The citric acid cycle (TCA cycle) occurs in the mitochondria and involves a series of reactions that oxidize acetyl groups from acetyl-CoA derived from carbohydrates, fats, and proteins, releasing carbon dioxide and reducing equivalents (NADH and FADH2) that are used to generate ATP through oxidative phosphorylation. The TCA cycle produces two GTP/ATP molecules per acetyl-CoA molecule oxidized and feeds reduced electron carriers into the electron transport chain to produce additional ATP. It is also an amphibolic pathway that generates precursors for various biosynthetic pathways.
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Citric Acid Cycle
1.
2. Other names-
Krebs cycle,
Tricarboxylic acid cycle
Definition-
It is a series of reactions in mitochondria that oxidize
acetyl residues (as acetyl-CoA) librating reducing
equivalents, which upon oxidation through ETC
generate ATP.
Site-
Mitochondrial matrix
6. Energetic of Glycolysis & Citric Acid Cycle
Energy yield (ATP)/glucose in
glycolysis under aerobic
condition.
Rea. no.1(Hexokinase) -1ATP
Rea. 3 (PFK) -1ATP
Rea. 6 (G 3-PDH) +2.5x2=5
Rea. 7 (BPGK) + 1x2=2
Rea. 10 (PK) + 1x2=2
Total=9-2=7ATP
Energy yield (ATP)/glucose
in TCA.
Isocitrate DH 2.5x2=5ATP
Ketoglutarate DH 2.5x2=5ATP
S. thiokinase 1x2=2ATP
S. DH 1.5x2=3ATP
Malate DH 2.5x2=5ATP
Total=20ATP
Net ATP generation in glycolysis 9-2 =7ATP
ATP generation in PDH 2.5x2=5ATP
ATP generation in Citric acid cycle 20ATP
Net generation of ATP from one glucose mol. 32ATP
7. The primary function is to provide energy (ATP).
It is the final common pathway for the oxidation
of carbohydrate, lipids, and protein via acetyl CoA
or intermediates of the cycle.
Citric acid cycle is an amphibolic process i.e, it
plays role in both oxidative (catabolic) and
synthetic (anabolic) processes. E.g….
Gluconeogenesis
Transamination
Fatty acid synthesis and
Porphyrin synthesis.
Significance of Citric Acid Cycle
9. Certain intermediates of TCA, can be removed from
cycle for the synthesis of many compounds
Succinate for haem (Porphyrin) synthesis.
α-ketoglutarate for glutamate, GABA.
α-ketoglutarate & oxaloacetate for many amino acids.
Because this rate of TCA would expected to decline.
However, intermediates of TCA can be replenished
again by other reactions.
The special enzyme reaction by which the pool of
TCA intermediates can be replenished are called
anaplerotic reactions.
Anaplerotic reactions of Citric Acid Cycle
10. Acetyl-CoA
Anaplerotic
reactions of
Citric Acid Cycle
Oxaloacetate
Citrate
α-Ketoglutarate
Succinyl-CoA
Malate
Fumarate
AspartatePyruvate
Glucogenic
AA
Pyruvate
Valine
isoleucine
methionine
threonine
propionyl CoA
Glutamate