Acetoacetyl-S-ACP

It is at this stage that carbon dioxide is evolved. The acetoacetyl-S-ACP is then reduced, dehydrated and then reduced again to form butyryl-S-ACP ... 

Steps 6-8 of Figure 29.5 Reduction and Dehydration The ketone carbonyl group in acetoacetyl ACP is next reduced to the alcohol /S-hydroxybutyry] ACP by yS-keto thioester reductase and NADPH, a reducing coenzyme closely related to NADH. R Stereochemistry results at the newly formed chirality center in the /3-hydroxy thioester product. (Note that the systematic name of a butyryl group is biitanoyl.)... 

STEP S Condenantion of maionyl ACP with an ncety) group occurs, fbffowed by dceartmxylation, to give acetoacetyl ACP. 

Claisen-like condensation of malonyl ACP with acetyl synthase occurs, followed by decarboxylation to yield acetoacetyl ACP, a /S-keto thioester. 

STEP S Condensation of malonyl AGP with an acetyl group occurs, followed by decarboxylation, to give acetoacetyl ACP. 

S Acetoacetyl ACP -P NADPH -P D-3-hydroxybutyryl ACP 4 NADP [3-KeToacyt ACP reductase... 

The acetyl group is shown bound to the enzyme )3-ketoacyl-ACP synthase through a cysteine residue. The carbonyl group of the acetyl group is attacked by the central carbon on the malonyl group attached to ACP. Acetoacetyl-ACP is generated as the C—S bond is broken. 

Fig. 2.3 Biosynthesis pathway of A P(3HB) B P(3HB-co-3HV) C P(3HB-co-3HHx) via fatty acid /S-oxidation and D P(3HB-co-3HHx) via fatty acid de novo synthesis. PhaA, f -ketothiolase PhaB, NADPH dependent acetoacetyl-CoA reductase PhaC, PHA synthase PhaG, 3-hydroxyl-ACP-CoA transferase PhaJ, (J )-enoyl-CoA hydratase FabG, 3-ketoacyl-CoA reductase (Sudesh et al. 2000)...

The cenjienin insensitive KAS III catalyzes the first condensing step of the fatty acid biosynthesis between acetyl-CoA and malonyl-ACP producing acetoace-tyl-ACP, which is further reduced to butyryl-ACP in the course of the FAS-reac-tion using NADH and NADPH as reducing equivalents. The current assay for KAS Ill-activity is based on the incorporation, in presence of 100 pM cerulenin, of radioactively labeled acetyl-CoA into acetoacetyl-ACP, which is precipitated by acid. NADH and NADPH are not present in this "normal" assay. However, the supplementation of NADH and NADPH, with the consequent reduction of acetoacetyl-ACP to butyryl-ACP, leads to a decrease in the I S Ill-activity, as shown in Table 1. This decrease in KAS Ill-activity can be directly attributed to the presence of butyryl-ACP (Table 1). Similarly as described for spinach leaf... 

Jackowski, S. and Rock, C.O. (1987) Acetoacetyl-ACP synthase, a potential regulator of fatty acid biosynthesis in bacteria. J. Biol. Chem., 262,7927-7931. 

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