Glycolysis occurs in all animals (even bacteria). All of glycolysis occurs in the cytosol. The end
product of glycolysis is pyruvate (this occurs in the cytosol). Then, pyruvate is oxidized in the
mitochondrial matrix to acetyl-coenzyme A.
Acetyl-CoA: C-S is a “high-energy bond”, so acetyl-CoA is an energy carrier like ATP; it allows
transfer of that 2-carbon group to another molecule at the start of the citric acid cycle. This “high-
energy bond” can again be generated by the large amount of free energy released by the
pyruvate/NAD+ redox reaction. Acetyl-CoA is a key molecule in metabolism, because sugars, fats
and many amino acids are all broken down to acetyl-CoA, and acetyl-CoA also serves as the
building block for making fats and other metabolites.
Glycolysis occurs in the cytoplasm. The pyruvate move to the mitochondria matrix where it gets
oxidized and then goes into the citric acid cycle (crebbs cycle) in the mitochondria matrix still.
Citric acid cycle: It is there to finish the oxidation of glucose by converting the 2- carbon compound
acetyl-CoA to 2 CO2. C2 added in first reaction, 2 CO2 split off in following oxidations. All of these
reactions (they are catalyzed by dehydrogenases) are redox reactions. They result in reduced electron
carriers (FADH2 and NADH). The only reaction that is not redox is 1 GTP(ATP): this is the only step that’s
not a redox reaction. When we oxidize Fumarate, we use FAD instead of NAD, because oxidation of succinate does not release
enough energy to reduce NAD (see table of redox potentials). If we used NAD the electrons would flow
away from NAD to Fumarate (from fumarate to succinate instead of the opposite). When we use FAD,
the electrons go from succinate to FAD. The concentration of fumarate is very low.
We finish the citric Acid Cycle with 3 NADH & 1 FADH (reduced electron carriers), and 1 GTP (ATP). Glycolysis, pyruvate oxidation, and the citric cycle are generally very favourable. We end up with a lot of
electron carriers (NADH and FADH). The cell does not need them so it converts NADH to ATP (in the
electron transport chain).
The next step, the electron transport chain (ETC), is required to get rid of all the NADH, because a cell
needs much more ATP than NADH, and the cell needs NAD+ for the citric acid cycle to continue. The
electron transport chain occurs in the inner membrane of the mitochondrion (membranes of cristae).
The electron transport chain is a series of redox reactions coupled to the transport of protons across the
inner mitochondrial membrane into the intermembrane space. Membrane proteins help the oxidation
NADH binds to complex 1, there is a shape change in complex 1 so it oxidizes the electron and transports
the protons across the membrane (against the concentration gradient). Because FADH carrries less
energie than NADH, it gives its electron to complex 2. Complex 2 cannot oxidize the electron and pass
the protons across the membrane so it passes the electron to a hydrophobic carrier (ubiquinone) that
passes it on to the complex 3. Complex 3 oxidizes it and passes the protons across the membrane.
Another hydrophobic carrier(cytochrome c) moves to complex IV. At complex 4, the oxygen waits until
all 4 electrons are accepted. Then there are redox reactions that allow the transport of more protons
across the membrane. Ubiquinone and cytochrome c are hydrophobic electron carriers shuttling electrons from one complex
to the next by diffusing laterally within the membrane.
Why doesn’t NADH deliver its electrons directly to O2? Because these reactions need to be catalyzed;
this ensures gradual release of energy that is used to transport H+ across the membrane in each
Redox reaction releases the necessary energy for conformation change required for H+ transport across
Oxygen is found in the cytochrome c oxidase protein, in a cofactor (non-amino acid components). There
are two heme groups (cofactors) and oxygen is in one of them. Oxygen held in place by a heme group in
complex IV until all four electrons are accepted. this is important because intermediate stages are highly
reactive radicals with free electrons. If cyanide binds to the active site, it irreversibly binds the active site instead of O2, therefore the
electron transport chain is shut down.
The electrochemical proton gradient induces protons to move back across the