6-Deoxyerythronolide biosynthesis

Kato, Y., Bai, L., Xue, Q. et al. (2002) Functional expression of genes involved in the biosynthesis of the novel polyketide chain extension unit, methoxymalonyl-acyl carrier protein, and engineered biosynthesis of 2-desmethyl-2-methoxy-6-deoxyerythronolide B. Journal of the American Chemical Society, 124, 5268. 

Ward, S.L., Desai, R.P., Hu, Z. et al. (2007) Precursor-directed biosynthesis of 6-deoxyerythronolide B analogues is improved by removal of the initial catalytic sites of the polyketide synthase. Journal of Industrial Microbiology and Biotechnology, 34, 9-15. 

Deoxyerythronolide B synthase (DEBS) is a modular Type I PKS involved in erythromycin biosynthesis (see page 96) and its structure and function are illustrated in Figure 3.85. The enzyme contains three subunits (DEBS-1, 2, and 3), each encoded by a gene (eryA-l, II, and III). It has a linear organization of six modules, each of which... 

D Hunziker, N Wu, K Kinoshita, DE Cane, C Khosla. Precursor directed biosynthesis of novel 6-deoxyerythronolide B analogs containing non-natural oxygen substituents and reactive functionalities. Tetrahedron Lett 40 635-638, 1999. 

The biosynthesis of erythromycin can be divided into two phases (Scheme 1). In the first constructive phase of the pathway a set of key enzymes, collectively known as the polyketide synthase (PKS), assembles the typical polyketide chain by sequential condensation of one unit of propionyl-CoA and six units of methylmalonyl-CoA 6. The initially formed chain is cyclised to give the first macrocychc lactone (macrolide) intermediate 6-deoxyerythronolide B 7 [6,7]. In the second phase 6-deoxyerythronohde B is elaborated by a series of tailoring enzymes which carry out regiospecific hydroxylations, glycosylations and a methylation (of an added sugar residue) to give finally erythromycin A. The core polyketide structure is generated by the PKS in phase one, but the later steps of phase two are essential to produce active antibiotics. 

Chemical degradation of erythromycin A yielded its aglycone, erythronolide A (16, R = R = OH), whereas erythronolide B (16, R = H, R = OH) was obtained from fermentation (63,64). Biosynthesis of erythromycin proceeds via 6-deoxyerythronolide B (16, R = R/ = H) and then erythronolide B (64,65). The first total synthesis of erythromycin-related compounds was erythronolide B (66) syntheses of erythronolide A and 6-deoxyerythronolide B soon followed (67,68). 

Shafiee, A. Hutchinson, C. R. Macrolide antibiotic biosynthesis isolation and properties of two forms of 6-deoxyerythronolide B hydroxylase from Saccharopolyspora erythraea (Streptomyces erythreus). Biochemistry 26, 6204-6210 (1987). 

Jacobsen, J.R., Cane, D.E. Khosla, C. Spontaneous priming of a downstream module in 6-deoxyerythronolide B synthase leads to poly-ketide biosynthesis. Biochemistry 37, 4928-4934 (1998). 

Manipulation of the DEBS system has led to the most impressive demonstration of combinatorial biosynthesis to date. McDaniel and coworkers have utilized specific module-swapping strategies to access a variety of 6-deoxyerythronolide B analogs with modifications at each carbon of the macrolide backbone [26]. Modules 1-6 of DEBS were systematically replaced with individual rapamycin synthase components to alter oxidation state and methylation in the final polyketide product. The study produced 60 unique structures at yields ranging from 1 to 70% of that of 6-deoxyerythronolide B (Fig. 9.2-5). However, each new compound required independent synthase engineering, which made library construction quite tedious. 

Fig. 18. Precursor-directed biosynthesis of novel polyketides containing synthetic building blocks [145]. NAC=A-acetylcystearmne, DEBS=deoxyerythronolide B synthase.

The first enzyme-free intermediate in the biosynthesis of erythromycin is the polyketide 6-deoxyerythronolide B. All of the carbons come from either acetate or propanoate. 

Biosynthesis of erythromycin represents a typical polyketide pathway and structural organization of PKSs. Erythromycin PKS (6-deoxyerythronolide B synthase, DEBS) uses propionyl-CoA as a priming unit and six methylmalonyl-CoA molecules as extender units. Each module in a modular PKS contains domains for one round of chain elongation (KS, AT, ACP) and p-keto modification (ketoreductase, KR dehy-drase, DH enoylreductase, ER). DEBS consists of six modules (mod 1 -6) encoded on three separate polypeptide subunits. 

Andersen JF, Tatsuta K, Gunji H, Ishiyama T, Hutchinson CR (1993) Substrate specificity of 6-deoxyerythronolide B hydroxylase, a bacterial cytochrome P450 of erythromycin a biosynthesis. Biochemistry 32 1905-1913... 

CYP107A1 (P450 EiyF) Saccharopofyspora erythraea E.g., lOXA, 1Z80, lEGY, lEUP E.g., 6-deoxyerythronolide B, androstenedione, 9-aminophenan-threne, ketoconazole 6-Deoxyerythronolide B hydroxylase in erythromycin biosynthesis E.g., [34, 36, 773]... 

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