Biosynthesis of corrins. II. Incorporation of 13C-labeled substrates in vitamins B12

Biosynthetic studies of novel polyketides from the marine sponge-derived fungusStachylidiumsp. 293K04

A methylation of the acetate starter unit is part of the polyketide biosynthesis of phthalides and phthalimidines from the marine-derived fungusStachylidiumsp. 293K04.

Lysoquinone-TH1, a New Polyphenolic Tridecaketide Produced by Expressing the Lysolipin Minimal PKS II in Streptomyces albus

The structural repertoire of bioactive naphthacene quinones is expanded by engineering Streptomyces albus to express the lysolipin minimal polyketide synthase II (PKS II) genes from Streptomyces tendae Tü 4042 (llpD-F) with the corresponding cyclase genes llpCI-CIII. Fermentation of the recombinant strain revealed the two new polyaromatic tridecaketides lysoquinone-TH1 (7, identified) and TH2 (8, postulated structure) as engineered congeners of the dodecaketide lysolipin (1). The chemical structure of 7, a benzo[a]naphthacene-8,13-dione, was elucidated by NMR and HR-MS and confirmed by feeding experiments with [1,2-¹³C₂]-labeled acetate. Lysoquinone-TH1 (7) is a pentangular polyphenol and one example of such rare extended polyaromatic systems of the benz[a]napthacene quinone type produced by the expression of a minimal PKS II in combination with cyclases in an artificial system. While the natural product lysolipin (1) has antimicrobial activity in nM-range, lysoquinone-TH1 (7) showed only minor potency as inhibitor of Gram-positive microorganisms. The bioactivity profiling of lysoquinone-TH1 (7) revealed inhibitory activity towards phosphodiesterase 4 (PDE4), an important target for the treatment in human health like asthma or chronic obstructive pulmonary disease (COPD). These results underline the availability of pentangular polyphenolic structural skeletons from biosynthetic engineering in the search of new chemical entities in drug discovery.

Biosynthesis of Rishirilide B

Rishirilide B was isolated from Streptomyces rishiriensis and Streptomyces bottropensis on the basis of its inhibitory activity towards alpha-2-macroglobulin. The biosynthesis of rishirilide B was investigated by feeding experiments with different ¹³C labelled precursors using the heterologous host Streptomyces albus J1074::cos4 containing a cosmid encoding of the gene cluster responsible for rishirilide B production. NMR spectroscopic analysis of labelled compounds demonstrate that the tricyclic backbone of rishirilide B is a polyketide synthesized from nine acetate units. One of the acetate units is decarboxylated to give a methyl group. The origin of the starter unit was determined to be isobutyrate.

The SMUL_1544 Gene Product Governs Norcobamide Biosynthesis in the Tetrachloroethene-Respiring Bacterium Sulfurospirillum multivorans

The tetrachloroethene (PCE)-respiring bacterium Sulfurospirillum multivorans produces a unique cobamide, namely, norpseudo-B12, which, in comparison to other cobamides, e.g., cobalamin and pseudo-B12, lacks the methyl group in the linker moiety of the nucleotide loop. In this study, the protein SMUL_1544 was shown to be responsible for the formation of the unusual linker moiety, which is most probably derived from ethanolamine-phosphate (EA-P) as the precursor. The product of the SMUL_1544 gene successfully complemented a Salmonella enterica ΔcobD mutant. The cobD gene encodes an l-threonine-O-3-phosphate (l-Thr-P) decarboxylase responsible for the synthesis of (R)-1-aminopropan-2-ol O-2-phosphate (AP-P), required specifically for cobamide biosynthesis. When SMUL_1544 was produced in the heterologous host lacking CobD, norpseudo-B12 was formed, which pointed toward the formation of EA-P rather than AP-P. Guided cobamide biosynthesis experiments with minimal medium supplemented with l-Thr-P supported cobamide biosynthesis in S. enterica producing SMUL_1544 or S. multivorans Under these conditions, both microorganisms synthesized pseudo-B12 This observation indicated a flexibility in the SMUL_1544 substrate spectrum. From the formation of catalytically active PCE reductive dehalogenase (PceA) in S. multivorans cells producing pseudo-B12, a compatibility of the respiratory enzyme with the cofactor was deduced. This result might indicate a structural flexibility of PceA in cobamide binding. Feeding of l-[3-(13)C]serine to cultures of S. multivorans resulted in isotope labeling of the norpseudo-B12 linker moiety, which strongly supports the hypothesis of EA-P formation from l-serine-O-phosphate (l-Ser-P) in this organism.

IMPORTANCE

The identification of the gene product SMUL_1544 as a putative l-Ser-P decarboxylase involved in norcobamide biosynthesis in S. multivorans adds a novel module to the assembly line of cobamides (complete corrinoids) in prokaryotes. Selected cobamide-containing enzymes (e.g., reductive dehalogenases) showed specificity for their cobamide cofactors. It has recently been proposed that the structure of the linker moiety of norpseudo-B12 and the mode of binding of the EA-P linker to the PceA enzyme reflect the high specificity of the enzyme for its cofactor. Data reported herein do not support this idea. In fact, norpseudo-B12 was functional in the cobamide-dependent methionine biosynthesis of S. enterica, raising questions about the role of norcobamides in nature.

Epoxomicin and Eponemycin Biosynthesis Involves gem-Dimethylation and an Acyl-CoA Dehydrogenase-Like Enzyme

The α',β'-epoxyketone moiety of proteasome inhibitors confers high binding specificity to the N-terminal threonine in catalytic proteasome β-subunits. We recently identified the epoxomicin and eponemycin biosynthetic gene clusters and have now conducted isotope-enriched precursor feeding studies and comprehensive gene deletion experiments to shed further light on their biosynthetic pathways. Leucine and two methyl groups from S-adenosylmethionine were readily incorporated into the epoxyketone warhead, suggesting decarboxylation of the thioester intermediate. Formation of the α',β'-epoxyketone is likely mediated by conserved acyl-CoA dehydrogenase-like enzymes, as indicated by complete loss of epoxomicin and eponemycin production in the respective knockout mutants. Our results clarify crucial questions in the formation of epoxyketone compounds and lay the foundation for in vitro biochemical studies on the biosynthesis of this pharmaceutically important class of proteasome inhibitors.

p-hydroxyacetophenone amides from Cystobacter ferrugineus, strain Cb G35

A family of six novel p-hydroxyacetophenone amides, 1-6, was isolated from Cystobacter ferrugineus, strain Cb G35. Their structures were elucidated by ESI-TOF mass spectrometry and NMR spectroscopy. Feeding experiments with labeled [¹³C₉,¹⁵N]-tyrosine and [d₁₀]-leucine identified the biosynthetic precursors of 1.

Evidence for the combined participation of a C10 and a C15 precursor in the biosynthesis of moenocinol, the lipid part of the moenomycin antibiotics

Upon feeding of [2-(13)C,4-(2)H]-1-deoxy-D-xylulose to Streptomyces ghanaensis, the deuterium label was retained exclusively at positions C-7 and C-17 in the moenocinol part of the moenomycin antibiotics. This result vindicates the hypothesis that the C(25) structure of moenocinol is assembled from a C(10) and a C(15) precursor, each of which requires for its formation the involvement of a dimethylallyl diphosphate starter unit.

Purification and characterization of S-adenosyl-L-methionine: uroporphyrinogen III methyltransferase from Pseudomonas denitrificans

S-Adenosyl-L-methionine:uroporphyrinogen III methyltransferase (SUMT), the enzyme of the cobalamin biosynthetic pathway which catalyzes C methylation of uroporphyrinogen III, was purified about 150-fold to homogeneity from extracts of a recombinant strain of Pseudomonas denitrificans derived from a cobalamin-overproducing strain by ammonium sulfate fractionation, anion-exchange chromatography, and hydroxyapatite chromatography. The purified protein has an isoelectric point of 6.4 and molecular weights of 56,500 as estimated by gel filtration and 30,000 as estimated by gel electrophoresis under denaturing conditions, suggesting that the active enzyme is a homodimer. It does not contain a chromophoric prosthetic group and does not seem to require metal ions or cofactors for activity. SUMT catalyzes the two successive C-2 and C-7 methylation reactions involved in the conversion of uroporphyrinogen III to precorrin-2 via the intermediate formation of precorrin-1. In vitro studies suggest that the intermediate monomethylated product (precorrin-1) is released from the protein and then added back to the enzyme for the second C-methylation reaction. The pH optimum was 7.7, the Km values for S-adenosyl-L-methionine and uroporphyrinogen III were 6.3 and 1.0 microM, respectively, and the turnover number was 38 h-1. The enzyme activity was shown to be completely insensitive to feedback inhibition by cobalamin and corrinoid intermediates tested at physiological concentration. At uroporphyrinogen III concentrations above 2 microM, SUMT exhibited a substrate inhibition phenomenon. It is suggested that this property might play a regulatory role in cobalamin biosynthesis in the cobalamin-overproducing strain studied.

5-Aminolevulinic acid formation from glutamate via the C5 pathway in Clostridium thermoaceticum

A cell-free extract of the anaerobic eubacterium, Clostridium thermoaceticum, catalyzes the synthesis of 5-aminolevulinic acid (ALA) from glutamate via the C5 pathway. The enzyme reaction resembles that of higher plants and algae in cofactor requirements and sensitivity to ribonuclease. From the phylogenetic distribution it is proposed that the C5 pathway evolved earlier than the ALA synthase pathway.

Carbon-13 nuclear magnetic resonance studies of cobalamins

The carbon-13 nuclear magnetic resonance spectra of aquocobalamin, adenosylcobalamin, methylcobalamin, and (carboxymethyl)cobalamin have been interpreted. The assignments were made by a comparison of the spectra with that of cyanocobalamin, by a study of the pH dependence of the chemical shifts, by an analysis of the effect of the axial ligands on the carbon atoms of the corrin ring, and by a study of the specific line broadening effect of the paramagnetic ions Mn2+ and Gd3+. The chemical shift changes that accompany the "base-on"----"base-off" conversion of the organocobalamins demonstrate that the conformation of the "western" half of the corrin ring and the conformations of the a, b, c, d, f, and g side chains are relatively constant. In contrast, the conformations of the "eastern" half of the corrin ring and the e propionamide side chain are highly variable.