Microorganisms tryptophan metabolism

Among the most remarkable features of tryptophan metabolism is the widespread use of a common pathway by a wide range of organisms. Thus many microorganisms growing on tryptophan as sole carbon source metabolize it by a route very similar to that used in man, animals, insects, and probably also to a considerable extent in plants. In this discussion, therefore, evidence leading to the elucidation of the pathway will be drawn from many fields, and more detailed consideration of the individual steps will cover a range of species. 

Experiments with mutants of microorganisms and insects, which made it likely that hydroxykynurenine is an intermediate in tryptophan metabolism, have been described above. Further evidence for the occurrence of hydroxykynurenine in insect larvae has siirce been reported (575, 844, 845), and its relation to eye-pigments is discussed below. It is also formed in plants (932). Strong support for participation in mammalian metabolism was provided by its identification in mammalian urine (171, 176, 292, and cf. further discussion later). 

The largest number of indole derivatives which have been structurally characterized are the indole alkaloids these mainly arise from plant sources. Not unexpectedly, in view of tryptophan s status as one of the essential amino acids, there are also diverse derivatives found in microorganisms. Indole-3-acetic acid plays a major role in plant metabolism, being a growth regulator (74MI30600). 

Generous sources of this vitamin include pork. lamb, and beef livers hog kidneys yeasts pork beef tongue heans lean meats wheat germ peanut meal and green peas. Nicotinic acid can be synthesized by almost all planLs and animals. Tryptophan can be metabolized to a nicotinic acid nucleotide in animrds. but the efficiency of this mullistep process varies from species to species. Plants and many microorganisms synthesize this vitamin through alternative routes by use of aspartic acid. 

Diagram 16. Interrelationships of some side-chain metabolic reactions used by microorganisms. Ar can be phenyl, p-hydroxyphenyl (as in tyrosine), indol-3-yl-(as in tryptophan), 3,4-dihydroxyphenyl, etc. Names given above are in general for substances derived from phenylalanine. 

Tryptophan also gives rise to the important plant hormone, indoleacetic acid, and microorganisms and especially plants metabolize the aromatic amino acids to a wide range of natural products, for example, certain antibiotics, alkaloids (e.g., diagrams 25-28), flavonoids, and possibly lignin. These are briefly considered. 

Plant secondary metabolites are biosynthesized from rather simple building blocks supplied by primary metabolism. Two important metabolic routes in this are the shikimate pathway and the isoprenoid biosynthesis. The shikimate pathway leads to the synthesis of phenolic compounds and the aromatic amino acids phenylalanine, tyrosine and tryptophan. The isoprenoid biosjmthesis is a heavily branched pathway leading to a broad spectrum of compounds (fig. 1). From plants and microorganisms more than 37,000 isoprenoid compounds have been isolated so far [1]. 

The pivotal position occupied by chorismic acid in the shikimic acid pathway has been established in several higher plants as well as microorganisms (Fig. 2) (Edwards and Jackman, 1965 Cotton and Gibson, 1968 Schmit and Zalkin, 1969 Gilchrist et al., 1972). By action of chorismate mutase [Fig. 3 (8)], chorismate is converted to prephenate which is subsequently metabolized by two independent pathways [Fig. 3 (9 and 11)] to form phenylalanine and tyrosine. Alternatively, chorismate serves as a substrate for anthranilate synthase, the first enzyme in the pathway branch leading to the synthesis of tryptophan [Fig. 4 (13)]. 

Certain microorganisms degrade L-tryptophan (in some cases also D-trypto-phan) via kynurenic acid to compounds of primary metabolism (Fig. 265, quinolinic pathway of tryptophan degradation, in contrast to the aromatic pathway via 3-hydroxyanthranilic acid shown in Fig. 244). 

The oxidation of tryptophol to indoleacetaldehyde is not necessarily its only pathway of metabolism. The microorganism Pseudomonas fluorescens. for example, contains a tryptophan side-chain oxidase . This oxidase, at least in vitro, attacks a variety of substrates including tryptophol, to form indole-3-glycol and indole-3-ketol [Fig. 2 23] ... 

The aroma of butter made from sweet cream is affected primarily by free fatty acids (especially capric and lauric acids), S- and y-lactones, dimethylsulfide, (Z)-hept-4-enal and the degradation products of tryptophan (indole and skatole). The butter obtained from sour cream contains mainly metabolic products of microorganisms (so-called starter cultures). Especially important compounds are biacetyl, lactic and acetic acids. 

Tryptophan is metabolized by several different pathways (Fig. 1) each yielding biologically important substances such as tryptamine and in particular serotonin (5-hydroxytryptamine), which seems to be involved in certain mental disorders. Indole-3-acetic acid is a plant growth hormone its precursor is tryptamine or indole-3-pyruvic acid. In humans, the microorganisms of the large intestine can further degrade indole-3-acetic acid to yield indole, skatole (3-methyl-indole) and other substances. 

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