BCHM2972
2006
Lecture 6
CYTOCHROME C OXIDASE
Revision: Fill in the blanks.
Cytochrome c oxidase (Complex IV) catalyses the final irreversible reduction in the
electron transport chain. Overall, the reaction is ________________(insert equation)
and involves _____ (how many?) electrons (passed _____ (how many?) at a time)
from reduced __________ to cytochrome c oxidase to complete the _______(how
many?) -electron (oxidation/reduction?) of one _________ molecule.
Complex IV is composed of multiple subunits and contains _______(how many?)
redox centers. Two redox centers, haem a and a pair of _______ atoms known as the
____ center, are on subunit II and are the first to accept electrons from ________.
Electrons are then passed to the next two redox centers , CuB and _______, which are
very closely associated and form the ________ on subunit I.
To start the cycle, _____ (how many?) electrons are passed from cytochrome C to
reduce haem and-CuA. (so then cytochrome c has become oxidised) These electrons
are then passed to the oxidised binuclear center [Fe3+—Cu2+] to fully
(reduce/oxidise?) it  [Fe2+—Cu+]. At the same time, ______ from the mitochondrial
matrix and a _______ group from the binuclear center form the first _____ molecule
produced in the electron transport chain. The _______ gets regenerated later in the
cycle. Then a ________ molecule binds, shared by the Fe and Cu in the _______.
This arrangement ensures that reactive oxygen that will be produced as an
intermediate in the reaction does not get released to cause trouble!
The reduction of O2  O22-  ? H2O requires the nearly simultaneous
rearrangement of _____ (how many?) electrons. However, the fully reduced [Fe2+—
Cu+] binuclear complex can readily donate only _____ (how many?) electrons to its
bound O2 before reaching its fully, fully oxidized [Fe4+—Cu2+] state. (Don’t get
confused, the [Fe3+—Cu2+] form of the binuclear center at the start of the cycle is
also considered to be fully oxidised!).
The (1st /2nd /3rd /4th?) electron required to (reduce/oxidise?) O2  O22- is temporarily
donated (with a _______) by the phenolic –OH on a _______ residue that is located
close to the binuclear center. Remember, the other _____ (how many?) electrons were
provided by metal ions in the binuclear complex: reduced Fe2+ (accepts/donates?)
____ (how many?) electrons Fe4+ and Cu + (accepts/donates?) _____ (how many?)
electrons  Cu2+. These _____ (how many?) electrons are given to oxygen. (at this
point in the cycle, only _____ (how many?) electrons have entered the complex).
This redistribution of electrons yields the ‘____________’ complex.
A (1st /2nd /3rd /4th?) electron (transferred from cytochrome c via haem a and CuA) and
_____ (how many?) protons from the matrix complete the (reduction/ oxidation?) of
oxygen, releasing the second ______ molecule and regenerating the ________
(paying back the debt from earlier when it participated in the electron rearrangement).
A ?th and final electron transfer (and proton from the matrix) yields the oxidized
[Fe3+ —Cu2+] complex, and regenerates the ________, thereby completing the
catalytic cycle.
Note that in the (first/second?) H2O, the O and one of the Hs came from the hydroxyl
(OH) which is later regenerated by one of the oxygen atoms from the incoming O2
while the other the oxygen atom from the incoming O2 leaves as _________?
So, _____ (how many?) protons have come from the matrix to form _____ (how
many?) H2O (one proton hangs out in the hydroxyl group of the binuclear center for a
while!) and _____ (how many?)
protons are meanwhile pumped from
_______(where?) to form the ____________ which provides energy for oxidative
phosphorylation.