DH, Dehydratase

Figure 11.2 Biosynthesis of the nine-membered enediynes. Members of this family share a common biosynthetic pathway for the enediyne core intermediate. Domains are shown in circles with abbreviations (KS, ketosynthase AT, acyltransferase KR, ketoreductase DH, dehydratase TE, thioesterase ACP, acyl carrier protein PPT, phosphopantetheine transferase)...

L, loading module DH, dehydratase KS, p-ketosynthase KR, ketoreductase MT methyltransferase PS, pyran synthase DHh and KRh are DH and KR-like sequences, together with the FkbH domain, they are involved in the formation of D-lactate starter unit HMG-CS, hydroxy-methyl-glutaryl CoA synthase. Acyl-carrier-protein domains are shown as small filled balls with chain attached by the thiol group. The box shows the HMG-CS pathway for the formation of exocyclic enoate. 

ACP acyl carrier protein DH dehydratase ER enoylreductase KR P-ketoacylreductase KS P-ketoacylsynthase MAT malonyl/acetyltransferase TE thioesterase... 

ACP acyl carrier protein AT acyltransferase DH dehydratase ER enoyl reductase KR p-ketoacyl reductase KS p-ketoacyl synthase TE thioesterase... 

Figure 1 Polyketide biosynthesis. Polyketide backbones are formed via condensations from acyl-CoA thioesters of carboxylic acids. The (3-ketone which results from each condensation can undergo a series of reductive steps analogous to fatty acid biosynthesis. However, either none or only some of the reductive activities may occur in a given cycle. This allows PKSs to generate diversity through selection of priming and extender units, variation of the reductive cycle, and stereoselectivity. (ACP, acyl carrier protein AT, acyl transferase KS, ketosynthase DH, dehydratase ER, enoylreductase KR, ketoreductase TE, thioesterase.) The structure depicted in the lower right-hand corner is representative of the possible structural variations that can arise during polyketide biosynthesis.

Figure 3 The fatty acid biosynthetic cycle (ACP, acyl carrier protein KS, P-ketoacyl synthase KR, P-ketoacyl reductase DH, dehydratase ER, enoyl reductase TE, thioes-terase).

Figure 5 Domain organization of the erythromycin polyketide synthase. Putative domains are represented as circles and the structural residues are ignored. Each module incorporates the essential KS, AT, and ACP domains, while all but one include optional reductive activities. AT, acyltransferase ACP, acyl carrier protein KS, (3-ketoacyl synthase KR, P-ketoacyl reductase DH, dehydratase ER, enoyl reductase TE, thioesterase.

AT = acyl transferase DH = dehydratase ER = enoyl reductase KR = ketoreductase KS = ketosynthase mAT = methylmalonyl-specific acyl transferase. 

Figure 10.2 The PKS/NRPS biosynthetic paradigm, showing the most common domains and their relative positions within a modular PKS/NRPS enzyme. A = adenylation AT = acyl transferase C = condensation DH = dehydratase Ep = epimerase ER = enoyl reductase KR = ketoreductase KS = ketosynthase MT = methyltransferase PCP = peptidyl carrier protein TE = thioesterase.

Figure 6 HMGS containing biosynthetic pathways. Portions of the PKS and PKS/NRPS pathways where the HMGS and related enzymes are located. Abbreviations A - Adenylation, AGP - acyl carrier protein, AT - acyltransferase, Cy - cyciization, DH - dehydratase, ER - enoyl reductase, GNAT -CCN5-related N-acetyltransferase, KS - ketosynthase, KR - ketoreductase, MT - methyltransferase. Ox - Oxidase, Oxy - Oxygenase, PGP - peptide carrier protein, PhyH - phytanoyl-CoA dioxygenase, PS - pyrone synthase, TE - thioesterase, - unknown function, - inactive domain.

DH Dehydratase, found in fatty acid syntheases and polyketide syntheases, dehydrates the 3-OH of acyl thioester... 

Figure 5 Fatty acid biosynthesis catalyzed by fatty acid synthases. The growing acyl chain is tethered to the phosphopantetheinylated ACP domain, which enabies it to undergo cycles of condensation, ketone reduction, dehydration, and enol reduction catalyzed by different domains. AT, acyltransferase ACP, acyi-carrier protein KS, ketosynthase KR, ketoreductase DH, dehydratase ER, enoyireductase.

KS = /3-Ketoacyl synthase MT = malonyl transacylase AT = acetyl transacylase DH = dehydratase ER = enoyl reductase KR = /3-ketoacyl reductase ACP = acyl carrier site TE = thioesterase. [Reproduced with permission from S. J. Wakil, J. K. Stoops, and V. C. Joshi, Fatty acid synthesis and its regulation. Annu. Rev. Biochem. 52, 537 (1983). 1983 by Annual Reviews Inc.]... 

ACP = acyl carrier protein KS = p-ketoacyl synthase KR = p-ketoacyl reductase ER = enoyl reductase DH = dehydratase TE = thioesterase... 

Figure 3. Relationship between polyketide and fatty acid biosynthesis. The simplest ( minimaV) PKSs possess ketosynthase activity and produce linear polyketide products. In contrast, FASs also catalyze successive ketoreduction-dehydration-enoyl reduction reactions following each condensation. Diverse PKSs may perform none, part, or all of this reductive sequence. KS, ketosynthase KR, ketoreductase DH, dehydratase ER, enoyl reductase.

Figure 1. (A) Domain organization of the NcsB naphthoic acid synthase and (B) proposed pathway for biosynthesis of the naphthoic acid intermediate (2) from the acyl CoA substrates by NcsB and its subsequent conversion to 3 by NcsB3 and NcsB 1 and incorporation into neocarzinostatin (I) by NcsB2. ACP, acyl carrier protein AT, acyltransferase DH, dehydratase KR, ketoreductase ...

ACP, acyl carrier protein AT, acyltransferase DH, dehydratase KR, ketoreductase KS, ketoacyl synthase TD, terminal domain that most likely encodes a phosphopantetheinyl transferase. 

A) DAHP synthase (B) DHQ synthase (C) DHQ dehydratase (D) shikimate dehydrogenase (E) quinate dehydrogenase (F) DHS dehydratase (G) protocatechuate decarboxylase (H) catechol 1,2-dioxygenase. 

Fig. 5. Predicted domain organization and biosynthetic intermediates of the erythromycin synthase. Each circle represents an enzymatic domain as follows ACP, acyl carrier protein AT, acyl-transferase DH, dehydratase ER, P-ketoacyl-ACP enoyl reductase KR, [3-ketoacyl-ACP reductase KS, p-ketoacyl-ACP synthase TE, thioesterase. Zero indicates dysfunctional domain.

KR - ketoreductase, ER - enoyl reductase, DH - dehydratase, ACP - acyl carrier protein, TE - thioesterase. 

FATTY ACID CHAIN EXTENSION CYCLE KS Ketoacyl Synthase ACP Acyl Carrier Protein KR = Ketoreductase DH = Dehydratase ER = Enoylreductase AT = Acyhransferase TE = Thloesterase... 

Catalyzed conversion of D-glucose into c/s, c/s-muconic acid (27) required creation of a biosynthetic pathway not known to exist naturally (Figure 5). This pathway relied on DHS dehydratase (Figure 5, enzyme A) (44,45) to couple aromatic biosynthesis to... 

Figure 5. The biocatalytic pathway (boxed arrows) created for microbial conversion of D-glucose into cis, cw-muconate from the perspective of the biochemical pathways from which the enzymes were recruited. Conversion of D-glucose into DHS requires transketolase (tkt) from the pentose phosphate pathway and DAHP synthase (aroF, aroG, aroH)y DHQ synthase aroB and DHQ dehydratase aroD) from the common pathway of aromatic amino acid biosynthesis. Conversion of DHS into catechol requires DHS dehydratase (aroZ, enzyme A) from hydroaromatic catabolism, protocatechuate decarboxylase aroY, enzyme B), and catechol 1,2-dioxygenase (caM, enzyme C) from the benzoate branch of the p-ketoadipate pathway. (Adapted and reproduced with permission from ref. 21.)...

Figure 1, DHS as a chemical building block. Boxed structures are of current industrial value. Catalysts (a) DHS dehydratase (aroZ) (b) reflux (c) catechol-O-methyltransferase (comt) (d) aryl-aldef de dehydrogenase (e) PCA decarboxylase (aroY) (f) catechol 1,2-dioxygenase (catA) (g) Pt/C, H2 (h) p-hydroxybenzoate hydroxylase (pobA ) (i)Cu(OAc) 2, HOAc.

Comparison of the two strategies for PCA synthesis from DHS depends upon the ultimate goal of die process. If synthesis of PCA as a final product is the goal, the two-step conversion that relies on thermal dehydration of DHS is superior. However, if PCA is needed only as an intermediate which will undergo further catalysis, biocatalytic synthesis using DHS dehydratase is superior, since the need for separate, sequential biocatalytic processes can be avoided. 

Vanillin is the second largest food additive used each year. As a consequence of limited natural sources of this ubiquitous flavor and fragrance, the bulk of commercial vanillin is synthesized from phenol (19). A two-step synthesis of vanillin from glucose via DHS intermediacy was recently reported (Figure 3) (17). DHS dehydratase-catalyzed conversion of DHS into PCA is followed by formation of the methyl ether, catalyzed by catechol-O-methyltransferase, an enzyme not native to E. coli but for which the gene (comt) has been cloned from rat liver (20). In a separate step, incubation of the resulting vanillic acid with aryl-aldehyde dehydrogenase (21) affords vanillin. 

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