Biosynthesis of Fatty AcidsFatty acids are long carbon chain acids. The location of fatty acid synthesis is liver and adipose tissue. Fatty acid synthesis takes place in smooth endoplasmic reticulum. Biosynthesis requires the participation of a three-carbon intermediate, malonyl-CoA.

Malonyl-CoA Is Formed from Acetyl-CoA and Bicarbonate:-

The formation of malonyl-CoA from acetyl-CoA is an irreversible process, catalyzed by acetyl-CoA carboxylase. the enzyme contains a biotin prosthetic group covalently bound in amide linkage to the amino group of a Lys residue in one of the threepolypeptides or domains of the enzyme molecule. The carboxyl group, derived from bicarbonate (HCO3-), is first transferred to biotin in an ATP dependent reaction. The biotinyl group serves as a temporary carrier of CO2, transferring it to acetyl-CoA in the second step to yield malonyl-CoA.

Pathway of fatty acid biosynthesis

The long carbon chains of fatty acids are assembled in a repeating four-step sequence (Fig. 21–2). A saturated acyl group produced by this set of reactions becomes the substrate for subsequent condensation with an activated malonyl group. With each passage through thecycle, the fatty acyl chain is extended by two carbons. All the reactions in the synthetic process are catalyzed by a multienzyme complex, fatty acid synthase.

Step 1 Condensation

The first reaction in the formation of a fatty acid chain is condensation of the activated acetyl and malonyl groups to form acetoacetyl-ACP, an acetoacetyl group bound to ACP through the phosphopantetheine OSH group; simultaneously, a molecule of CO2 is produced. In this reaction, catalyzed by ketoacyl-ACP synthase (KS), the acetyl group is transferred from the Cys OSH group of the enzyme to the malonyl group on the OSH of ACP, becoming the methyl-terminal two-carbon unit of the new acetoacetyl group. The carbon atom of the CO2 formed in this reaction is the same carbon originally introduced into malonyl- CoA from HCO3 _ by the acetyl-CoA carboxylase reaction. Thus CO2 is only transiently in covalent linkage during fatty acid biosynthesis; it is removed as each two-carbon unit is added. The use of activated malonyl groups rather than acetyl groups is what makes the condensation reactions thermodynamically favorable. The methylene carbon (C-2) of the malonyl group, sandwiched between carbonyl and carboxyl carbons, is chemically situated to act as a good nucleophile. In the condensation step (step 1 ), decarboxylation of the malonyl group facilitates the nucleophilic attack of the methylene carbon on the thioester linking the acetyl group to _-ketoacyl-ACP synthase, displacing the enzyme’s OSH group. The extra energy required to make fatty acid synthesis favorable is provided by the ATP used to synthesize malonyl-CoA from acetyl-CoA and HCO3 _.

Step 2 Reduction of the Carbonyl Group

The acetoacetyl-ACP formed in the condensation step now undergoes reduction of the carbonyl group at C-3 to form D-_- hydroxybutyryl-ACP. This reaction is catalyzed by _- ketoacyl-ACP reductase (KR) and the electron donor is NADPH. Notice that the D-_-hydroxybutyryl group does not have the same stereoisomeric form as the L-_- hydroxyacyl intermediate in fatty acid oxidation.

Step 3 Dehydration

The elements of water are now removed from C-2 and C-3 of D-_-hydroxybutyryl-ACP toyield a double bond in the product, trans-_2- butenoyl-ACP. The enzyme that catalyzes this dehydration is _-hydroxyacyl-ACP dehydratase (HD).

Step 4 Reduction of the Double Bond

Finally, the double bond of trans-_2-butenoyl-ACP is reduced (saturated) to form butyryl-ACP by the action of enoyl-ACP reductase (ER); again, NADPH is the electron donor.Production of the four-carbon, saturated fatty acyl–ACP completes one pass through the fatty acid synthase

Production of the four-carbon, saturated fatty acyl–ACP completes one pass through the fatty acid synthase complex. To start the next cycle of four reactions that lengthens the chain by two more carbons, another malonyl group is linked to the now unoccupied phosphopantetheine OSH group of ACP. Seven cycles of condensation and reduction produce the 16-carbon saturated palmitoyl group. We can consider the overall reaction for the synthesis of palmitate from acetyl-CoA in two parts. First, the formation of seven malonyl-CoA molecules:

then seven cycles of condensation and reduction:

The overall process is:-

Eicosanoids Are Formed from 20-Carbon Polyunsaturated Fatty Acids

Eicosanoids are a family of very potent biological signaling molecules that act as short-range messengers, affecting tissues near the cells that produce them. In response to hormonal or other stimuli, phospholipase A2, present in most types of mammalian cells, attacks membrane phospholipids, releasing arachidonate from the middle carbon of glycerol. Enzymes of the smooth ER then convert arachidonate to prostaglandins, beginning with the formation of prostaglandin H2 (PGH2), the immediate precursor of many other prostaglandins andof thromboxanes. The two reactions that lead to PGH2 are catalyzed by a bifunctional enzyme, cyclooxygenase (COX), also called prostaglandin H2 synthase. In the first of two steps, the cyclooxygenase activity introduces molecular oxygen to convert arachidonate to PGG2. The second step, catalyzed by the peroxidase activity of COX, converts PGG2 to PGH2.

Pathway 1

Biosynthesis of Triacylglycerols

Most of the fatty acids synthesized or ingested by an organism have one of two fates: incorporation into triacylglycerols for the storage of metabolic energy or incorporation into the phospholipid components of membranes. The partitioning between these alternative fates depends on the organism’s current needs. During rapid growth, synthesis of new membranes requires the production of membrane phospholipids; when an organism has a plentiful food supply but is not actively growing, it shunts most of its fatty acids into storage fats. Both pathways begin at the same point: the formation of fatty acyl esters of glycerol.

In animal tissues, triacylglycerols and glycerophospholipids such as phosphatidylethanolamine share two precursors (fatty acyl–CoA and L-glycerol 3-phosphate) and several biosynthetic steps. The vast majority of the glycerol 3-phosphate is derived from the glycolytic intermediate

Pathway 2

dihydroxyacetone phosphate (DHAP) by the action of the cytosolic NAD-linked glycerol 3-phosphate dehydrogenase; in liver and kidney, a small amount of glycerol 3-phosphate is also formed from glycerol by the action of glycerol kinase. The other precursors of triacylglycerols are fatty acyl–CoAs, formed from fatty acids by acyl-CoA synthetases, the same enzymes responsible for the activation of fatty acids for _ oxidation.

The first stage in the biosynthesis of triacylglycerols is the acylation of the two free hydroxyl groups of Lglycerol 3-phosphate by two molecules of fatty acyl–CoA to yield diacylglycerol 3-phosphate, more commonly called phosphatidic acid or phosphatidate.

Phosphatidic acid is present in only trace amounts in cells but is a central intermediate in lipid biosynthesis; it can be converted either to a triacylglycerol or to a glycerophospholipid.

In the pathway to triacylglycerols, phosphatidic acid is hydrolyzed by phosphatidic acid phosphatase to form a 1,2-diacylglycerol.

Diacylglycerols are then converted to triacylglycerols by transesterification with a third fatty acyl–CoA.

Oxidation of fatty acidsFatty Acids Are Activated and Transported into Mitochondria:

The enzymes of fatty acid oxidation in animal cells are located in the mitochondrial matrix, as demonstrated in 1948 by Eugene P. Kennedy and Albert Lehninger. The fatty acids with chain lengths of 12 or fewer carbons enter mitochondria without the help of membrane transporters. Those with 14 or more carbons, which constitute the majority of the FFA obtained in the diet or released from adipose tissue, cannot pass directly through the mitochondrial membranes—they must first undergo the three enzymatic reactions of the carnitine shuttle.

The first reaction is catalyzed by a family of isozymes (different isozymes specific for fatty acids having short, intermediate, or long carbon chains) presentin the outer mitochondrial membrane, the acyl-CoA synthetases, which promote the general reaction.

Thus, acyl-CoA synthetases catalyze the formation of a thioester linkage between the fatty acid carboxyl group and the thiol group of coenzyme A to yield a fatty acyl–CoA, coupled to the cleavage of ATP to AMP and PPi. The formation of a fatty acyl–CoA is made more favorable by the hydrolysis of two high-energy bonds in ATP; the pyrophosphate formed in the activation reaction is immediately hydrolyzed by inorganic pyrophosphatase, which pulls the preceding activation reaction in the direction of fatty acyl–CoA formation.

The overall reaction is

Fatty acids destined for mitochondrial oxidation are transiently attached to the hydroxyl group of carnitine to form fatty acyl–carnitine—the second reaction of the shuttle. This transesterification is catalyzed by carnitine acyltransferase I (Mr 88,000), in the outer membrane.

The fatty acyl–carnitine ester then enters the matrix by facilitated diffusion through the acyl-carnitine/carnitine transporter of the inner mitochondrial membrane.

In the third and final step of the carnitine shuttle, the fatty acyl group is enzymatically transferred from carnitine to intramitochondrial coenzyme A by carnitine acyltransferase II. This isozyme, located on the inner face of the inner mitochondrial membrane, regenerates fatty acyl–CoA and releases it, along with free carnitine, into the matrix.

The β Oxidation of Saturated Fatty Acids

1.) Four enzyme-catalyzed reactions make up the first stage of fatty acid oxidation (Fig. 17–8a). First, dehydrogenation of fatty acyl–CoA produces a double bond between the α and β carbon atoms (C-2 and C-3), yielding a trans-_2-enoyl-CoA. This first step is catalyzed by three isozymes of acyl-CoA dehydrogenase.

2.) In the second step of the β-oxidation cycle, water is added to the double bond of the trans-_2-enoyl-CoA to form the L stereoisomer of_-hydroxyacyl-CoA (3-hydroxyacyl-CoA). This reaction,catalyzed by enoyl-CoA hydratase, is formally analogous to the fumarase reaction in the citric acid cycle, in which H2O adds across an α–β doublebond.

3.) In the third step, L,β-hydroxyacyl-CoA is dehydrogenatedto form β-ketoacyl-CoA, by the action of β-hydroxyacyl-CoA dehydrogenase; NAD+ is the electron acceptor. This enzyme is absolutely specific for the L stereoisomer of hydroxyacyl-CoA. The NADH formed in the reaction donates its electrons to NADH dehydrogenase, an electron carrier of the respiratory chain, and ATP is formed from ADP as the electrons pass to O2. The reaction catalyzed by β-hydroxyacyl-CoA dehydrogenase is closely analogous to the malate dehydrogenasereaction of the citric acid cycle.

4.) The fourth and last step of the β-oxidation cycle is catalyzed by acyl-CoA acetyltransferase, more commonly called thiolase, which promotes reaction of β-ketoacyl-CoA with a molecule of free coenzyme A to split off the carboxyl-terminal two-carbon fragment of the original fatty acid as acetyl-CoA. The other product is the coenzyme A thioester of the fatty acid, now shortened by two carbon atoms (Fig. 17–8a). This reaction is called thiolysis, by analogy with the process of hydrolysis, because the _-ketoacyl-CoA is cleaved by reaction with the thiol group of coenzyme A.

In one pass through the β-oxidation sequence, one molecule of acetyl-CoA, two pairs of electrons, and four protons are removed from the long-chain fatty acyl–CoA, shortening it by two carbon atoms. The equation for one pass, beginning with the coenzyme A ester of our example, palmitate, is

The overall equation is

The acetyl-CoA produced from the oxidation of fatty acids can be oxidized to CO2 and H2O by the citric acid cycle. The following equation represents the balance sheet for the second stage in the oxidation of palmitoyl- CoA, together with the coupled phosphorylations of the third stage:

Combining both Equations, we obtain the overall equation for the complete oxidation of palmitoyl- CoA to carbon dioxide and water:

Oxidation of Unsaturated Fatty Acids

Two auxiliary enzymes are needed for β oxidation of the common unsaturated fatty acids: an isomerase and a reductase.

Oleate is an abundant 18-carbon monounsaturated fatty acid with a cis double bond between C-9 and C-10 (denoted _9).

In the first step of oxidation, oleate is converted to oleoyl-CoA and, like the saturated fatty acids, enters the mitochondrial matrix via the carnitine shuttle. Oleoyl-CoA then undergoes three passes through the fatty acid oxidation cycle to yield three molecules of acetyl-CoA and the coenzyme A ester of a _3, 12-carbon unsaturated fatty acid, cis-β-3-dodecenoyl-CoA. This product cannot serve as a substrate for enoyl-CoA hydratase, which acts only on trans double bonds.

The auxiliary enzyme β-3,_2-enoyl-CoA isomerase isomerizes the cis-_3-enoyl-CoA to the trans-β_2-enoyl-CoA, which is converted by enoyl-CoA hydratase into the corresponding Lβ_-hydroxyacyl-CoA (trans-_2-dodecenoyl-CoA).

This intermediate is now acted upon by the remaining enzymes of β oxidation to yield acetyl-CoA and the coenzyme A ester of a 10- carbon saturated fatty acid, decanoyl-CoA.

The latter undergoes four more passes through the pathway to yield five more molecules of acetyl-CoA. Altogether, nine acetyl-CoAs are produced from one molecule of the 18-carbon oleate

.

Oxidation of polyunsaturated fatty acids

The oxidation of polyunsaturated fatty acids is similar to monounsaturated fatty acids, as both requires β oxidation. In case of polyunsaturated fatty acids, there are more than one doubles bond present there. In oxidation process initially β oxidation processes till it reaches double bond. Here isomerase enzyme converts it into single bond and process futher continuoue as such.

The ω Oxidation of Fatty Acids

The enzymes unique to ω oxidation are located (in vertebrates) in the endoplasmic reticulum of liver and kidney, and the preferred substrates are fatty acids of 10 or 12 carbon atoms. In mammals ω oxidation is normally a minor pathway for fatty acid degradation, but when β oxidation is defective (because of mutation or a carnitine deficiency, for example) it becomes more important.

The first step introduces a hydroxyl group onto the ω_ carbon (Fig. 17–16). The oxygen for this group comes from molecular oxygen (O2) in a complex reaction that involves cytochrome P450 and the electron donor NADPH. Reactions of this type are catalyzed by mixedfunction oxidases. Two more enzymes now act on the _ carbon: alcohol dehydrogenase oxidizes the hydroxyl group to an aldehyde, and aldehyde dehydrogenase oxidizes the aldehyde group to a carboxylic acid, producing a fatty acid with a carboxyl group at each end. In each pass through the _-oxidation pathway, the “double-ended” fatty acid yields dicarboxylic acids such as succinic acid, which can enter the citric acid cycle, and adipic acid.

α Oxidation in Peroxisomes

The presence of a methyl group on the α_ carbon of a fatty acid makes α_ oxidation impossible, and these branched fatty acids are catabolized in peroxisomes of animal cells by α_ oxidation. In the oxidation of phytanic acid, for example, phytanoyl- CoA is hydroxylated on its α _ carbon, in a reaction that involves molecular oxygen; decarboxylated to form an aldehyde one carbon shorter; and then oxidized to the corresponding carboxylic acid, which now has no substituenton the _α carbon and can be oxidized further by β_ oxidation.

Refrences-

Biochemistry-Lehninger

Biochemistry- Satyanarayan