C(sp3 )-H Hydroxylation in Diiron β-Hydroxylase CmlA Transpires by Amine-Assisted O2 Activation Avoiding FeIV 2 O2 Species.

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Through QM/MM modeling, we discovered that C(sp3 )-H β-hydroxylation in the diiron hydroxylase CmlA transpires by traceless amine-assisted O2 activation. Different from the canonical diiron hydroxylase sMMO, this aliphatic-amine-assisted O2 activation avoids generating the high-valent diferryl FeIV 2 O2 species, but alternatively renders a diferric FeIII 2 O species as the reactive oxidant. From this unprecedented O2 activation mode, the derived C(sp3 )-H hydroxylation mechanism in CmlA also differs drastically from the toluene aromatic C(sp2 )-H hydroxylation in the diiron hydroxylase T4MO. This substrate-modulated O2 activation in CmlA has rich mechanistic implications for other diiron hydroxylases with an amine group adjacent to the C-H bond under hydroxylation in substrates, such as hDOHH. Furthermore, the adapted coordination environment of the diiron cofactor upon O2 binding in CmlA opens up more structural and mechanistic possibilities for O2 activation in non-heme diiron enzymes.