Figure 168.
Conversion of metastable “active” oxidized intermediate OH (far left) to EH (far right) through the reduction of CuB or the cross-linked tyrosine.916 The numbering scheme for CcO isolated from T. thermophilus is used. Structures for proposed high potential CuB sites through changes in coordination number are proposed. (A) Dissociation of the coordinated hydroxide moiety leads to formation of a planar 3-coordinate CuI complex following protonation and reduction. (B) Deligation of H283 and H233 and addition of an electron leads to formation of a linear H282- CuI-OH moiety. (C) Dissociation of hydroxide and H283 gives a linear Nε coordinated H282-Cu-H233 Cub state following protonation and reduction. (D) A tautomeric change in H283 and H282 coordination and dissociation of H233 and hydroxide leads to formation of a N<5 ligated H283-Cu-H282 complex after proton and electron transfer. Recall (section 3.1) that for adjacent bis-histidine dipeptides and single-copper sites, formation of a linear 2-coordinate copper(I) structure is highly favored when both imidazole groups ligate via their Nδ nitrogen-atoms. The inspiration for proposals B and c derive from the computational studies of Noodleman and co-workers.916 See text for details.