Functional and protective hole hopping in metalloenzymes

Abstract

Hole hopping through tryptophan and tyrosine residues in metalloenzymes facilitates catalysis and prolongs survival.

Theory of Protein Charge Transfer: Electron Transfer between Tryptophan Residue and Active Site of Azurin

“…[1][2][3] The reason for conductivity is sought in chains of aromatic residues that can be oxidized and serve as relay elements to direct oxidizing electron holes to the protein surface to avoid oxidative damage of active sites of enzymes. [4][5][6] Each hop in this sequence is viewed as a single electron-transfer step 7 as traditionally described by the Marcus theory of electron transfer reactions. 8 This article reports molecular dynamics (MD) simulations and theoretical modeling of a single step in such a relay involving electron transfer between the active site of azurin protein (Figure 1, PDB 1AZU) and the nearby tryptophan residue as studied experimentally by Shih et al 4 Trp Cu R e ≃ 9.2 − 10.…”

The histidine-brace active site of a copper monooxygenase is redox active and forms part of an in-built enzyme repair mechanism

“…It is also clear that other strategies, such as addition of excess reducing agents taken to 'extinguish' histidyl radicals generated during the turnover of oxygenases, may offer themselves as means of increasing e ciency by prolonging the life of the catalyst 36 , be it an enzyme or-for that matter-a small molecule mimic. Our studies reported herein on LPMOs can therefore direct future research efforts on maximizing the e ciency of oxidative catalysts 13 .…”

“…In addition to these theoretical studies, evidence that LPMOs do indeed employ a high-valent copper species comes from experimental studies in which the enzyme has been 'shunted' with hydrogen peroxide (H 2 O 2 ) to perform the same selective oxidative chemistry on saccharidic substrates as with O 2 (analogous to 'shunt' studies on P450 enzymes) 12 . As with all oxygenase and peroxygenase enzymes, however, these intermediates cause damage to the enzyme through oxidative cleavage of bonds on adjacent amino acids 13 , an action which compromises signi cantly the e ciency of the enzyme 14 − 16 . An outstanding question about the mode of action of LPMOs is, therefore, what is the nature of this damage, and how does the enzyme mitigate/prevent it?…”

“…Moreover, rapid charge transfer associated with these pathways reduced the lifetimes of any species formed immediately after reactions with peroxides, thereby making their experimental observation challenging. Thus, in expectation that the lifetime of any reactive species is a function of the distance between the histidine brace and any redox-active amino acids 13 , we sought to take advantage of the fact that some structures of LPMOs exhibit the putative redox-active tryptophan at a distance of ca. 10.3 Å away from the active site, as opposed to the ca.…”

CbpD crystal structure adds intrigue to substrate-specificity motifs in chitin-active lytic polysaccharide monooxygenases

“…This includes a phenylalanine (Phe176) axial to the active site and a universally conserved alanine (Ala102) (Figure 3C) Forsberg, Røhr et al, 2014) that is believed to restrict coordination of cosubstrates in the solvent-exposed axial position and increase the likelihood an oxygen cosubstrate coordinated at the equatorial position will be activated for catalysis, thus conferring C1/C4 oxidizing specificity (Forsberg, Mackenzie et al, 2014;Borisova et al, 2015). A well conserved tryptophan (Trp167) among LPMOAA10s (Forsberg, Røhr et al, 2014;Meier et al, 2018) thought to protect the active site against oxidative inactivation during uncoupled turnovers (Loose et al, 2018;Paradisi et al, 2019;Gray & Winkler, 2021) is also present (Figures 3B,C).…”