Nature has devised intrinsic electric fields (IEFs) that
are engaged
in electrostatic catalysis of enzymes. But, how does the IEF target
its function in enzymes that involve several reaction steps in catalytic
cycles? To decipher the impact of the IEF on the catalytic cycle of
an enzyme system, we have performed molecular dynamics and quantum-mechanical/molecular-mechanical
(QM/MM) simulations on tyrosine hydroxylase (TyrH). The catalytic
cycle of TyrH involves two reaction stages: the activation of H2O2 to form the active species of compound I (Cpd
I), in the first stage, and the Cpd I-mediated hydroxylation of l-tyrosine to l-DOPA, in the second stage. For the
first stage, the QM/MM calculations show that a heme-propionate group
functions as a base to catalyze the O–O heterolysis reaction.
For the second stage, the study reveals that the reaction is initiated
by the His88-mediated proton-coupled electron transfer followed by
the oxygen atom transfer from compound II (Cpd II) to the l-Tyr substrate. Importantly, our calculations demonstrate that the
IEF in TyrH is optimized to promote the O–O bond heterolysis
that generates the active species of the enzyme, Cpd I. However, the
same IEF slows down the subsequent aromatic hydroxylation. Thus, the IEF in the TyrH enzymes does not catalyze the product
formation step, but will selectively boost one or more challenging
steps in the catalytic cycle. These findings have general implications
on O2/H2O2-dependent metalloenzymes,
which can expand our understanding of how nature has used electric
fields as “smart reagents” in modulating the catalytic
reactivity.