PS8 - Recommended reading: 24, 27.1-27.7 Ch 102 Problem Set...

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1 Recommended reading: 24 , 27.1-27.7 Ch 102 Problem Set 8 Due: Tuesday, May 31, before class Problem 1 (15 points) Laccase is part of an important class of multi-copper proteins found in plants and fungi, and which catalyze the synthesis (in plants) and degradation (in fungi) of lignin coupled to reduction of dioxygen to water. Shown below is the X-ray diffraction crystal structure of the fungal laccase from Trametes hirsuta . Figure 1. Structure of fungal laccase ( PDB: 3PXL ). Cu atoms shown as bronze spheres. Laccase contains four copper atoms in two distinct locations. Three Cu atoms, bound by histidine residues, are found in a triangular arrangement. A fourth Cu atom (bound by two histidines and one cysteine, and labeled as Cu-S Cys ) is located ~13 Å away from the tricopper active site. Shown below is a schematic representation of the catalytic cycle involving the full tetracopper moiety for the reaction with dioxygen and substrate. Assume that the organic substrates (ArO - ) bind to the protein but do not coordinate to copper. Cu I S N N Cu I Cu I Cu I Cys His His H O H Cu I S N N Cu II Cu II Cu I Cys His His O O Cu II S N N Cu II Cu II Cu II Cys His His HO HO 4 4 Reduced Oxidized H 2 O Ar O Ar O O 2 Cu II S N N II Cu II Cu II Cys His His O 2 H + H 2 O 2 H + H 2 O H 2 O Intermediate Ia Intermediate IIa Scheme 1. Catalytic cycle of laccase
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2 a) Write the balanced chemical equation for the overall catalytic reaction. How many protons and electrons are formally transferred in this reaction? b) Label the O 2 -derived oxygen ligands from the above scheme as dioxygen adduct, superoxide, (hydro)peroxide, hydroxide, or oxide. c) How many electron-transfer processes take place in the reaction from the Intermediate Ia state of the enzyme to the Intermediate IIa state? Are these electron-transfers inner-sphere or outer-sphere?
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PS8 - Recommended reading: 24, 27.1-27.7 Ch 102 Problem Set...

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