The catalytic reaction of cytokinin oxidase/dehydrogenase (EC 126.96.36.199) was studied in detail using the recombinant flavoenzyme from maize. Determination of the redox potential of the covalently linked flavin cofactor revealed a relatively high potential dictating the type of electron acceptor that can be used by the enzyme. Using 2,6-dichlorophenol indophenol, 2,3-dimethoxy-5-methyl-1,4-benzoquinone or 1,4-naphthoquinone as electron acceptor, turnover rates with N6-(2-isopentenyl)adenine of approx. 150 s−1 could be obtained. This suggests that the natural electron acceptor of the enzyme is quite probably a p-quinone or similar compound. By using the stopped-flow technique, it was found that the enzyme is rapidly reduced by N6-(2-isopentenyl)adenine (kred=950 s−1). Re-oxidation of the reduced enzyme by molecular oxygen is too slow to be of physiological relevance, confirming its classification as a dehydrogenase. Furthermore, it was established for the first time that the enzyme is capable of degrading aromatic cytokinins, although at low reaction rates. As a result, the enzyme displays a dual catalytic mode for oxidative degradation of cytokinins: a low-rate and low-substrate specificity reaction with oxygen as the electron acceptor, and high activity and strict specificity for isopentenyladenine and analogous cytokinins with some specific electron acceptors.
Catalytic reaction of cytokinin dehydrogenase: preference for quinones as electron acceptors
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Jitka FRÉBORTOVÁ, Marco W. FRAAIJE, Petr GALUSZKA, Marek ŠEBELA, Pavel PEČ, Jan HRBÁČ, Ondřej NOVÁK, Kristin D. BILYEU, James T. ENGLISH, Ivo FRÉBORT; Catalytic reaction of cytokinin dehydrogenase: preference for quinones as electron acceptors. Biochem J 15 May 2004; 380 (1): 121–130. doi: https://doi.org/10.1042/bj20031813
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