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intracellular

Thermotoga neopolitana

Dong and Zeikus (110)

bacteria include B. methylotrophicum, Eubacterium limo-sum, Peptostreptococcus productus, C. thermoaceticum, C. ljungdahlii, and Methanosarcina barkeri. Synthesis gas represents a cheap feedstock for microbial conversion to higher-value commodity products. Recent work has focused at syngas fermentation to liquid-fuel additives. The feasibility of ethanol production from syngas fermentation by C. ljungdahlii (118) and ethanol plus butanol production by B. methylotrophicum (119,120) has been established. However, these processes need to be developed further for their commercialization.

Recently, anaerobes have also been examined for a wide range of specialty fermentation products including antimicrobials, bioflavors, biopigments, biopesticides, and anticancer agents. Antimicrobial compounds such as antibiotics and bacteriocins are produced by some of the anaerobic species. The most common examples are that of nisin and pediocin production by Lactococcus lactis and Pe-diococcus acidilactici, respectively (121). Some strains of lactobacilli catalyze the decarboxylation of glutamate to y-aminobutyrate (122).

Many chiral compounds can be synthesized by microbial hydrogenation using hydrogen (or formate) and hydroge-nase-containing microorganisms. Oxidoreductase enzymes are involved in electron-transfer reactions and can be applied in a stereoselective catalysis. The best known alcohol dehydrogenase is that present in both T. ethanolicus and other thermoanaerobes, the most notable being T. brockii. This NADP-linked secondary alcohol (aldehyde/ketone) de-hydrogenase, found in thermoanaerobes, exhibits a wide substrate specificity toward linear and cyclic secondary alcohols and thioesters (116,123,124). Table 4 summarizes novel oxidoreductases present in some clostridial species that are of commercial interest (125). The biotransformation reactions can be carried out either by whole-cell fermentation (which eliminates coenzyme regeneration but may be subject to interference by competing enzymes), or by crude or purified enzyme in batch or continuous-flow systems using an immobilized enzyme column. C. sporo-genes performs the stickland reaction, in which pairs of amino acids are fermented, one amino acid acting as an electron donor (e.g., valine, leucine, isoleucine), and the other acting as an electron acceptor (e.g., proline, glycine). Synthesis of pyruvate and other 2-oxacids from acyl phosphate derivatives have been shown with permeabilized cells of C. sporogenes (125). In amino acid-fermenting anaerobic bacteria, a set of unusual dehydratases is found that use 2-hydroxyacetyl-CoA, 4-hydroxybutyryl-CoA, or 5-hydroxyvaleryl-CoA as substrates. These anaerobic bacteria include C. propionicum, C. aminobutyricum, and C. aminovalericum (126).

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