Butyryl-S-ACP

The biosynthesis of fatty acids is accomplished two carbons at a time by an enzyme complex called fatty acid synthetase. The biochemical reactions involved in fatty acid synthesis are described in Special Topic E WileyPLUS). Each of these biochemical reactions has a counterpart in synthetic reactions you have studied. Consider the biochemical reactions involved in adding each —CH2CH2— segment during fatty acid biosynthesis (those in Special Topic E that begin with acetyl-S-ACP and malonyl-S-ACP, and end with butyryl-S-ACP). Write laboratory synthetic reactions using reagents and conditions you have studied (not biosynthetic reactions) that would accomplish the same sequence of transformations (i.e., the condensation-decarboxylation, ketone reduction, dehydration, and alkene reduction steps). 

Crotonyl-S-ACP -I- NADPH 4- H+ rotonyl-ACP.rejg butyryl-S-ACP NADP+... 

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.)... 

Subsequent dehydration of /3-hydroxybutyryl ACP by an ElcB reaction in step 7 yields trans-ciotonyl ACP, and the carbon-carbon double bond of crotonyl ACP is reduced by NADPH in step 8 to yield butyryl ACP. The doublebond reduction occurs by conjugate addition of a hydride ion from NADPH to the /S carbon of fraus-crotonyl ACP. In vertebrates, the reduction occurs by an overall syn addition, but other organisms carry out similar chemistry with different stereochemistry. 

Step (4) Reduction of the Double Bond Finally the double bond of 1/rm.s-A2-butenoyl-ACP is reduced (saturated) to form butyryl-ACP by the action of enoyl-ACP reductase (ER) again, NADPH is the electron donor. 

In biological pathways, dehydrations rarely occur with isolated alcohols but instead normally take place on substrates in w luch the -OH is positioned two carbons away from a carbonyl group. In the biosynthesis of fats, for instance, /S-hydroxs butyryl ACP is converted by dehydration to fnz/rs-crotonyl AGP, w hcie AGP is an abbreviation for acyl carrier protein. Wcdl see the reason for this requirement in Section 11.10. 

In step 8 of fatty-acid biosynthesis (Figure 29.5), reduction of hoz/.s-crotonyl ACP gives butyryl ACP. A hydride from NADPH adds to C3 of the crotonyl grou 3 from the Re face, and protonation on C2 occurs on the Si face. Is the reduction a svn addit ion or an anti addition ... 

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... 

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