Double enzymatic formation

The latter material has been used in the synthesis of asymmetrically labelled L-homoserine in a study on the mechanism of the enzymatic formation of L-threonine (9). However, we soon realized that under the experimental conditions used, the reduction of the carbonyl carbon and the saturation of the double bond are only two of the synthetic manifestations that are possible using an a-B-unsaturated aldehyde (10). Since then, we have been exploring this area and it now appears that a -unsaturated aldehydes can be reduced by baker s yeast to yield the synthetically useful chiral products indicated in Scheme 1 in a manner that depends upon the fermentation conditions and the nature of the a and Ysubstituents. 

In keeping with its biogenetic origin m three molecules of acetic acid mevalonic acid has six carbon atoms The conversion of mevalonate to isopentenyl pyrophosphate involves loss of the extra carbon as carbon dioxide First the alcohol hydroxyl groups of mevalonate are converted to phosphate ester functions—they are enzymatically phosphorylated with introduction of a simple phosphate at the tertiary site and a pyrophosphate at the primary site Decarboxylation m concert with loss of the terti ary phosphate introduces a carbon-carbon double bond and gives isopentenyl pyrophos phate the fundamental building block for formation of isoprenoid natural products... 

This concept of reversible chemical crosslinking of the chains to drastically decrease the enthalpic penalty of the folding process and to exploit the highly favored disulfide loop formation in the A-chain with m = 4, has been further developed into artificial peptide linker chains that can be excised selectively by enzymatic processing, to allow for bioexpression of the artificial proinsulins on an industrial scale. 95,96 However, to apply this approach rationally to other double-stranded cystine peptides, knowledge about their three-dimensional structure is essential. 

Mitomycin C, 1, is a potent antitumor antibiotic discovered by Japanese scientists in fermentation cultures of Streptomyces caespitosus. It has been described as "small, fast and deadly (but very selective)" and has an extraordinary ability to crosslink the complementary strands of the DNA double helix with high efficiency and absolute specificity. It is so lethal that one crosslink per genome is sufficient to cause death of a bacterial cell. Mitomycin C, which is widely used clinically as an antitumor drug, does not react with DNA, but enzymatic reduction of the quinone induces a cascade of transformations which results, ultimately, in formation of the DNA crosslink 2. 

Enzymatic enantioselectivity in organic solvents can be markedly enhanced by temporarily enlarging the substrate via salt formation (Ke, 1999). In addition to its size, the stereochemistry of the counterion can greatly affect the enantioselectivity enhancement (Shin, 2000). In the Pseudomonas cepacia lipase-catalyzed propanolysis of phenylalanine methyl ester (Phe-OMe) in anhydrous acetonitrile, the E value of 5.8 doubled when the Phe-OMe/(S)-mandelate salt was used as a substrate instead of the free ester, and rose sevenfold with (K)-maridelic acid as a Briansted-Lewis acid. Similar effects were observed with other bulky, but not with petite, counterions. The greatest enhancement was afforded by 10-camphorsulfonic acid the E value increased to 18 2 for a salt with its K-enanliomer and jumped to 53 4 for the S. These effects, also observed in other solvents, were explained by means of structure-based molecular modeling of the lipase-bound transition states of the substrate enantiomers and their diastereomeric salts. 

The enzymatic reduction of a double bond is a key step in the formation of a fatty acid that is ultimately incorporated into the cell wall of the bacterium that causes tuberculosis. The antituberculosis drug isoniazid blocks this enzyme, preventing reduction of the double bond. Without an intact cell wall, the bacteria die. 

H) Dihydrozeatin formation The double bond of the tZ side chain could be enzymatically reduced by zeatin reductase, an enzyme characterized from immature seeds of P. vulgaris, resulting in DZ.429 Zeatin reductase uses tZ as a substrate, but not cZ, tZR, iP, or tZ O-glucoside, and requires NADPH as a cofactor (Figures 15 and 18).429 The responsible genes for this reaction have not yet been identified. 

The resulting dynamic aminonitrile systems were first subjected to lipase mediated resolution processes at room temperature. A-Methy] acetamide was observed as a major product from the lipase amidation resolution. In this case, free methylamine A was generated during the dynamic transimination process and transformed by the lipase. To avoid this by-reaction, the enzymatic reaction was performed at 0 °C, and the formation of this amide was thus detected at less than 5% conversion. To circumvent potential coordination, and inhibition of the enzyme by free Zn(II) in solution [54], solid-state zinc bromide was employed as a heterogeneous catalyst for the double dynamic system at 0 °C. The lipase-catalyzed amidation resolution could thus be used successfully to evaluate /V-substituted a-aminonitrile substrates from double dynamic systems in one-pot reactions as shown in Fig. 7d. Proposedly, the heterogeneous catalyst interfered considerably less or not at all in the chemo-enzymatic reaction because the two processes are separated from each other. Moreover, the rate of the by-reaction was reduced due to strong chelation between the amine and zinc bromide in the heterogeneous system. 

---