454 IRON-CONTAINING PROTEINS AND ENZYMES
possibly leading to cytochrome c oxidase - type P or F transient intermediates
(Figures 7.40 and 7.42 ) relevant to the enzyme ’ s O 2 reduction pathway. Biome-
metic studies on Fe II – Cu I complexes and the reaction products of these species
with dioxygen are areas of continuing study. Functional modeling studies of
dioxygen - binding to model compounds using electrochemical approaches has
been reviewed by Collman et al.^168 The work of Karlin ’ s research group pre-
paring FeIII−−()O^22 − CuII products from heterobinuclear Fe II – Cu I complexes
has been discussed in some detail in this section. Naruta ’ s X - ray crystallo-
graphic structure (reference 155 ) showing peroxo side - on ( η^2 ) binding to the
heme ’ s Fe ion and end - on ( η^1 ) binding to the copper ion confi rms earlier
conclusions about the peroxo bonding modes garnered from spectroscopic
evidence. The question of whether this bonding mode exists in the enzyme
itself remains unanswered. Biomemetic studies of the his – tyr covalent cross-
link at the heme a 3 – Cu B center is a new exciting direction for cytochrome c
oxidase modeling not discussed here. A few references are included to start
the reader into this research area.^169 Finally, modeling studies of the Cu A dicop-
per center, which serves as a one - electron transfer conduit for many copper
oxidases (and nitrous oxide reductase), continue into the present time. The
mixed - valence delocalized Cu 1.5 · · · Cu 1.5 center with cysteine bridging ligands
has provided challenges for chemists synthesizing workable models that mimic
the enzyme ’ s structure and function. Better Cu A models are needed, especially
those that mimic the sulfur and nitrogen ligand coordination sphere of the
CcO enzyme like those of Tolman (reference 166 ) but that also have the short
Cu – Cu distance found in the native cytochrome c oxidase.
7.9 NON - HEME IRON - CONTAINING PROTEINS
7.9.1 Introduction,
The preceding sections of Chapter 7 have discussed iron - containing proteins
and enzymes having a porphyrin ring system. Section 7.9 presents a short
introduction to the many non - heme iron - containing proteins and enzymes.
Two of these are iron – sulfur proteins (Section 7.9.2) and iron – oxo proteins
(Section 7.9.3).
7.9.2 Proteins with Iron – Sulfur Clusters
One large class of non - heme iron - containing biomolecules involves proteins
and enzymes containing iron – sulfur clusters. Iron – sulfur clusters are described
in Sections 1.7 (Bioorganometallic Chemistry) and 1.8 (Electron Transfer) as
well as in Section 3.6 (M ö ssbauer Spectroscopy). See especially Table 3.2 and
the descriptive examples discussed in Section 3.6.4. Iron – sulfur proteins include
rubredoxins, ferrodoxins, and the enzymes aconitase and nitrogenase. The
nitrogenase enzyme was the subject of Chapter 6 in the fi rst edition of this
text — see especially Section 6.3 for a discussion of iron – sulfur clusters.^1 In this