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Rhizobium radiobacter C58 was found to convert 4-hydroxyisoleucine (HIL) and 2-amino-3-methyl-4-ketopentanoate (AMKP), bioactive oxidative derivatives of l-isoleucine, in both cases producing 2-aminobutyrate. Three native enzymes involved in these metabolisms were purified by column chromatography and successfully identified. In this strain, HIL was converted to acetaldehyde and 2-aminobutyrate by coupling action of the transaminase rrIlvE and the aldolase HkpA. AMKP was also converted to acetate and 2-aminobutyrate by coupling action of rrIlvE and a hydrolase DkhA. In the multi-enzymatic reactions, HkpA catalyzes the retro-aldol reaction of 4-hydroxy-3-methyl-2-ketopentanoate into acetaldehyde and 2-ketobutyrate, and DkhA catalyzes hydrolytic cleavage of the carbon-carbon bond of 2,4-diketo-3-methylpentanoate into acetate and 2-ketobutyrate. rrIlvE catalyzes reversible transamination between HIL and 4-hydroxy-3-methyl-2-ketopentanoate, AMKP and 2,4-diketo-3-methylpentanoate, and 2-ketobutyrate and 2-aminobutyrate. The results suggested that the conversion activity of Rhizobium bacteria plays an important role in the complex biological metabolic networks associated with HIL and AMKP. |
