Mycolactones are polyketide-derived lipid virulence factors made by the slow-growing human pathogen, Mycobacterium ulcerans. Three unusually large and homologous plasmid-borne genes (mlsA1: 51 kb, mlsB: 42 kb and mlsA2: 7 kb) encode the mycolactone type I polyketide synthases (PKS). The extreme size and low sequence diversity of these genes has posed significant barriers for exploration of the genetic and biochemical basis of mycolactone synthesis. Here, we have developed a truncated, more tractable 3-module version of the 18-module mycolactone PKS and we show that this engineered PKS functions as expected in the natural host M. ulcerans to produce an additional polyketide; a triketide lactone (TKL). Cell fractionation experiments indicated that this 3-module PKS and the putative accessory enzymes encoded by mup045 and mup038 associated with the mycobacterial cell wall, a finding supported by confocal microscopy. We then assessed the capacity of the faster growing, Mycobacterium marinum to harbor and express the 3-module Mls PKS and accessory enzymes encoded by mup045 and mup038. RT-PCR, immunoblotting, and cell fractionation experiments confirmed that the truncated Mls PKS multienzymes were expressed and also partitioned with the cell wall material in M. marinum. However, this heterologous host failed to produce TKL. The systematic deconstruction of the mycolactone PKS presented here suggests that the Mls multienzymes are necessary but not sufficient for mycolactone synthesis and that synthesis is likely to occur (at least in part) within the mycobacterial cell wall. This research is also the first proof-of-principle demonstration of the potential of this enzyme complex to produce tailored small molecules through genetically engineered rearrangements of the Mls modules.

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8) was prepared in multistep syntheses, starting from the enantiopure silicon-containing α-methylated α-amino acids (R)-or (S)-α-[(trimethylsilyl)methyl]alanine ((R)-4 or (S)-4). For the preparation of (R)-4 and (S)-4, a robust synthesis on a multigram scale was developed, using an enzymecatalyzed stereoselective ester hydrolysis as the key step. Coupling of (R)-4 and (S)-4 with different S-configured aspartate residues yielded a series of dipeptides that can be best described as silicon analogues of the artificial sweeteners aspartame and neotame. The identity of these dipeptides was established by elemental analyses and NMR spectroscopic studies ( 1 H, 13 C, 15 N, 29 Si). Some of the title compounds and some of their precursors were structurally characterized by single-crystal X-ray diffraction. The silicon-containing dipeptides were shown to display attractive ADME properties, including good solubility and plasma protein binding capabilities, no CYP inhibition, and no metabolic stability liabilities. Thus, the silicon-containing α-amino acids (R)-4 and (S)-4 proved to be promising building blocks for the design of biologically active peptides. Attempts to characterize the title compounds for their T1R2/R3 activating properties by testing their ability to induce GLP-1 secretion from the intestinal endocrine cell line STC-1 failed for unknown reasons. Sensory evaluation of the silicon-containing dipeptides demonstrated the aspartame analogue (S,R)-5 and the neotame analogue (S,R)-8 to be very potent artificial sweeteners, whereas the corresponding diastereomers tasted bitter. The silicon compounds (S,R)-5 and (S,R)-8 are approximately 50 and 600 times, respectively, as sweet as sucrose on a weight basis.

The nonproteinogenic silicon-containing, R,R-disubstituted R-amino acids (R)-and (S)-2-amino-2-methyl-3-(trimethylsilyl)propanoic acid [(R)-and (S)-R-[(trimethylsilyl)methyl]alanine, (R)-1 and (S)-1] were synthesized by multistep syntheses, starting from diethyl malonate, and both were isolated with an enantiomeric purity of >99% ee. Compound rac-1 was also prepared. The key step of the asymmetric syntheses of (R)-1 and (S)-1 was an enantioselective enzyme-catalyzed (porcine liver esterase) ester cleavage of the prochiral substrate diethyl methyl[(trimethylsilyl)methyl]malonate. The absolute configurations of (R)-1 and (S)-1 were established by crystal structure analyses of (R)-1 and of its rhodium complex (S Rh ,R C )-16.