Screening for inhibitors of 2-oxoglutarate-dependent dioxygenases: Flavanone 3β-hydroxylase and flavonol synthase

2-Oxoglutarate-dependent dioxygenases (2-ODDs) catalyze numerous steps in biosynthetic pathways of plants. Prohexadione-Ca is a known inhibitor of such reactions, due to its structural similarity to 2-oxoglutarate. In apple (Malus domestica) and pear (Pyrus communis) leaves, the transient inhibition of 2-ODDs flavanone 3β-hydroxylase (FHT) and flavonol synthase (FLS) by prohexadione-Ca results in distinct changes in the flavonoid spectrum, which are responsible for an enhanced resistance against two major pome fruit diseases, fire blight (caused by Erwinia amylovora) and apple scab (caused by Venturia inaequalis). We used recombinant apple and pear FHT and apple FLS for screening 23 structural analogues of 2-oxoglutarate, mostly cyclohexanediones, pyridine dicarboxylic acids and N-heterocycles with carbonyl functions for other dioxygenase inhibitors. Activations, which were also observed for some compounds, are interpreted as in vitro effects due to Fe2+-chelating ability. Apart from structural similarity to 2-oxoglutarate, close structural similarity of cyclohexanediones and some pyridine dicarboxylic acids to flavonoid substrates was identified. Beyond the competitive inhibition for the co-substrate 2-oxoglutarate, flavonoid converting 2-ODDs may also be inhibited at the substrate binding site by these inhibitors. All compounds found to be active as inhibitors may prove useful for studying the reaction mechanisms and substrate specificities of various 2-ODDs.