Impact of Local Electrostatics on the Redox Properties
of Tryptophan Radicals in Azurin: Implications for Redox-Active Tryptophans
in Proton-Coupled Electron Transfer
posted on 2020-03-12, 13:06authored byKristin
J. Tyson, Amanda N. Davis, Jessica L. Norris, Libero J. Bartolotti, Eli G. Hvastkovs, Adam R. Offenbacher
Tyrosine
and tryptophan play critical roles in facilitating proton-coupled
electron transfer (PCET) processes essential to life. The local protein
environment is anticipated to modulate the thermodynamics of amino
acid radicals to achieve controlled, unidirectional PCET. Herein,
square-wave voltammetry was employed to investigate the electrostatic
effects on the redox properties of tryptophan in two variants of the
protein azurin. Each variant contains a single redox-active tryptophan,
W48 or W108, in a unique and buried protein environment. These tryptophan
residues exhibit reversible square-wave voltammograms. A Pourbaix
plot, representing the reduction potentials versus pH, is presented
for the non-H-bonded W48, which has potentials comparable to those
of tryptophan in solution. The reduction potentials of W108 are seen
to be increased by more than 100 mV across the same pH range. Molecular
dynamics shows that, despite its buried indole ring, the N–H
of W108 hydrogen bonds with a water cluster, while W48 is completely
excluded from interactions with water or polar groups. These redox
properties provide insight into the role of the protein in tuning
the reactivity of tryptophan radicals, a requirement for controlled
biological PCET.