Kinetic and spectroscopic investigations of copper metalloenzymes : mechanisms of oxygen activation in mono and tri-nuclear active sites
Keywords: Chemistry.

Nature uses a wide variety of Cu metalloenzymes to achieve biologically important oxidase and oxygenase reactions. Two such classes of these Cu enzymes are the multicopper oxidases (MCOs) and the lytic polysaccharide monooxygenases (LPMOs). The MCOs couple the four electron oxidation of a range of substrates to the four electron reduction of oxygen to water. They contain a minimum of four Cu ions arranged as a mononuclear Type 1 (T1) Cu site, and a trinuclear Cu cluster (TNC). The TNC is itself composed of a mononuclear Type 2 (T2) Cu site and a binuclear Type 3 (T3) Cu site. The LPMOs utilize a mononuclear Cu active site with an unusual T-shaped ligand geometry known as the His brace for the oxidative cleavage of polysaccharides in carbohydrate degradation by fungi and bacteria. The T1 sites of MCOs span a range from 400-800 mV (vs NHE), grouped into high and low potential MCOs. In both groups, the typical resting form of the enzyme is the Resting Oxidized (RO) form where all four Cu's are fully oxidized (4xCu(II)). In some high potential MCOs, which are important for their role as cathode materials in biofuel cells, there exists an Alternative Resting (AR) form, which is instead a stable, partially reduced form of the enzyme (2xCu(II), 2xCu(I)). The AR form of the TNC is incapable of accepting electrons from the T1 Cu, which is the typical mechanism for the enzyme to enter the catalytic cycle. We have characterized the electron-accepting behavior of the RO and AR forms in a fungal laccase from Posospora anserina (PaL). Selective reduction of the T1 Cu(II) enabled spectroscopic characterization of the singly oxidized Cu of the AR TNC, identifying it as a half-reduced T3 species. This AR form of the TNC was also shown to be generated by outersphere oxidation of a fully reduced TNC via the high potential T1 site, but could not be similarly generated in an enzyme with a low potential T1 site. A reduction titration was evaluated computationally, revealing a mechanism where the protein constraints play an important role in guiding the reduction behavior of the T3 site and avoiding formation of a thermodynamically favored (but inactivated) form of the TNC. In the MCO catalytic mechanism the first 2 electron reduction step of the mechanism forms the Peroxide Intermediate (PI) while the second generates the fully oxidized (4xCu(II)) Native Intermediate (NI). Previous work on the low potential laccase from Rhus vernicifera (RvL) has established that the NI is then rapidly reduced back to the FR form where it can react with another molecule of oxygen, continuing the catalytic cycle. The NI form, rather than RO, is therefore the catalytically relevant fully oxidized form of the enzyme responsible for rapid turnover. In the fungal laccase from Trametes versicolor (TvL), the impact of the increased T1 reduction potential was evaluated in the context of the accepted MCO mechanism. Rapid formation of NI was characterized, followed by its slow decay to the RO form at a rate similar to that observed in RvL. The measured rate of IET to the NI in TvL confimred that NI reduction does occur in the catalytic cycle of the high potential MCOs. However, in contrast to the findings in RvL where the rate determining step is substrate oxidation, the rate determining step in TvL was found to be IET to the NI TNC. MCOs can be broadly divided into two groups based on substrate selectivity: the organic oxidases and the metallooxidases. The organic oxidases utilize organic substrates and have higher substrate turnover frequencies (TOFs). The metallooxidases are selective for transition metal ions and exhibit lower TOFs. To understand the nature of the slow turnover in the metallooxidases, the MCO Fet3p from Saccharomyces cerevisiae was studied to determine the factors responsible for its slow ferroxidase reactivity. The catalytic cycle was found not to proceed through the reduction of NI, but rather via rapid, pH-dependent decay of NI to the RO form. The reduction of RO was therefore found to be the rate limiting process in the catalytic cycle. Constructing a model for RO reduction revealed that slow turnover in this metallooxidase was functionally significant for efficient Fe metabolism while avoiding generation of reactive oxygen species. The LPMOs are a class of enzymes containing a mononuclear Cu active site capable of utilizing oxygen for hydroxylation of C--H bonds in polysaccharide. Many LPMO sequences also contain additional carbohydrate binding modules (CBMs), which are small domains attached to the core catalytic domain by flexible linkers. The enzyme HjLPMO9A contains a CBM1 domain. A variant lacking this CBM was expressed for structural and spectroscopic characterization. Crystal structures of the truncated enzyme revealed the importance of a peptide linker on the structure of the core catalytic domain. Further analysis confirmed that the CBM was important for increasing binding affinity for substrate-protein interactions, but did not impact the active site structure or the regioselectivity of the monooxygenase reaction. Although initially proposed to utilize oxygen directly in the C--H hydroxylation reaction, LPMOs have recently been found to be catalytically competent for regioselective hydroxylation with hydrogen peroxide as cosubstrate. Several mechanisms have been proposed for this reactivity, but a dearth of experimental data persists in the literature. We have therefore undertaken a kinetic analysis and have identified two amino acid radical intermediates coupled to the Cu(II) that form and decay rapidly during single turnover of the reduced enzyme with peroxide. Importantly, these intermediates represent a minor reaction pathway, with the majority of the Cu(I) reaction proceeding through homolytic O-O cleavage. Finally, the use of non-native organic peroxides as cosubstrates excludes the possibility of a ping-pong mechanism. This provides important insight into pathways of LPMO catalysis