Iron(II)- and 2-oxoglutarate-dependent (Fe/2OG) oxygenases generate iron(IV)-oxo (ferryl) intermediates that can abstract hydrogen from aliphatic carbons (R-H). Hydroxylation proceeds by coupling of the resultant substrate radical (Râ€¢) and oxygen of the Fe(III)-OH complex ("oxygen rebound"). Nonhydroxylation outcomes result from different fates of the Fe(III)-OH/R•state; for example, halogenation results from R•coupling to a halogen ligand cis to the hydroxide. We previously suggested that halogenases control substrate-cofactor disposition to disfavor oxygen rebound and permit halogen coupling to prevail. Here, we explored the general implication that, when a ferryl intermediate can ambiguously target two substrate carbons for different outcomes, rebound to the site capable of the alternative outcome should be slower than to the adjacent, solely hydroxylated site. We evaluated this prediction for (i) the halogenase SyrB2, which exclusively hydroxylates C5 of norvaline appended to its carrier protein but can either chlorinate or hydroxylate C4 and (ii) two bifunctional enzymes that normally hydroxylate one carbon before coupling that oxygen to a second carbon (producing an oxacycle) but can, upon encountering deuterium at the first site, hydroxylate the second site instead. In all three cases, substrate hydroxylation incorporates a greater fraction of solvent-derived oxygen at the site that can also undergo the alternative outcome than at the other site, most likely reflecting an increased exchange of the initially O2-derived oxygen ligand in the longer-lived Fe(III)-OH/R•states. Suppression of rebound may thus be generally important for nonhydroxylation outcomes by these enzymes.
|Number of pages||13|
|Journal||Journal of the American Chemical Society|
|State||Published - Sep 25 2019|
Bibliographical noteFunding Information:
This work was supported by the National Institutes of Health (GM113389 to C.J.P., GM113106 to J.M.B. and C.K., and GM127079 to C.K.). We thank Prof. Hung-wen Liu and Dr. Richiro Ushimaru for supplying the deuterated and hydroxylated hyoscyamine compounds and Dr. Jeffrey W. Slater for technical assistance. We dedicate this study to Larry Que, a pioneer in the field of nonheme iron chemistry, on the occasion of his 70th birthday.
© 2019 American Chemical Society.
ASJC Scopus subject areas
- Chemistry (all)
- Colloid and Surface Chemistry