Tetrahydroxynaphthalene, structural

Streptomycetes also produce a variety of fused ring aromatic polyketides, the carbon skeletons of some of which closely parallel those of their fimgal counterparts. However, with very few exceptions, such as the melanin spore pigment precursor 1,3,6,8-tetrahydroxynaphthalene (10, Scheme 5) of Alternaria alternate and Streptomyces griseus, fimgi and streptomycetes have not been found to produce identical fiised ring structures. 

The strucmral and kinetic data on chalcone synthase and related PKS Ills have been used to develop a model that explains how these enzymes control loading, condensation, and cyclization reactions in one active site. The attention of readers is drawn to an excellent review by Austin and Noel in which the structure and mechanism of plant and bacterial PKS III homologs have been compared and contrasted. Several noteworthy points from their analyses include a discussion on the bacterial PKS III enzymes that contain multiple cysteine residues in their active sites, such as tetrahydroxynaphthalene synthase and DpgA, a PKS III involved in dihydroxyphenylglycine synthesis, and the possibility that some bacterial PKS III enzymes may use AGP as the substrate carrier. 

Table 20.2 Structures of inhibitors of 1,3,6,8-tetrahydroxynaphthalene reductase in DHN melanin biosynthesis.

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