Acyl CoA molecules

FIGURE 25.3 In the acetyl-CoA carboxylase reaction, the biotin ring, on its flexible tether, acquires carboxyl groups from carbonylphos-phate on the carboxylase subunit and transfers them to acyl-CoA molecules on the transcarboxylase subunits. 

Acetyl-CoA carboxylase is an allosteric enzyme and is activated by citrate, which increases in concentration in the well-fed state and is an indicator of a plentiful supply of acetyl-CoA. Citrate converts the enzyme from an inactive dimer to an active polymeric form, having a molecular mass of several milhon. Inactivation is promoted by phosphorylation of the enzyme and by long-chain acyl-CoA molecules, an example of negative feedback inhibition by a product of a reaction. Thus, if acyl-CoA accumulates because it is not esterified quickly enough or because of increased lipolysis or an influx of free fatty acids into the tissue, it will automatically reduce the synthesis of new fatty acid. Acyl-CoA may also inhibit the mitochondrial tricarboxylate transporter, thus preventing activation of the enzyme by egress of citrate from the mitochondria into the cytosol. 

In P-oxidation (Figure 22-2), two carbons at a time are cleaved from acyl-CoA molecules, starting at the carboxyl end. The chain is broken between the 0t(2)- and P(3)-carbon atoms—hence the name P-oxidation. The two-carbon units formed are acetyl-CoA thus, palmi-toyl-CoA forms eight acetyl-CoA molecules. 

Figure 18-5 A current concept of the electron transport chain of mitochondria. Complexes I, III, and IV pass electrons from NADH or NADPH to 02, one NADH or two electrons reducing one O to HzO. This electron transport is coupled to the transfer of about 12 H+ from the mitochondrial matrix to the intermembrane space. These protons flow back into the matrix through ATP synthase (V), four H+ driving the synthesis of one ATP. Succinate, fatty acyl-CoA molecules, and other substrates are oxidized via complex II and similar complexes that reduce ubiquinone Q, the reduced form QH2 carrying electrons to complex III. In some tissues of some organisms, glycerol phosphate is dehydrogenated by a complex that is accessible from the intermembrane space.

In higher plants, animals, protozoa, and fungi, saturated fatty acids are acted upon by desaturases to introduce double bonds, usually of the cis (Z) configuration. The substrates may be fatty acyl-ACP, fatty acyl-CoA molecules, membrane phospholipids,97 or glycolipids.98 The A9 desaturase, isolated from liver or from yeast, converts stearoyl-CoA to oleoyl-CoA (Eq. 21-3).99-102 This membrane-associated enzyme system... 

Transport into Small- and medium-chain acyl CoA molecules (up to 10 carbon atoms) are readily... 

This step leaves two cleavage products. The first, derived from the two carbons at the carboxyl end of the fatty acid, is acetyl-CoA, which can be further metabolized in the TCA cycle. The second cleavage product is a shorter fatty acyl-CoA. Thus, for example, the initial step of digesting a fatty acid with 16 carbons is an acyl-CoA molecule where the acyl group has 14 carbons and a molecule of acetyl-CoA. The P-oxidation scheme may be used to accommodate unsaturated fatty acids also. The reactions occur as described previously for the saturated portions of the molecule. Where a trans carbon-carbon double bond occurs between the %- and p-carbons of the acyl-CoA, the accommodation is fairly simple reaction 1 isn t needed. Where the double bonds are in the cis configuration, or are between the P and y carbons, isomerase enzymes change the location of the double bonds to make recognizable substrates for P-oxidation. 

Answer The first three reactions in the /3 oxidation of fatty acyl-CoA molecules are analogous to three reactions of the citric acid cycle. 

Answer Activation of carboxyl groups by ATP could in theory be accomplished by three types of reactions the formation of acyl-phosphate + ADP of acyl-ADP + P, or of acyl-AMP + PP,. All these reactions are readily reversible. To create an activation reaction with a highly negative AG ° (effectively irreversible), the third type of reaction can be coupled to a pyrophosphatase reaction, as in the synthesis of fatty acyl-CoA molecules. 

Answer The transport of fatty acid molecules into mitochondria requires a shuttle system involving a fatty acyl-carnitine intermediate. Fatty acids are first converted to fatty acyl-CoA molecules in the cytosol (by the action of acyl-CoA synthetases) then, at the outer mitochondrial membrane, the fatty acyl group is transferred to carnitine (by the action of carnitine acyl-transferase I). After transport of fatty acyl-carnitine through the inner membrane, the fatty acyl group is transferred to mitochondrial CoA. The cytosolic and mitochondrial pools of CoA are thus kept separate, and no labeled CoA from the cytosolic pool enters the mitochondrion. 

Transport of the fatty acyl-CoA into the mitochondrion is accomplished via an acyl-carnitine intermediate inside the mitochondrion the fatty acyl-CoA molecule is regenerated. 

The process of fatty acid oxidation is termed /S-oxidation since it occurs through the sequential removal of 2-carbon units by oxidation at the /S-carbon position of the fatty acyl-CoA molecule. Each round of /S-oxidation produces one molecule of NADH, one molecule of FADH2 (which in turn produces one molecule of QH2) and one molecule of acetyl-CoA. The acetyl-CoA, the end product of each round of /S-oxidation, then enters the TCA cycle, where it is further oxidised to CO2 with the concomitant generation of three molecules of NADH, one molecule of FADH2 (QH2) and one molecule of ATP. The NADH and FADH2 (QH2) generated during fat oxidation and acetyl-CoA oxidation in the TCA cycle then enter the mitochondrial respiratory pathway for the production of ATP. 

Fatty acids are activated on the outer mitochondrial membrane, whereas they are oxidized in the mitochondrial matrix. A special transport mechanism is needed to carry long-chain acyl CoA molecules across the inner mitochondrial membrane. Activated long-chain fatty acids are transported across the membrane by conjugating them to carnitine, a zwitterionic alcohol. The acyl group is transferred from the sulfur atom of CoA to the hydroxyl group of carnitine to form acyl carnitine. This reaction is catalyzed by carnitine acyltransferase I (also called carnitine palmitoyl transferase I), which is bound to the outer mitochondrial membrane. 

The desaturase catalyzing this final step in the synthesis of a plasmalogen is an endoplasmic reticulum enzyme akin to the one that introduces double bonds into long-chain fatty acyl CoA molecules. In both cases, O2 and NADH are reactants, and cytochrome b 5 participates in catalysis (Section 22.6). 

In the last step of P-oxidation, p-ketoacyl CoA reacts with coenzyme A in the presence of the enzyme, thiolase. The products of this reaction are acetyl CoA and an acyl CoA containing two carbons less than the original acyl CoA molecule that underwent oxidation. 

Triacylglycerol synthesis (referred to as lipogenesis) is illustrated in Figure 12.2. Glycerol-3-phosphate or dihydroxyacetone phosphate reacts sequentially with three molecules of acyl-CoA (fatty acid esters of CoASH). Acyl-CoA molecules are produced in the following reaction ... 

Each acyl-CoA molecule is converted to an acylcamitine derivative ... 

The /J-oxidalion of acyl-CoA molecules includes four reactions that occur in the mitochondrial matrix. Each cycle of reactions forms acetyl-CoA and an acyl-CoA that is shorter by two carbons. 

Acyl-CoA molecules are desaturated in ER membrane in the presence of NADH and 02. All components of the desaturase system are integral membrane proteins that are apparently randomly distributed on the cytoplasmic surface of the ER. The association of cytochrome b5 reductase (a flavoprotein), cytochrome b5, and oxygen-dependent desaturases constitutes an electron transport system. This system efficiently introduces double bonds into long-chain fatty acids (Figure 12.15). Both the flavoprotein and cytochrome b5 (found in a ratio of approximately 1 30) have hydrophobic peptides that anchor the proteins into the microsomal membrane. Animals typically have A9, A6, and A5 desaturases that use electrons supplied by NADH via the electron transport system to activate the oxygen needed to create the double bond. Plants contain additional desaturases for the A12 and A15 positions. 

Carnitine tablets are sold in health food stores. It is claimed that carnitine will enhance the breakdown of body fat. Carnitine is a tertiary amine found in mitochondria, cell organelles in which food molecules are completely oxidized and ATP is produced. Carnitine is involved in transporting the acyl groups of fatty acids from the cytoplasm into the mitochondria. The fatty acyl group is transferred from a fatty acyl CoA molecule and esterified to carnitine. Inside the mitochondria the reachon is reversed and the fatty acid is completely oxidized. The structure of carnitine is shown here ... 

The p-oxidation cycle (steps 2-5, Figure 23.7) consists of a set of four reactions whose overall form is similar to the last four reactions of the citric acid cycle. Each trip through the sequence of reactions releases acetyl CoA and returns a fatty acyl CoA molecule that has two fewer carbons. One molecule of FADH2, equivalent to two ATP molecules, and one molecule of NADH, equivalent to three ATP molecules, are produced for each cycle of p-oxidation. 

Reaction 1. The first step is an activation reaction that results in the production of a fatty acyl CoA molecule. A thioester bond is formed between coenzyme A and the fatty acid ... 

Fatty acids are degraded to acetyl CoA in the mitochondria by the ( -oxidation pathway, which involves five steps (1) the production of a fatty acyl CoA molecule, (2) oxidation of the fatty acid by an FAD-dependent dehydrogenase, (3) hydration, (4) oxidation by an NAD+-dependent dehydrogenase, and (5) cleavage of the chain with release of acetyl CoA and a fatty acyl CoA that is two carbons shorter than the beginning fatty acid. The last four reactions are repeated until the fatty acid is completely degraded to acetyl CoA. 

Thioesterases preferentially cleave the thioester bond of acyl-CoA molecules to produce CoA and free fatty acid. E. coli contains two well-characterized thioesterases. Thioesterase I (encoded by tesA) is a periplasmic enzyme of 20.5 kDa, with a substrate specificity for acyl chains >12 carbon atoms. Thioesterase 1 hydrolyzes synthetic substrates used in the assay of chymotrypsin, which led to the initial conclusion that TesA was a protease ( protease F). However, the purified protein does not cleave peptide bonds. Thioesterase... 

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