Hydroxyacyl-ACP dehydrase

coli possesses two p-hydroxyacyl-ACP dehydratases. One is encoded by fabZ, and is active on all chain lengths of saturated and unsaturated intermediates. This enzyme is distinct from the dual-function P-hydroxydecanoyl-ACP dehydratase/isomerase (encoded by fabA) first described by Bloch and coworkers. The FabA enzyme dehydrates saturated, but not unsaturated, fatty acid intermediates and catalyzes a key isomerization reaction at the point where the biosynthesis of unsaturated fatty acids diverges from saturated fatty 

Saturated Fatty Adds Unsaturated Fatty Acids 

---

The Reduction Reactions. The object of the next three reactions (steps 4 to 6 in fig. 18.12a) is to reduce the 3-carbonyl group to a methylene group. The carbonyl is first reduced to a hydroxyl by 3-ketoacyl-ACP reductase. Next, the hydroxyl is removed by a dehydration reaction catalyzed by 3-hydroxyacyl-ACP dehydrase with the formation of a trans double bond. This double bond is reduced by NADPH catalyzed by 2,3-trans-enoyl-ACP reductase. Chemically, these reactions are nearly the same as the reverse of three steps in the j6-oxidation pathway except that the hydroxyl group is in the D-configuration for fatty acid synthesis and in the L-configuration for /3 oxidation (compare figs. 18.4a and 18.12a). Also remember that different cofactors, enzymes and cellular compartments are used in the reactions of fatty acid biosynthesis and degradation. 

Figure 3.8 One complete cycle and the first step in the next cycle of the events during the synthesis of fatty acids. ACP = acyl carrier protein, a complex of six enzymes i.e. acetyl CoA-ACP transacetylase (AT) malonyl CoA-ACP transferase (MT) /3-keto-ACP synthase (KS) /J-ketoacyl-ACP reductase (KR) / - hydroxyacyl-ACP-dehydrase (HD) enoyl-ACP reductase (ER).

Four enzymes participate in each iterative cycle of chain elongation (Fig. 3). The acetoacyl-ACP formed from the initiating FabH condensation is reduced by an NADPH-dependent P-ketoacyl-ACP reductase (fabG), and a water molecule is then removed by a P-hydroxyacyl-ACP dehydrase (fabA otfabZ). The last step is catalyzed by enoyl-ACP reductase (fabl or fabK) to form a saturated acyl-ACP, which serves as the substrate for another condensation reaction or when the chain length reaches 16-18 carbons is utilized for membrane phospholipid synthesis. p-Ketoacyl-ACP synthase I or II (fabB or fabF) initiates additional... 

Dehydration between the alpha and Beta by Beta-hydroxyacyl-ACP dehydrase. A molecule of water is removed to introduce a double bond between carbons 2 and 3 (the a- and yff-carbons) ... 

Once condensation has taken place, the /3-ketoacyl-ACP undergoes a cycle of reduction, dehydration, and reduction to yield a new fatty acid. The enzymes concerned are )8-ketoacyl-ACP reductase, )8-hydroxyacyl-ACP dehydrase, and enoyl reductase, and detailed information is provided in recent reviews. Both of the reductases have been reported in isoforms from several tissues. Thus, in avocado, an NADH form of )8-ketoacyl-ACP reductase has been resolved from an NADPH form. In seed tissues, two forms of enoyl reductase appear to be present they differ in the chain-length activity and also in whether they use NADH only or use NADPH in preference to NADH only the NADH form has been purified from leaves. ... 

---