Nature typically employs metal centers within enzymes to active molecular oxygen and to carry out various processes involved in metabolism, oxidative phosphorylation and biodegradation. Our knowledge of iron-containing enzymes will help us better understand above natural processes which of great importance to agriculture, industry, environment, and bio-medicine. Structural study has been provided significant insight into the mechanisms and functions of enzymes of our interest.
Extradiol dioxygenase catalyzes the oxygenolytic ring opening of aromatic compounds involved in microbial catabolism. Its substrates are catechol analogues in most cases, but noncatecholic compounds, e.g. 2-aminophenol can be cleaved by 2-aminophenol 1,6-dioxygenase (APD), which represents a minor subset of extradiol dioxygenases. Two baffling but intriguing questions remain to be investigated: 1, if enzymes catalyzing noncatecholic substrates rely on the same mechanistic strategy as the canonical one deduced from previous structural studies? 2, how does APD distinguish its own substrate with catecholic compounds that are so structurally similar?
Recently, Chinese scientists, Li et al. determined two crystal structures of APD, one in complex with a lactone intermediate and the product, and the other with a suicide inhibitor. Although they did not straightforwardly observe substrate binding, its binding manner could indubitably be deduced from the determined complex structures, and hence a solid conclusion that APD use a similar enzymatic mechanism on catalyzing its substrate could be drawn. More interestingly, by comparison with published structures, they observed that the configuration of Fe coordination in APD was effectively enantiomeric to other extradiol dioxygenases reported so far, which results in anunique linear O--Fe2+-O- species at the active site. All these data implicates thatthe chirality of the iron centerin these enzymes forms the structural determinant of distinctive substrate specificity.
This study revealed a lactone intermediate binding to FeII for the first time, and inspiringly links the enantiomeric Fe coordination to the amazing substrate selectivity of a minor group in the huge family of non-hemeFeII enzymes. These results expand our understanding from the canonical extradiol dioxygenases to those that catalyze noncatechol compounds and account well for the high substrate specificity of the enzyme.
This work waspublished as a research paper entitled “Structures of aminophenol dioxygenase in complex with intermediate, product and inhibitor” on Acat Crystallographic Section D. It was a fruit of collaboration among scientists from the Institute of Biophysics, Institute of Microbiology, and the Graduate School of the Chinese Academy of Sciences and the Third Military Medical University, China.
This work was supported by grants from the Ministry of Science and Technology and the National Natural Science Foundation of China.
Fig. Comparison of Fe–ligand coordination geometry at the active site prior to O2 binding in some 2-His-1-carboxylate enzymes indicates that the enantiomeric Fe coordination is the structural determinant for high degree of substrate specificity.
