Phenoxazinone chromophore

Snznki H, Y Fnrnsho, T Higashi, Y Ohnishi, S Horinonchi (2006a) A novel o-aminophenol oxidase responsible for formation of the phenoxazinone chromophore of grixazone. J Biol Chem 281 824-833. 

Aromatization can also be accomplished by oxidative cascade reactions-a strategy often utilized by natural product biosynthesis [33]. For example, the phenoxazinone chromophore of actinomycin was proposed to be generated by a six-electron oxidation of aminophenols catalyzed by a copper-containing oxidase (Scheme 7.8) [34]. 

Naturally-occurring actinomycins show as a common feature the presence of the phenoxazinone chromophore, doubly-linked to a pentapeptide which forms a macrocyclic structure. Differences among actinomycins arise from variations in composition of the pentapeptide. The precursors of the aminoacids, forming the pentapeptide, have been singled out [176-178]. 

Barry CE III. Pannesh G, Nayar PG, Begley T. Phenoxazinone synthase Mechanism for the formation of the phenoxazinone chromophore of actinomycin. Biochem 1989 ... 

Weissbach, H., and E. Katz Studies on the biosynthesis of actinomycin Enzymic synthesis of the phenoxazinone chromophore. J. Biol. Chem. 236, PC 17 (1961). 

Almost all actinomycins have the same chromophore, a planar phenoxazinone dicarboxylic acid called actinociiL In dactinomycin, the structure of which is shown in Figure 12, the two pendent pentapeptide lactones are identical, but in other actinomycins these lactones may be different. In other actinomycins the first amino acid, amide linked with actinocin, is usually L-threonine, as in dactinomycin the second position is sometimes D-allo-isoleucine instead of D-valine the third position may be sarcosine or oxoproline the fourth position is sarcosine and the fifth position is sometimes iV-methyl isoleucine instead of N-methylvaline. The lactone ring is always present. 

The blue oxidases-related enzymes include phenoxazinone synthase from S. antibioticus This enzyme is a copper-containing oxidase that catalyzes the coupling of 2-aminophenols to form the 2-aminophenoxazinone chromophore. This reaction constitutes the final step in the biosynthesis of the potent antineoplastic agent actinomycin. The crystal structure of the oxidized form phenoxazinone synthase from S. anibioticus has been determined. It has been solved in his hexameric form. One monomer is very similar to Iaccase or ascorbate oxidase but it contains a long loop, which connects two domains and stabilizes the hexameric structure. Bound... 

Actinomycins are chromopeptides, generally containing the same chro-mophore, based on the tricyclic system of phenoxazinone (phenoxazone), and known as actinocin, Fig. (8). The chromophore is responsible for the intense yellow, orange or red colours of actinomycins. The first known actinomycin was isolated from a culture medium of an actinomyces, Streptomyces sp. [174], and caused some interest on account of its antibiotic activity. Many other actinomycins were isolated later and characterised by their antibiotic and/or antitumor activities [175]. Among these, the best known is actinomycin D (Dactinomycin), Fig. (8). 

Fig. 1. Actinomycin IV(d). The chromophore, actinocin (2-amino-4,6-dimethyl-3-phenoxazinone-i,9-dicarboxylic acid), is identical in all of the actinomycins examined. The sequence of amino acids is L-threonine, D-valine (n-val), L-proline (L-pro), sarcosine (sar), N-methyl-L-valine...

Evidence obtained with radioisotopically labeled tryptophan as well as with certain tryptophan analogues have established that tryptophan is a precursor of the actinomycin chromophore (Katz, 196O Sivak, Meloni, Nobili and Katz, 1962 Sivak and Katz, I962). In experiments with benzene ring-labeled tryptophan it was shown that the C-label of the amino acid was efficiently (11.7%) and preferentially incorporated into the phenoxazinone moiety of actinomycin (Table 3). The chromophore derivatives, actinocinin and desaminoactinocylthreonine dimethyl ester, obtained from actinomycin by acid hydrolysis (Brockmann and Grone,... 

The enzymatic synthesis of the actinomycin chromophore also has been described (Weissbach and Katz, 1961 Katz and Weissbach 1962 Salzman, Weissbach and Katz, unpublished results). An enzyme, designated phenoxa-zinone synthetase, was obtained from sonic extracts of S. antihioticus and was shown to catalyze the oxidative condensation of o-aminophenols to the corresponding phenoxazinone. When 3-hydroxy-4-methylanthranilic acid (4MHAA) was used as substrate, actinocin was synthesized. The enzyme has been purified approximately 25- to 30-fold from sonic or French pressure cell extracts of the organism by means of standard protein fractionation procedures (ammonium... 

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