The Krebs Cycle
Uploaded by
filzah haziqahThe Krebs Cycle
Uploaded by
filzah haziqahThe Krebs cycle is a closed-loop set of reactions in eight steps:
1. The two-carbon acetyl CoA is combined with a four-carbon oxaloacetic acid and
hydrolyzed to produce a six-carbon compound called citric acid or citrate.
2. Citrate is then converted into isocitrate, a six-carbon isomer of citrate by
dehydrating and then hydrating the molecule to reshuffle its structure.
3. Isocitrate is oxidized and decarboxylation occurs with a carbon dioxide molecule
released. The coenzyme NAD+ is reduced to form another dinucleotide, NADH.
With the removal of the carbon molecule, the five-carbon molecule α-
ketoglutarate is produced.
4. The α-ketoglutarate molecule is oxidized, NAD+ is reduced to form NADH and
another carbon molecule is released. The four-carbon molecule produced
combines with Coenzyme A, forming the unstable succinyl CoA compound.
5. A phosphate group replaces the Coenzyme A in succinyl CoA, which is then
transferred to ADP (adenosine diphosphate) to form ATP. The transference of
the phosphate groups occurs between GDP (guanosine diphosphate) to form
GTP (guanosine triphosphate) in some cells. The four-carbon molecule that
remains is called succinate.
The remaining steps of the Krebs cycle regenerate oxaloacetic acid from succinate:
1. Succinate is oxidized to form the four-carbon molecule called fumerate. The
electron carrier FAD (flavin adenine dinucleotide), is reduced to FADH2 by the
transference of two hydrogen atoms.
2. Fumerate is converted into the four-carbon molecule called malate by the
addition of a water molecule.
3. The original reactant oxaloacetic acid is regenerated by the oxidation of malate.
The coenzyme NAD (nicotinamide adenine dinucleotide) is reduced to NADH by
the transference of one hydrogen atom
Products and Functions of the Krebs Cycle
For one cycle, two molecules of carbon, three molecules of NADH, one molecule of
FADH2 and one molecule of ATP or GTP are produced. Each glucose molecule
produces two molecules of acetyl CoA, enough for two cycles. These products can be
multiplied by two to produce the per-glucose yield. Though only one ATP (or GTP) is
produced directly per cycle, the products NADH and FADH2, can produce ATP (or
GTP) in a further process of cellular respiration called oxidative phosphorylation.
1
The main function of the Krebs cycle is to produce energy, stored and transported as
ATP or GTP. The cycle is also central to other biosynthetic reactions where the
intermediates produced are required to make other molecules, such as amino acids,
nucleotide bases and cholesterol. The Krebs cycle is found in all cells that use oxygen.
Combined with the process of oxidative phosphorylation, the Krebs cycle produces the
majority of energy used by aerobic cells with the percentage energy provided for
humans being greater than 95%.