CYTOCHROME c OX IDASE 449
models may have analogies to CcO enzyme behavior, because the basicity of
O 2 - derived intermediates may be critical to proton translocation in the enzyme.
Finally, in this section we discuss the model compounds for the Type III
binuclear Cu A center. Electron transfer (eT) to and from Cu A is remarkably
fast, considering the distances to be traveled. Intermolecular eT from cyto-
chrome c→ Cu A , over a distance of ∼ 18 – 20 Å , has a rate constant, k , equal to
6 × 10^4 s − 1 , while intramolecular eT from Cu A → heme a, also over an ∼ 18 - to
20 - Å distance, has a rate constant of 1 × 10^4 s − 1. The Cu A center exists in two
redox states: the reduced form with two cuprous ions (Cu I · · · Cu I ) and the fully
delocalized mixed valent center (Cu II · · · Cu I ↔ Cu I · · · Cu II = Cu 1.5 · · · Cu 1.5 ). In the
Figure 7.47 Acid – base behavior of (A) [(^6 L)Fe III – O – Cu II ] + versus (B) [(^5 L)Fe III – O –
Cu II ] + as described in reference 159.
N NN FeIIIOArFNNCuIIN
N(O)NFF(^6) Ar
(^6) L
F
- N N
N FeIII
O
ArF
N
N
CuII
N
N
(O)
N
F
F
Ar
5
F
[(^6 L)FeIII-O-CuII]+
Fe - O = 1.75 Å, Cu - O = 1.84 Å
Fe.. .Cu = 3.58 Å,
Fe - O - Cu angle = 171o
[(^5 L)FeIII-O-CuII]+
Fe - O = 1.77 Å, Cu - O = 1.84 Å
Fe.. .Cu = 3.40 A,
Fe - O - Cu angle = 141o
N N
N FeIII
O
ArF
N
N
CuII
N
N
OH
N
F
F
ArF
2+
H+
H+
B
A
[(^6 L)FeIII-OH-CuII]2+
broken bridge
(^5) L