Microbial conversions

As will become apparent in what follows, some oxygenates are manufactured by established, commercial, microbial processes using biomass feedstocks. Others can be manufactured by microbial conversion of biomass, but are currently produced using thermochemical conversion methods, usually with fossil feedstocks because of economic or technical factors. A few other oxygenates must be manufactured by thermochemical conversion of fossil feedstocks because suitable microbial processes do not yet exist to produce the oxygenate from biomass. Still others can be produced by a combination of thermochemical and microbial conversion. Microbial conversion systems with biomass feedstocks are emphasized here, but thermochemical methods are briefly reviewed to present a perspective on the options available and what advancements are necessary to perfect suitable processes. 

Enzymatic Method. L-Amino acids can be produced by the enzymatic hydrolysis of chemically synthesized DL-amino acids or derivatives such as esters, hydantoins, carbamates, amides, and acylates (24). The enzyme which hydrolyzes the L-isomer specifically has been found in microbial sources. The resulting L-amino acid is isolated through routine chemical or physical processes. The D-isomer which remains unchanged is racemized chemically or enzymatically and the process is recycled. Conversely, enzymes which act specifically on D-isomers have been found. Thus various D-amino acids have been... 

In another process, diosgenin is degraded to 16-dehydropregnenolone by chemical methods. Conversion of 16-dehydropregnenolone to 11-deoxycortisol (125) can be accompHshed in 11 chemical steps. These steps result in hydroxylations at C21 and C17, oxidation at C3, and to double-bond isomeri2ation (175). Microbial oxidation of (125) also produces cortisol (29). 

Fermentation. The term fermentation arose from the misconception that black tea production is a microbial process (73). The conversion of green leaf to black tea was recognized as an oxidative process initiated by tea—enzyme catalysis circa 1901 (74). The process, which starts at the onset of maceration, is allowed to continue under ambient conditions. Leaf temperature is maintained at less than 25—30°C as lower (15—25°C) temperatures improve flavor (75). Temperature control and air diffusion are faciUtated by distributing macerated leaf in layers 5—8 cm deep on the factory floor, but more often on racked trays in a fermentation room maintained at a high rh and at the lowest feasible temperature. Depending on the nature of the leaf, the maceration techniques, the ambient temperature, and the style of tea desired, the fermentation time can vary from 45 min to 3 h. More highly controlled systems depend on the timed conveyance of macerated leaf on mesh belts for forced-air circulation. If the system is enclosed, humidity and temperature control are improved (76). 

These organisms have been used frequently in the elucidation of the biosynthetic pathway (37,38). The mechanism of riboflavin biosynthesis has formally been deduced from data derived from several experiments involving a variety of organisms (Fig. 5). Included are conversion of a purine such as guanosine triphosphate (GTP) to 6,7-dimethyl-8-D-ribityUuma2ine (16) (39), and the conversion of (16) to (1). This concept of the biochemical formation of riboflavin was verified in vitro under nonen2ymatic conditions (40) (see Microbial transformations). 

In a biotechnology-based approach, Japanese workers have reported on the microbial conversion of 2-methylnaphthalene to both 2-methyl-1-naphthol and menadione by Jiodococcus (64). The intermediate 2-methyl-1-naphthol can readily be converted to menadione by a variety of oxidizing agents such as heteropoly acids (65) and copper chloride (66). A review of reagents for oxidizing 2-methylnaphthalene and naphthol is available (67). 

Increasingly, biochemical transformations are used to modify renewable resources into useful materials (see Microbial transformations). Fermentation (qv) to ethanol is the oldest of such conversions. Another example is the ceU-free enzyme catalyzed isomerization of glucose to fmctose for use as sweeteners (qv). The enzymatic hydrolysis of cellulose is a biochemical competitor for the acid catalyzed reaction. 

Malted barley contains a- and P-amylases along with proteases and phytases. Most standardi2ed microbial en2yme preparations for industrial starch conversion contain approximately 100 times more amylase activity than malt. In beermaking, malt is not just valuable for its en2ymes but also for flavor compounds. 

Acarbose and Miglitol These agents are specific inhibitors of intestinal glucosidases and reduce the conversion of sucrose and starch to glucose. Their main effect is a delay, not a complete inhibition, of the absorption of carbohydrates. Postprandial blood sugar excursions are effectively reduced. Because a small portion of the carbohydrates enters the colon, their microbial degradation frequently causes flatulence or... 

Izuka H, Naito A (1981) Microbial conversion of steroids and alkaloids. University of Tokyo Press/Springer, Berlin... 

Lonza, for example, has commercialized processes for highly chemo- and regioselective microbial ring hydroxylation and side-chain oxidation of heteroaromatics (see Fig. 2.32 for examples) (Kiener, 1995, 1999). The pharmaceutical intermediate 5-methylpyrazine-2-carboxylic acid, for example, is manufactured by microbial oxidation of 2,5-dimethylpyrazine. Many conversions of the type shown in Fig. 2.32 would not be possible by conventional chemical means. 

Recently, exchange of metals between siderophores and phytosiderophores has been proposed as a primary mechanism for plant use of microbial siderophores (20,21). Conversely, it has also been shown that microbial siderophores... 

With the extraction procedure we employed (22), ferulic acid was isolated as the most inhibitory component in wheat straw. There could also be other unknown compounds in the straw which would not be evident with this procedure. In addition, we ignored the possible influence of toxin-producing microorganisms. Microorganisms may have influenced the phytotoxicity exhibited by the aqueous wheat extract in Table IX. Although the present study was not concerned with the phytotoxic effects of microbially decomposed wheat straw, an influence of microbial activity on ferulic acid phytotoxicity was observed. From the results shown in Table XI, it appears that the presence of the prickly sida seed carpel enhanced the inhibitory effects of ferulic acid. In addition to ferulic acid in test solutions containing prickly sida seeds with carpels, a second compound, 4-hydroxy-3-methoxy styrene, was also found to be present. This compound is formed by the decarboxylation of ferulic acid and was produced by a bacterium present on the carpel of prickly sida seed. The decarboxylation of ferulic acid was detected in aqueous solutions of ferulic acid inoculated with the bacterium isolated from the carpels of prickly sida seed. No conversion occurred when the bacterium was not present. 

Tea oxidation is generally referred to as fermentation because of the erroneous early conception of black tea production as a microbial process.66 Not until 1901 was there recognition of the process as one dependent on an enzymically catalyzed oxidation.67 This step and further reactions result in the conversion of the colorless flavanols to a complex mixture of orange-yellow to red-brown substances and an increase in the amount and variety of volatile compounds. Extract of oxidized leaf is amber-colored and less astringent than the light yellow-green extract of fresh leaf and the flavor profile is considerably more complex. 

Conversion of a toxic organic compound to a nontoxic organic compound. The pesticide 2,4-D can be detoxified microbially to 2,4-dichlorophenol. 

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