Kreb`s Cycle

D
Krebs cycle
4.3 The link reaction and Krebs cycle as stages of aerobic respiration in the mitochondrion

The link reaction
The first stage in respiration, whether it’s aerobic or anaerobic, is glycolysis. This produces two molecules of ATP, two
molecules of reduced NAD and two molecules of pyruvate. It is these pyruvate molecules which we are interested in for
the next stage of aerobic respiration. The link reaction does not concern anaerobic respiration. The link reaction takes
pyruvate and involves dehydrogenation and decarboxylation to produce acetate.

(3C)
pyruvate


CO2 NAD (ox)
NADH2 (red)
Link reaction (2C)
acetyl coenzyme A


Coenzyme A
(2C)
acetate




(6C)
citrate
(4C)
oxaloacetate

CO2 NAD (ox)
NADH2 (red)
NADH2 (red)
NAD (ox)
5C acid
FADH2 (red)

FAD (ox) CO2
NAD (ox)
4C acid
NADH2 (red)
4C acid




ATP
ADP + Pi
During the link reaction, the pyruvate molecule must undergo decarboxylation and dehydrogenation. The enzymes
pyruvate decarboxylase and pyruvate dehydrogenase remove the carboxyl group (which will become a carbon dioxide
molecule) and the hydrogen atoms respectively from the pyruvate molecule.

The coenzyme NAD (nicotinamide adenine dinucleotide) accepts the two hydrogen atoms becoming reduced NADH2 and
the coenzyme-A accepts acetyl (from pyruvate) to become acetyl coenzyme A. The function of coenzyme-A is simply to
carry the acetyl from the link reaction to Krebs cycle.



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