Characterization of the PLP-dependent aminotransferase NikK from Streptomyces tendae and its putative role in nikkomycin biosynthesis

Authors


Peter Macheroux, Institute of Biochemistry, Graz University of Technology, Petersgasse 12, A-8010 Graz, Austria
Fax: +43 316 873 6952
Tel: +43 316 873 6450
E-mail: peter.macheroux@tugraz.at
Karl Gruber, Institute of Molecular Biosciences, University of Graz, Humboldtstr. 50/3, A-8010 Graz, Austria
Fax: +43 316 380 9897
Tel: +43 316 380 5483
E-mail: karl.gruber@uni-graz.at

Abstract

As inhibitors of chitin synthase, nikkomycins have attracted interest as potential antibiotics. The biosynthetic pathway to these peptide nucleosides in Streptomyces tendae is only partially known. In order to elucidate the last step of the biosynthesis of the aminohexuronic building block, we have heterologously expressed a predicted aminotransferase encoded by the gene nikK from S. tendae in Escherichia coli. The purified protein, which is essential for nikkomycin biosynthesis, has a pyridoxal-5′-phosphate cofactor bound as a Schiff base to lysine 221. The enzyme possesses aminotransferase activity and uses several standard amino acids as amino group donors with a preference for glutamate (Glu > Phe > Trp > Ala > His > Met > Leu). Therefore, we propose that NikK catalyses the introduction of the amino group into the ketohexuronic acid precursor of nikkomycins. At neutral pH, the UV-visible absorbance spectrum of NikK has two absorbance maxima at 357 and 425 nm indicative of the presence of the deprotonated and protonated aldimine with an estimated pKa of 8.3. The rate of donor substrate deamination is faster at higher pH, indicating that an alkaline environment favours the deamination reaction.

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