Download Krebs Cycle

Krebs Cycle: oxidative decarboxylation of the C2 Acetyl group (CH3CO). This cycle has been broken down
into 4 steps. The carbons from the original glucose molecule are shown in purple and those of
mitochondria molecules in blue.

Acetyl CoA joins with the C4(acceptor)group

CoA is released to transport more pyruvate into the matrix

A C6 fragment is formed (citric acid)

C6 (Citric Acid) is oxidatively decarboxylated.

A C5 group is formed.

The Carbon is given off as Carbon Dioxide

NAD+ is reduced to NADH + H+

The C5 fragment is oxidised and decarboxylated further to a C4
compound.

Again the carbon removed forms carbon dioxide.

NAD+ is further reduced to NADH + H+.

The final stage in the cycle has the C4 acceptor regenerated.

There is a reduction of NAD+ to NADH + H+.

FAD (Coenzyme) is reduced to FADH2 .

ADP is reduced to ATP
The krebs cycle is an example of the metabolic cycles mentioned in section 7.6.1 . Each step in the cycle
requires enzymes to reduce the activation energy. The reactions all take place in the matrix of the
mitochondria and are usually represented as a circular diagram. Try to overcome the idea that the
molecules are going in a circle but more that this reaction is taking place within the confined space of the
matrix where each intermediate becomes the substrate for the next step.
(a) Pyruvate (3C)
(b) Link reaction
(c) C4 + C2= C6
(d) Recycling of CoA
(e) Decarboxylation
C6 to C5 and the
reduction of NAD
(f) Decarboxylation
C5 to C4 and the
reduction of NAD
(g) C4 to C4 with the
reduction of
coenzymes FAD and
NAD. ATP is made
directly.
(h) C4 to C4 acceptor
This cycle follows one acetyl group.
Each glucose that enters glycolysis will produce 2 acetyl groups.