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Author(s): Changjiang Dong [1]; Fanglu Huang [2]; Hai Deng [1]; Christoph Schaffrath [1]; Jonathan B. Spencer [2]; David O'Hagan [1]; James H. Naismith (corresponding author) [1] Fluorine is the thirteenth [...]

The multifunctional enzyme complex from Penicillium patulum called 6-methylsalicylic acid synthase, that catalyzes the biosynthesis of the tetraketide 6-methylsalicylic acid, was altered by thiol-specific inhibitors and cross-linking reagents. Treatment with 1,3-dibromopropan-2-one caused rapid enzyme inactivation and formation of cross-linked dimers. Two types of cross-linked dimers were formed. Within each dimer, the beta-ketoacyl synthase and acyl carrier protein components are juxtaposed to enable the respective cysteine and 4'-phosphopantetheine thiols to interact during condensation.

Absolute stereochemistries of precorrin-3x, a novel hydroxy-gamma-lactone intermediate formed during vitamin B12 biosynthesis in the aerobe Pseudomonas denitrificans, have been determined. In contrast to the anaerobic pathway in Propionibacterium shermanii, no oxygen exchange occurs in P. denitrificans between precorrin-3x and the peripheral carboxylic acid at the C-20 center. Nuclear magnetic resonance spectroscopy showed that the C-20 hydroxyl is cis to the oxygen terminus of the gamma-lactone at C-1, resulting in ring contraction and subsequent loss of acetic acid.

6-methylsalicylic acid was synthesized from (R)-(1-13C;2-2H)- and (S)-(1-13C;2-2H)malonates using succinyl-CoA transferase to study the stereochemical course of hydrogen elimination during catalysis by 6-methylsalicylic acid synthase. The results indicate that the hydrogen atom at the 5-position of 6-methylsalicylic acid is derived from the H(sub Re) of malonyl-CoA or H(sub Si) in the polyketide intermediate. Additionally, the enzyme catalyzes stereospecific exchange of hydrogen atoms in the polyketide intermediates.

Research has been conducted on the cis-arylalkenes. The isomerization of these arylalkenes to their trans isomers has been carried out via the use of palladium(II) catalyst and the results are reported.

The biosynthesis of the vancomycin group of antibiotics is described.

The amphipolar characteristics of the palladium hydrogen bond in both homogeneous and heterogeneous hydrogenation of alkenes have been studied. The primary factors regulating the removal sequence of various benzyl-protecting groups were found to be the electronic characteristics of the aromatic ring and its affinity to the palladium surface. This had resulted into the successful development of the 2-naphthylmethyl protecting group, which is more labile to catalytic hydrogenolysis than the benzoyl group.

A novel method for investigating the hydrometalation step in homogeneous hydrogenation that has determined how the metal hydrogen addition occurs in organometallic chemistry is presented. The regioselectivity of hydrometalation on the metal surface was determined. Results indicate that the hydrometalation on the metal surface resembles the one established in organometallic chemistry that takes place by two possible electronic modes.

A study was conducted on the influence of substrate polarity on the mechanism of catalytic hydrogenation with homogeneous palladium and rhodium catalysts. The findings suggest that this mechanism is a two electron process that can be strongly influenced by the Coulombic nature of the substrate. The methodology readily determines electronic properties, which makes it useful for the rational design of new catalysts.

The stereoselective deoxygenation of myo-inositol monotosylates using lithium triethylborohydride (LTBH) provides a convenient entry into a series of compounds that are inaccessible by conventional methods. The deoxygenation reaction produces a ketone intermediate. LTBH deoxygenates primary alcohols by the reduction of their p-toluenesulfonyl esters. The observed stereoselectivity of the products depends on the reduction of free ketone.

The origin and fate of oxygen atoms in precorrin-3 are analyzed to study the ring contraction process in vitamin B12 biosynthesis. Ring contraction proceeds through two different mechanisms in the two pathways to B12. The anaerobic pathway loses 18(O) by gamma-lactone hydrolysis while the aerobic pathway retains the carboxyl oxygen up to precorrin-5.

Precorrin-3 is converted to precorrin-3x by the enzyme CobG from Pseudomonas denitrificans. Precorrin-3x forms the substrate of enzyme CobJ, which performs both S-adenosylmethionine and ring contraction. Catalytic turnover of percorrin-3 and precorrin-3x is based on the presence of CobG and O2. The participation of molecular oxygen in P. denitrificans is a unique feature, since vitamin B12 is biosynthesized anaerobically. Anaerobas use redox chemistry of cobalt to facilitate oxidative ring contraction.

Investigations of the functions of biosynthetic genes of the vitamin B12 pathway in aerobic bacterium Pseudomonas denitrificans reveal the structure and synthesize genetically engineered pentamethylated intermediate of vitamin B12 biosynthesis, precorrin-5. This has enhanced available information on the stages between precorrin-3 and precorrin-6x and the intermediates between (4-C)-5-aminolevulinic acid and hydrogenobyrinic acid.