoA, gets loaded onto the acyl-carrler protein (ACP/absent in the structure) via the malonyl-CoA-/acetyl-CoA-ACP transacylase (MAT). Then, the ketoacyl synthase (KS) catalyzes a decarboxylative condensation reaction and forms the B-ketoacyl-ACP. This is followed from a reduction reaction catalyzed by the B-ketoacyl reductase (KR). Subsequently, the Intermediate gets dehydrated by a dehydratase (DH) and additionally reduced by a B-enoyl reductase (ER). The product gets released from the ACP by a thloesterase (absent in the structure), (b) Module 3 of 6-deoxyerthronolide B synthase at 2.6 A resolution (PDB 2qo3) bound to the inhibitor cerulin. The ketosynthase (KS) - acyltransferase (AT) di-domain is part of the large homodimeric polypeptide involved in biosynthesis of erythromycin from Saccharopolyspora erythraea...

Iwig DF, Grippe AT, McIntyre TA, Booker SJ. Isotope and elemental effects indicate a rate-limiting methyl transfer as the initial step in the reaction catalyzed by Escherichia coli cyclopropane fatty acid synthase. Biochemistry 2004 43 13510-13524. 

The enzyme system that catalyzes the synthesis of saturated long-chain fatty acids from acetyl CoA, malonyl CoA, and NADPH is called the fatty acid synthase. The constituent enzymes of bacterial fatty acid synthases are dissociated when the cells are broken apart. The availability of these isolated enzymes has facilitated the elucidation of the steps in fatty acid synthesis (Table 22.2). In fact, the reactions leading to fatty acid synthesis in higher organisms are very much like those of bacteria. 

The major product of the fatty acid synthase is palmitate. In eukaryotes, longer fatty acids are formed by elongation reactions catalyzed by enzymes on the cytosolic face of the endoplasmic reticulum membrane. These reactions add two-carbon units sequentially to the carboxyl ends of both saturated and unsaturated fatty acyl CoA substrates. Malonyl CoA is the two-carbon donor in the elongation of fatty acyl CoAs. Again, condensation is driven by the decarboxylation of malonyl CoA. 

The most important functions of pantothenic acid are its incorporation in coenzyme A and acyl carrier protein (AGP). Both CoA and AGP/4-phosphopantetheine function metabolically as carriers of acyl groups. Coenzyme A forms high-eneigy thioester bonds with carboxylic acids. The most important coenzyme is acetyl CoA. Acetic acid is produced during the metabolism of fatty acids, amino acids, or carbohydrates. The active acetate group of acetyl CoA can enter the Krebs cycle and is used in the synthesis of fatty acids or cholesterol. AGP is a component of the fatty acid synthase multienzyme complex. This complex catalyzes several reactions of fatty acid synthesis (condensation and reduction). The nature of the fatty acid synthase complex varies considerably among different species (91). 

The growing fatty acid chain on the fatty acid synthase complex is elongated, two carbons at a time, by the addition of the three-carbon compound, malonyl CoA, which is subsequently decarboxylated. With each two-carbon addition, the growing chain, which initially contains a P-keto group, is reduced in a series of steps that require NADPH. NADPH is produced by the pentose phosphate pathway and by the reaction catalyzed by the malic enzyme. 

Synthesis of fatty acid. ACP = Functional unit of acyl-carrier-protein segment of fatty acid synthase. The cysteinyl-SH group of /3-ketoacyI synthase accepts the acetyl group or the acyl group, and its catalytic site, which is adjacent to the -SH group, catalyzes the condensation reaction. 

The first step in de novo fatty acid synthesis is the production of malonyl-CoA from acetyl-CoA and bicarbonate. This committed step is catalyzed by acetyl-CoA carboxylase present in the cytoplasm of liver cells and adipocytes. After replacement of the CoA residue in acetyl-CoA by ACP (acyl carrier protein), malonyl-ACP is used to convert acetyl-ACP to butyryl-ACP by the fatty acid synthase complex. In this multistep reaction, NADPH is used as donor of hydrogen atoms and CO2 is produced. Butyryl-ACP is subsequently elongated to hexanoyl-ACP by a similar process in which malonyl-ACP serves as donor of two carbon atoms required for lengthening of the growing acyl chain. This process is repeated until palmitic acid... 

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