Amino acids microbial synthesis

Biotransformations are carried out by either whole cells (microbial, plant, or animal) or by isolated enzymes. Both methods have advantages and disadvantages. In general, multistep transformations, such as hydroxylations of steroids, or the synthesis of amino acids, riboflavin, vitamins, and alkaloids that require the presence of several enzymes and cofactors are carried out by whole cells. Simple one- or two-step transformations, on the other hand, are usually carried out by isolated enzymes. Compared to fermentations, enzymatic reactions have a number of advantages including simple instmmentation reduced side reactions, easy control, and product isolation. 

Enzyme preparations from liver or microbial sources were reported to show rather high substrate specificity [76] for the natural phosphorylated acceptor d-(18) but, at much reduced reaction rates, offer a rather broad substrate tolerance for polar, short-chain aldehydes [77-79]. Simple aliphatic or aromatic aldehydes are not converted. Therefore, the aldolase from Escherichia coli has been mutated for improved acceptance of nonphosphorylated and enantiomeric substrates toward facilitated enzymatic syntheses ofboth d- and t-sugars [80,81]. High stereoselectivity of the wild-type enzyme has been utilized in the preparation of compounds (23) / (24) and in a two-step enzymatic synthesis of (22), the N-terminal amino acid portion of nikkomycin antibiotics (Figure 10.12) [82]. 

Phenoxazines — The two main types of phenoxazines are the ommochromes and the microbial phenoxazines. The biosynthesis of ommochromes occurs via the kynurenine pathway. The tryptophan amino acid is converted to formylkynurenine and then to kynurenine and 3-hydroxykynurenine. Not all the steps of ommochrome synthesis are completely elucidated yet. Ommatins are dimers and ommins are oligomers of 3-hydroxykynurenine. - The papiliochromes are derived from tyrosine as well as from the tryptophan pathway. The key intermediate in the formation of papiliochromes is N-beta-alanyldopamine (NBAD). Papiliochromes are synthesized in special wing scale cells, before melanins. " "... 

The ease of the Strecker synthesis from aldehydes makes a-aminonitriles an attractive and important route to a-amino acids. Fortunately, the microbial world offers a number of enzymes for carrying out the necessary conversions, some of them highly stereoselective. Nitrilases catalyze a direct conversion of nitrile into carboxylic acid (Equation (11)), whereas nitrile hydratases catalyze formation of the amide, which can then be hydrolyzed to the carboxylic acid in a second step (Equation (12)). In a recent survey, with a view to bioremediation and synthesis, Brady et al have surveyed the ability of a wide range of bacteria and yeasts to grow on diverse nitriles and amides as sole nitrogen source. This provides a rich source of information on enzymes for future application. 

Maurer PJ, Miller Ml (1982) Microbial Iron Chelators Total Synthesis of Aerobactin and its Constituent Amino Acid, if -Acetyl-lf -hydroxylysine. J Am Chem Soc 104 3096... 

Both sulfonamides and trimethoprim (not a sulfonamide) sequentially interfere with folic acid synthesis by bacteria. Folic acid functions as a coenzyme in the transfer of one-carbon units required for the synthesis of thymidine, purines, and some amino acids and consists of three components a pteridine moiety, PABA, and glutamate (Fig. 44.1). The sulfonamides, as structural analogues, competitively block PABA incorporation sulfonamides inhibit the enzyme dihydropteroate synthase, which is necessary for PABA to be incorporated into dihydropteroic acid, an intermediate compound in the formation of folinic acid. Since the sulfonamides reversibly block the synthesis of folic acid, they are bacteriostatic drugs. Humans cannot synthesize folic acid and must acquire it in the diet thus, the sulfonamides selectively inhibit microbial growth. 

There are several other examples of enzyme peptide synthesis. The conversion of porcine insulin into a human insulin precursor, by replacing the B chain C-terminal with threonine, is still an important alternative to microbially produced human insulin. Peptides that are produced from ethyl esters of L-amino acids by BioEurope are used as ingredients of cosmetics. 

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). 

A. Y. Chernyak, G. V. Sharma, L. O. Kononov, P. Radha Krishna, A. V. Rama Rao, and N. K. Kochetkov, Synthesis of glycuronamides of amino acids, constituents of microbial polysaccharides, and their conversion into neoglycoconjugates of copolymer type, Glycoconj. J., 8 (1991) 82-89. 

PJ Maurer, MJ Miller. Microbial iron chelators total synthesis of aerobactin and its constituent amino acid,N6-acetyl-N6-hydroxylysine. J Am Chem Soc 105 240-245, 1983. 

The terminal amino acids are under strict metabolic control. Some act as feedback inhibitors or repressors. Their synthesis is in equilibrium with metabolic requirement. This equilibrium position prevents their accumulation and hence the yield of these compounds is low. By changing the growth requirement (environmental stimulus) or by genetic manipulation, mutants could be found with limited or removed feedback inhibitors and repressors, e.g. auxotrophic and regulatory mutants 49). This needed a better understanding of biosynthesis and regulation of amino acid production. By selection of these mutants it became possible to alter microbial metabolism which led to the accumulation of the desired amino acids. 

In 1872 the simplest a,a-disubstituted amino acid (2-aminoisobutyric acid, Aib) was described [3]. In 1908 the first optically active representative of this class of compounds, (/ )-2-ethylala-nine (D-isovaline), was isolated by microbial racemic resolution [4]. Synthetic chemists have therefore been interested in the enantiopure synthesis of a-alkylated a-amino acids for some time. Their powerful methods for the construction of chiral a-amino acids can in some cases also be used for the synthesis of the a-alkylated derivatives which has been the topic of recent reviews [5]. 

Our novel contributions to the enzymatic (microbial) synthesis of some relevant P-amino acids is summarized in the succeeding part of this chapter (Section 15.2). Such P-amino acids occur as key components in many natural products [4] and... 

Various commercial routes for the production of L-phenyalanine have been developed because of the utilization of this amino acid in the dipeptide sweetener Aspartame. One route that has been actively pursued is the synthesis of L-phenylalanine from trans-cinnamic acid using the enzyme phenylalanine ammonia lyase (105,106). This enzyme catalyzes the reversible, nonoxidative deamination of L-phenylalanine and can be isolated from various plant and microbial sources (107,108). 

It is evident that natural proteins are not a primary source of large amounts of amino acids, despite the fact that many of the acids are commercially significant chemicals and a few are commodity chemicals. The technical difficulties just alluded to include undesirable distributions of the amino acids in natural proteins, the sensitivity of proteins and amino acids to chemical hydrolysis conditions, racemization, the multiplicity of the product acids and the often low concentration of the desired acid or acids in the hydrolysate, and the consequent separation problems. Microbial synthesis of specific amino acids from biomass substrates or biomass-derived intermediates often has substantial advantages over thermochemical processing methods and is used for the commercial production of several of the amino acids. This is discussed in more detail in the next section. 

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