Enoyl CoA isomerase

FIGURE 24.23 )3-Oxidation of unsaturated fatty acids. In the case of oleoyl-CoA, three /3-oxidation cycles produce three molecules of acetyl-CoA and leave m-AAdodecenoyl-CoA. Rearrangement of enoyl-CoA isomerase gives the tran.s-A species, which then proceeds normally through the /3-oxidation pathway. 

Polyunsaturated fatty acids pose a slightly more complicated situation for the cell. Consider, for example, the case of linoleic acid shown in Figure 24.24. As with oleic acid, /3-oxidation proceeds through three cycles, and enoyl-CoA isomerase converts the cA-A double bond to a trans-b double bond to permit one more round of /3-oxidation. What results this time, however, is a cA-A enoyl-CoA, which is converted normally by acyl-CoA dehydrogenase to a trans-b, cis-b species. This, however, is a poor substrate for the enoyl-CoA hydratase. This problem is solved by 2,4-dienoyl-CoA reductase, the product of which depends on the organism. The mammalian form of this enzyme produces a trans-b enoyl product, as shown in Figure 24.24, which can be converted by an enoyl-CoA isomerase to the trans-b enoyl-CoA, which can then proceed normally through the /3-oxidation pathway. Escherichia coli possesses a... 

FIGURE 24.24 The oxidation pathway for polyunsaturated fatty adds, illustrated for linoleic add. Three cycles of /3-oxidation on linoleoyl-CoA yield the cis-A, d.s-A intermediate, which is converted to a tran.s-A, cis-A intermediate. An additional round of /S-oxi-dation gives d.s-A enoyl-CoA, which is oxidized to the trans-A, d.s-A species by acyl-CoA dehydrogenase. The subsequent action of 2,4-dienoyl-CoA reductase yields the trans-A product, which is converted by enoyl-CoA isomerase to the tran.s-A form. Normal /S-oxida-tion then produces five molecules of acetyl-CoA. 

The specific behaviour of unsaturated fatty acids under oxidation is determined by the position and the number of double bonds in the fatty acid molecule. The stepwise oxidation of an unsaturated acid to the position of a double bond in it proceeds in a manner similar to that of saturated acid oxidation. If the double bond retains the same configuration (trans-configuration) and position (A2,3) as those of the enoyl-CoA, which is produced during the oxidation of saturated fatty acids, the subsequent oxidation proceeds via conventional route. Otherwise, the oxidation reaction proceeds with the involvement of an accessory enzyme, A3,4-CiS-A2,3jrans-enoyl-CoA isomerase this facilitates the translocation of the double bond to an appropriate position and alters the double-bond configuration from cis to trans. 

Unsaturated fatty acids usually contain a cis double bond at position 9 or 12—e.g., linoleic acid (18 2 9,12). As with saturated fatty acids, degradation in this case occurs via p-oxida-tion until the C-9-ds double bond is reached. Since enoyl-CoA hydratase only accepts substrates with trans double bonds, the corresponding enoyl-CoA is converted by an iso-merase from the ds-A, cis- A isomer into the trans-A, cis-A isomer [1]. Degradation by p-oxidation can now continue until a shortened trans-A, ds-A derivative occurs in the next cycle. This cannot be isomerized in the same way as before, and instead is reduced in an NADPH-dependent way to the trans-A compound [2]. After rearrangement by enoyl-CoA isomerase [1 ], degradation can finally be completed via normal p-oxidation. 

Isomerase, enoyl-CoA isomerase (1SG4) [66] Glul36 Leu66 Glylll ... 

The active site of enoyl-CoA isomerase is a good example of an active site built on the framework of the crotonase fold. It is now weU established that this crotonase fold provides an active site framework that has been used by Nature for a wide range of different chemical reactions, as reviewed recently [73, 85]. The reaction of this enoyl-CoA isomerase is initiated by a catalytic base, Glul36, abstracting a proton from the Ca-carbon, generating the negatively charged enolate... 

Figure 4.5 The active site of enoyl-CoA isomerase, complexed with octanoyl-CoA (PDB 1SC4). In this structure the oxygen atom of Clul36 which acts as the catalytic base points to the C2 carbon of the ligand and is therefore well positioned for proton...

Scheme 4.6 The reaction mechanism and mode of catalysis by enoyl-CoA isomerase.

FIGURE 17-9 Oxidation of a monounsaturated fatty add. Oleic acid, asoleoyl-CoA (A9), is the example used here. Oxidation requires an additional enzyme, enoyl-CoA isomerase, to reposition the double bond, converting the cis isomer to a trans isomer, a normal intermediate in 13 oxidation. 

Oxidation of unsaturated fatty acids requires two additional enzymes enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase. Odd-number fatty acids are oxidized by the /3-oxidation pathway to yield acetyl-CoA and a molecule of propionyl-CoA This is carboxylated to methylmalonyl-CoA, which is isomerized to succinyl-CoA in a reaction catalyzed by methylmalonyl-CoA mutase, an enzyme requiring coenzyme B12. 

Unsaturated fatty acids. Mitochondrial P oxidation of such unsaturated acids as the A9-oleic acid begins with removal of two molecules of acetyl-CoA to form a A5-acyl-CoA. However, further metabolism is slow. Two pathways have been identified (Eq. 17-l).26 29b The first step for both is a normal dehydrogenation to a 2-fraus-5-czs-dienoyl-CoA. In pathway I this intermediate reacts slowly by the normal p oxidation sequence to form a 3-czs-enoyl-CoA intermediate which must then be acted upon by an auxiliary enzyme, a ds-AMra s-A2-enoyl-CoA isomerase (Eq. 17-1, step c), before P oxidation can continue. 

Polyunsaturated fatty acids are also degraded by [3 oxidation, but the process requires enoyl-CoA isomerase and an additional enzyme, 2,4-dienoyl-CoA reductase (fig. 18.6). For example, the degradation of linoleoyl-CoA (18 2A9-12) begins, like that of oleoyl-CoA, with three rounds of /3 oxidation and results in a A3-cis unsaturated fatty acyl-CoA that is not a substrate for acyl-CoA dehydrogenase. Isomerization of the double bond to the A2-trans position by enoyl-CoA isomerase allows the resumption of... 

Unsaturated fatty acids such as oleoyl-CoA can also be degraded to acetyl-CoA in mitochondria. Three cycles of /3 oxidation result in an acyl-CoA (A3-cis-dodecenoyl-CoA) that is not a substrate for acyl-CoA dehydrogenase. This problem is circumvented by isomerization of the double bond to the t -tmns position by enoyl-CoA isomerase. Complete oxidation of the remainder of the molecule by the enzymes of f3 oxidation is now possible. 

Polyunsaturated fatty acids such as linoleoyl-CoA are also oxidized in the mitochondria. In addition to the enzymes of oxidation and enoyl-CoA isomerase, the process requires 2,4-dienoyl-CoA reductase. 

A2-tran.v-isomer by enoyl-CoA isomerase (see fig. 18.6). With the double bond in the A2-tra s-configuration, /3 oxidation can resume. As with the oxidation of oleoyl-CoA, one less FADH2 and 1.5 fewer ATPs are produced in the oxidation of linoleoyl-CoA compared to stearoyl-CoA. In addition, one NADPH is required for the reduction of the dienoyl-CoA. Hence, in balancing the yield for complete oxidation of linoleoyl-CoA, one less NADH would be available to the respiratory chain and, therefore, 2.5 fewer ATPs would be produced. This is true since the two nucleotides can be interconverted ... 

Methyl (E)-2-octenoate was found for the first time in pineapple. It had been previously reported in pears (22) and soursop (2. Its formation from methyl (E)-3-octenoate (a pineapple constituent) by 2,3-(E,E)-enoyl-CoA-isomerase was postulated by Berger and Kollmannsberger ( )... 

Very long chain fatty acids are initially oxidized in the peroxisome where the initial oxidation step is catalyzed by acyl-CoA oxidase and the subsequent steps in fS-oxidation are catalyzed by a multi-enzyme complex with hydratase, dehydo-genase, and thiolase activities. Unsaturated fatty acids require additional enzymatic activities, including enoyl-CoA isomerase and dienoyl-CoA reductase. Readers are directed to Vance and Vance (2) for additional details regarding fi-oxidation, including the details of the metabolic reactions. 

Figure 22.10. Oxidation of Linoleoyl CoA. The complete oxidation of the diunsaturated fatty acid linoleate is facilitated by the activity of enoyl CoA isomerase and 2,4-dienoyl CoA reductase.

Two new enzymes are required to handle these situations Enoyl-CoA isomerase (isomerizes a cis-3,4-double bond to a frflns-2,3-double bond), and 2,4-Dienoyl-CoA reductase (reduces the ds-4,5-double bond in the trans-2,3-ds-4,5-dienoyl-CoA derivative formed during feta-oxidation). The resulting products are then broken down by the feta-oxidation enzymes. 

Glutaric acidemia type II is caused by defects in the ETF/ETF-QO proteins. The clinical manifestations of these disorders are similar to medium-chain acyl-CoA dehydrogenase deficiency (discussed later). The double bond formed by the acyl-CoA dehydrogenase has a trans configuration. The double bonds in naturally occurring fatty acids are generally in the cis configuration. The oxidation of unsaturated m-fatty acids requires two auxiliary enzymes, enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase. 

Oxidation of unsaturated fatty acids requires A -cis-,A -trans-enoyl-CoA isomerase and NADPH-dependent 2,4-dienoyl-CoA reductase, in addition to the enzymes of y3-oxidation. The enoyl-CoA isomerase produces the substrate for the hydration step. The reductase catalyzes the reduction of A -frans-,A -cjs-decadienoyl-CoA to A -rrans-decenoyl-CoA. The latter is isomerized to A -trans-decenoyl isomerase, which is a normal -oxidation intermediate. These reactions are illustrated for oxidation of oleic and linoleic acids in Figures 18-7 and 18-8. 

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