D-3-Hydroxyacyl-ACP

Ketoacyl-ACP + NADPH + H+ <=> D-3-Hydroxyacyl-ACP + NADP+ (catalyzed by / -ketoacyl-ACP reductase)... 

D-3-Hydroxyacyl-ACP <=> Trans- A2-euoyl-ACP + H20 (catalyzed by Hydroxylacyl-ACP Dehydrogeuase)... 

D-3-Hydroxyacyl-ACP is an intermediate in fatty acid biosynthesis. It participates in the reactions as follows (Figure 18.27) ... 

Step (3) Dehydration The elements of water are now removed from C-2 and C-3 of D-j3-hydroxybutyryl-ACP to yield a double bond in the product, trans-A2- butenoyl-ACP. The enzyme that catalyzes this dehydration is /3-hydroxyacyl-ACP dehydratase (HD). 

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. 

Dehydration of D-3-hydroxybutyryl-ACP to produce crotonyl-ACP (catalyzed by 3-hydroxyacyl-ACP dehydratase). 

Step 0 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-/3-hydroxybutyryl-ACP This reaction is catalyzed by /3-ketoacyl-ACP reductase (KR) and the electron donor is NADPH. Notice that the D-/3-hydroxybutyryl group does not have the same stereoisomeric form as the l-/3-hydroxyacyl intermediate in fatty acid oxidation (see Fig. 17-8). 

Figure 3.2 Enzyme systems associated with the de novo synthesis of fatty acids in the plastid. (a) Acetyl-CoA carboxylase (EC 6.4.1.2) (b) [acyl-carrier protein] (ACP) acetyl-transferase (EC 2.3.1.38) (c) [ACP] malonyltransferase (EC 2.3.1.39) (dl) 3-oxoacyl-[ACP] synthase I (d2) 3-oxoacyl-[ACP] synthase II (EC 2.3.1.41) (d3) 3-oxoacyl-[ACP] synthase III (e) 3-oxoacyl-[ACP] reductase (EC 1.1.1.100) (f) 3-hydroxyacyl-[ACP] dehydratase (g) enoyl-[ACP] reductase (EC 1.3.1.9/10) (d)-(g) constitutes fatty acid synthase...

Figure 2. Medium-chain-length (MCL) PHA production from non-related carbon sources. A. The fatty acid biosynthesis pathway. B. FabH and FabG mediated SCL-MCL PHA monomer supply. C. 3-hydroxyacyl-ACP CoA transacetylase (PhaG) mediated MCL PHA monomer supply. D. Thioesterase A (TesA) mediated MCL PHA monomer supply (via the P-oxidation pathway).

ACID-BASE RELATIONSHIPS OXYGEN, OXIDES 0X0 ANIONS d-2-HYDROXY-ACID DEHYDROGENASE (S)-2-HYDROXY-ACID OXIDASE 3-HYDROXYACYL-CoA DEHYDROGENASE /3-HYDROXYACYL THIOESTER (or, ACP) DE-HYDRASE... 

Product is acetyl-CoA 2. Malonyl-CoA is not involved no requirement for biotin 3. Oxidative process requires NAD+ and FAD and produces ATP 4. Fatty acids form thioesters with CoA-SH 5. Starts at carboxyl end (CHjCOg") 6. Occurs in the mitochondrial matrix, with no ordered aggregate of enzymes 7. P-Hydroxyacyl intermediates have the L configuration Precursor is acetyl-CoA Malonyl-CoA is source of two-carbon units biotin required Reductive process requires NADPH and ATP Fatty acids form thioesters with acyl carrier proteins (ACP-SH) Starts at methyl end (CHjCHg") Occurs in the cytosol, catalyzed by an ordered multienzyme complex P-Hydroxyacyl intermediates have the D configuration... 

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