Chapter 17 : Biosynthesis of Oligosaccharide, Isoprenoid, and C-P Antibiotic Classes

The three approved antibiotics shown represent aminoglycoside (kanamycin), pleuromutilin (retapamulin), and phosphonate (fosfomycin) families, respectively.

Glucose with its all-equatorial configuration of hydroxyl groups at carbons 2 to 6 is the most prominent hexose in cellular metabolism. (a) Glucose in aqueous solutions is a rapidly equilibrating mixture of α- and β-anomers, via the open-chain aldehyde form (not shown). (b) Once glucose has diffused into cells, it is trapped by phosphorylation at C6. (c) The phosphoryl group can be transferred between C6 and C1 by phosphoglucomutase action. The glucose-1-P is the α-isomer and is the starting point for conversion to NDP-sugars as sugar donors in metabolism.

NDP-sugars as glycosyl donors. (a) Conversion of glucose-1-P to UDP-glucose, a major glucosyl donor. (b) Transfer of glucosyl group from dTDP-glucose to the phenol-OH of PheGly4 in the vancomycin aglycone yields a monoglucosyl product. (c) Generation of a 1,3-β-disaccharyl linkage by attack of the C3-OH of a glucosyl moiety as nucleophile.

Three naturally occurring aminoglycoside antibiotics: streptomycin, an aminocyclitol derived from myoinositol; and kanamycin and tobramycin, with a core 2-deoxystreptose unit.

Biosynthesis of the trisaccharide framework in kanamycin: (a) glucose-6-P to the diamino sugar 2-deoxystreptose; (b) formation of the paromamine disaccharide; (c) formation of kanamycin B.

(a) Dehydroxylation of kanamycin generates tobramycin. (b) Conversion of paromamine to ribostamycin and neomycin.

Biosynthesis of streptomycin-6-P. Glucose-6-P is the source of streptidine-6-P; glucose-1-P is the source of dTDP-dihydrostreptose and CDP-N-Me-l-glucosamine. The three building blocks are joined to give dihydrostreptomycin-6-P. The alcohol on the central sugar is then dehydrogenated to the aldehyde in streptomycin-6-P, which is secreted by the producing S. griseus cells.

Export and activation of streptomycin-6-P. Enzymatic dephosphorylation to the active antibiotic occurs in the extracellular medium by a phosphatase also secreted by the producer (Sugiyama et al., 1981).

The pleuromutilins and abyssomicins are of isoprenoid origin.

(a) The Δ2- and Δ3-isoprenyl diphosphates are the biological isoprene reagents. (b) In head-to-tail chain elongation, the Δ2-isoprenyl-PP acts as the electrophile via an SN1-type dissociation to the allyl cation, while the Δ3 partner acts as the nucleophile through the π-electrons of its double bond. (c) The C10, C15, and C20 prenyl diphosphates known as geranyl, farnesyl, and geranylgeranyl diphosphates are then entryways to hundreds to thousands of natural isoprene scaffolds.

Geranyl diphosphate as a central substrate for many variants of monoterpene scaffolds via SN1 ionization and cyclization to the tertiary terpinyl cation.

So Many Natural Isoprenoids, So Few Antibiotics.

The C20 geranylgeranyl diphosphate is the entryway to the platencin and platensimycin class of antibiotics. (a) Cyclization and cation rearrangements via beyeran-16-yl cation generate ent-kaurene, the enantiomer of the plant terpene kaurene. (b) Oxidation of ent-kaurene and coupling to aminohydroxybenzoate yields platensimycin. (c) Rearrangement of the beyeran-16-yl cation instead produces ent-atiserene on the way to platencin.

Moenomycin, an isoprenyl-phosphoglyceryl pentasaccharide antibiotic. The terpenoid moenocinyl 25-carbon side chain is shown in red.

Biosynthetic pathway for moenomycin. (a) Reaction of farnesyl diphosphate with the 2-OH of 3-phosphoglycerate. (b) Three glycosyl transfers generate the farnesyl-glycerate-P-trisaccharide intermediate; then the isoprene chain is elongated and rearranged to the C25 moenocinyl before addition of the last two hexoses and the C5N unit.

Naturally occurring antibiotics with direct C-P bonds at both the phosphonate and phosphinate oxidation states. The phosphinate moiety is shown in red.

C-P-containing antibiotic biosynthesis. (a) Phosphonomutase catalyzes the first committed step, forming the C-P bond via rearrangement of PEP; the unfavorable equilibrium is managed by subsequent decarboxylation to phosphonoacetaldehyde. Rearrangement and the orientation of the enolpyruvate and metaphosphate ion are presumed to occur in the active site to go on to C-P bond formation. (b) Generation of the phosphonate antibiotic fosfomycin and (c) of the phosphonate antibiotic FR900098.

Biosynthesis of the antimetabolite phosphinothricin as the protected tripeptide.