Microorganisms, transamination

Although 2-phenylethanol can be synthesised by normal microbial metabolism, the final concentrations in the culture broth of selected microorganisms generally remain very low [110, 111] therefore, de novo synthesis cannot be a strategy for an economically viable bioprocesses. Nevertheless, the microbial production of 2-phenylethanol can be greatly increased by adding the amino acid L-phenylalanine to the medium. The commonly accepted route from l-phenylalanine to 2-phenylethanol in yeasts is by transamination of the amino acid to phenylpyruvate, decarboxylation to phenylacetaldehyde and reduction to the alcohol, first described by Ehrlich [112] and named after him (Scheme 23.8). 

Anthranilic acid and indole are precursors of tryptophan in numerous microorganisms and fungi (e.g., 5, 263, 264, 602, 741, 783, 785, 816, 854, 855, 876), and it is probable that anthranilic acid is derived, with intermediate steps, from the common precursor, CP of diagram 1. The conversion of anthranilic acid to indole and tryptophan has been shown unambiguously in Neurospora with the use of isotopic techniques (93, 663). There may, however, be other pathways for tryptophan biosynthesis (45, 702). Tryptophan can, for example, be formed by transamination of indolepyruvic acid (e.g., 470, 912), which might be formed other than via anthranilic acid. Thus aromatic-requiring mutants have been found which accumulate unidentified indole compounds (307). 

Some microorganisms in culture show methionine-dependent ethylene formation. In studies with Escherichia coli, 2-oxo-4-methylthiobutyrate (KMB) produced from methionine by transamination was suggested as the precursor of ethylene [19], and subsequently a cell-free system which produced ethylene from KMB in the presence of NAD(P)H, EDTA-Fe and oxygen was established [20]. An enzyme which catalysed a similar ethylene-forming activity was purified from Cryptococcus albidus [15]. The purified enzyme of molecular mass 62 kDa turned out to be NADH EDTA-Fe oxidoreductase. The proposed mechanism involves reduction of EDTA-Fe to EDTA-Fe by the enzyme, reduction of oxygen to superoxide by EDTA-Fe, of hydrogen peroxide to hydroxyl radical, and oxidation of KMB by hydroxyl radical to ethylene. However, an extensive physiological evaluation of this enzyme must be done before it can... 

L-Alanine, L-glutamine and other amino acids act as amino group donors. The amines given in Table 42 are synthesized by transamination in microorganisms or higher plants. 

Plants and many microorganisms are able to synthesise proteins from simple nitrogenous compounds such as nitrates. Animals cannot synthesise the amino group, and in order to build up body proteins they must have a dietary source of amino acids. Certain amino acids can be produced from others by a process known as transamination (see Chapter 9), but the carbon skeletons of a number of amino acids cannot be synthesised in the animal body these are referred to as essential or indispensible amino acids. 

The transamination of L-isoleucine to d-a-keto-/3-methylvalerate and of L-alloisoleucine to Z-a-keto-/3-methylvalerate by a hog heart preparation and the cell-free extracts of a number of microorganisms e.g., Lacto-bacillus arabinosus) has been demonstrated by Meister. ... 

Transamination in microorganisms is still less satisfactorily exploi-ed than in plants. Reaction GL— AL could not be detected in fresh or plas-molyzed yeast, in the writer s laboratory. In papers from Euler s institute scantily documented data are found on transamination in maceration juice from yeast and in E. coli (2,3). Cohen (61) states that E. eoli and Lebedew juice from brewer s yeast are active in the reaction GL, but not in AL— GL, while baker s yeast is inactive in both systems. Dicsfalusy (67) failed to detect the reactions mentioned in E. coli, Staphylococcus or B. mesenterieus, and Konikova (39) found no transamination of GL or AS with PU in heavy suspensions of B. brevis (Dubos strain BG). 

A final mechanism for the formation of carbonys via nticroorganisms involves the transamination and decarboxylation of free amino acids. Morgan [95] has demonstrated the production of 3-methyl butanal by S. lactis var. maltigenes (Figure 5.10). MacLeod and Morgan [96] have demonstrated the production of 2-methyl butanal, methional and phenylacetaldehyde from leucine, methionine and phenylalanine, respectively, by the same microorganism. 

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