Single thioester

Activated esters (see Section 2.9) Activated esters of peptides are rarely used because there is no general method available for converting an (V -protected peptide into the ester with a guarantee that it will be a single isomer. Attempts have been made to overcome this obstacle (see Section 7.8). However, solid phase synthesis allows the preparation of thioesters of segments (see Section 7.10). Once the ester is in hand, it can be aminolyzed without generation of a second isomer if suitable conditions are employed. 

Figure 11.5 Reactions of the fatty acid synthase complex. A single multi-subunit enzyme is responsible for the conversion of acetyl-CoA to palmitate. The subunits in the enzyme are (i) acetyltransferase, (ii) malonyltransferase, (iii) oxoacyl synthase, (iv) oxoacyl reductase, (v) hydroxyacyl dehydratase, (vi) enoyl reductase. Finally, a separate enzyme, thioester hydrolase, hydrolyses palmitoyl-CoA to produce palmitate (vii).

Figure 8-2. Pathway for synthesis of palmitate by the fatty acid synthase (FAS) complex. Schematic representation of a single cycle adding two carbons to the growing acyl chain. Formation of the initial acetyl thioester with a cysteine residue of the enzyme preceded the first step shown. Acyl carrier protein (ACP) is a component of the FAS complex that carries the malonate covalently attached to a sulfhydryl group on its phosphopantatheine coenzyme (-SH in the scheme).

Photolysis of bis-thioester 60 (Equation 22) affords the enol ether 61, which undergoes equilibration to a single enantiomer upon hydrolysis <1995JA10252>. 

The construction of the maleic anhydride moiety 44 in only five steps starts with the conversion of the amide into an appropriate thioester. Upon treatment with DBU, an aldol-type cyclization occurs to provide the /1-hydroxy thiolactone as a single diastereomer. After removal of the allylic protecting group, dehydration and decarboxylation are carried out simultaneously by simple heating. The thiobutenolide is oxidized to the corresponding thio-... 

Chalcone and stilbene synthases are related plant PKSs [ 132]. Chalcones, such as naringenin chalcone, are produced as the biosynthetic precursors of flavinoids, while stilbenes are produced for their antifungal properties. Plant PKSs are likely to have evolved independendy from any of the aforementioned PKS and FAS systems [133, 134] and are atypical in many respects. These homodimeric enzymes consist of a single 40 kDa gene product (Fig. 2) [135]. The two active sites of the dimer function independently of one another [136]. Plant PKSs lack an AGP component, are not phosphopantetheinlyated, and act direcdy on CoA thioesters [134,137]. 

Figure 5 Chemoenzymatic approaches for the production of novel bioactive compounds. In this example, the enzymatic buildup of the linear precursor of daptomycin by its NRPSs (DptA, DptBC, and DptD) is substituted by solid-phase synthesis (a). By using the 4 Ppan transferase Sfp and the CoA-thioester of the linear peptide, the opo-enzyme PCP-TE and be modified, and after trans-esterification cyclized by the TE domain (b). Because the resulting ho/o-enzyme cannot be modified again, this is a single turnover reaction. Another strategy uses thiophenole-esters of the linear peptides to be cyclized (c). When these compounds are used, no PCP domain is necessary. The TE domain is readily acylated, and regiospecific and stereospecific cyclization toward daptomycin or, depending on the linear peptide provided, toward variants thereof occurs. Because the enzyme is not altered in any way after product release, this setup results in a multiple turnover.

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