Proximal Pocket Hydrogen Bonds Significantly Influence the Mechanism of Chloroperoxidase Compound I Formation

The
influence of backbone hydrogen bonds to the sulfur atom of
the proximal thiolate (NH···S hydrogen bonds) on the
formation of compound I in chloroperoxidase is investigated with DFT
calculations. Reaction profiles for the transformation of the ferric
resting state into compound I in the presence of a peroxide substrate
are calculated for a model system incorporating the heme and key proximal
and distal amino acid residues. We find that NH···S
hydrogen bonds (1) reduce the barrier for the formation of compound
0 by 7.6 kcal/mol, (2) increase the stability of compound 0 by 5.2
kcal/mol, (3) reduce the stability of compound I relative to compound
0 by 6.2 kcal/mol, and (4) reduce the stability of protonated compound
0, favoring a hybrid homo-heterolytic relative to a classic heterolytic
mechanism for O–O bond scission. In general, the influence
of the NH···S hydrogen bonds can be traced to a reduction
in the pKa of the heme-bound substrate.
We find that the hydrogen bond networks on the proximal and distal
sides of the heme function together to modulate the mechanism of reaction.
These results confirm and extend long-standing theories that the NH···S
hydrogen bonds in heme thiolate proteins influence reactivity by tuning
the thiolate “push” effect.