The document summarizes the tricarboxylic acid (TCA) cycle, also known as the Krebs cycle or citric acid cycle. It discusses that the TCA cycle involves the oxidation of acetyl-CoA to carbon dioxide and water and is the final common pathway for carbohydrates, fats, and amino acids. The cycle occurs in the mitochondrial matrix and generates energy in the form of NADH and FADH2 that are used in the electron transport chain to produce ATP. Key enzymes and reactions in the cycle are described, including the generation of citrate, isocitrate, alpha-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate

1. TCA (krebs) Cycles
Ravish Yadav

2. TCA Cycle
•Also known as Krebs cycle
•TCA cycle essentially involves the oxidation of acetyl
CoA to CO2 and H2O.
•TCA cycle –the central metabolic pathway
•The TCA cycle is the final common oxidative
pathway for carbohydrates, fats, amino acids.

3. •TCA cycle supplies energy & also provides many
intermediates required for the synthesis of amino
acids, glucose, heme etc.
•TCA cycle is the most important central pathway
connecting almost all the individual metabolic
pathways.

4. •Definition
•Citric acid cycle or TCA cycle or tricarboxylic acid
cycle essentially involves the oxidation of acetyl
CoA to CO2 & H2O.
•Location of the TCA cycle
•Reactions of occur in mitochondrial matrix, in
close proximity to the ETC.

5. Reactions of TCA cycle
•Oxidative decarboxylation of pyruvate to acetyl
CoA by PDH complex.
•This step is connecting link between glycolysis and
TCA cycle.

6. An Overview of the Citric Acid Cycle
A four-carbon oxaloacetate condenses with a
two-carbon acetyl unit to yield a six-carbon
citrate.
An isomer of citrate is oxidatively
decarboxylated and five-carbon -
ketoglutarate is formed.
-ketoglutarate is oxidatively
decarboxylated to yield a four-carbon
succinate.
Oxaloacetate is then regenerated from
succinate.
Two carbon atoms (acetyl CoA) enter the
cycle and two carbon atoms leave the cycle
in the form of two molecules of carbon
dioxide.
Three hydride ions (six electrons) are
transferred to three molecules of NAD+, one
pair of hydrogen atoms (two electrons) is
transferred to one molecule of FAD.
The function of the citric acid
cycle is the harvesting of high-
energy electrons from acetyl CoA.

7. Pyruvate
Acetyl CoA
Citrate
Cis-Aconitase
Iso-citrate
Oxalosuccinate
ɑ-Ketoglutarate
Succinyl CoA
Succinate
Fumarate
Malate
Oxaloacatete
PDH
CO2, NADH + H+
NAD+
NADH + H+
NAD+
CO2, NADH + H+
NAD+
GDP+Pi
GTP
FADH2
FAD
- H2O
NADH + H+
NAD+
Citrate
synthase
Aconitase
Aconitase
SDH
Fumarase
TCA

8. Reactions of TCA Cycle
•Step:1 Formation of citrate
•Oxaloacetate condenses with acetyl CoA to form
Citrate, catalysed by the enzyme citrate synthase
•Inhibited by:
•ATP, NADH, Citrate - competitive inhibitor of
oxaloacetate.

9. Steps 2 & 3 Citrate is isomerized to
isocitrate
•Citrate is isomerized to isocitrate by the enzyme
aconitase
•This is achieved in a two stage reaction of
dehydration followed by hydration through the
formation of an intermediate -cis-aconiase

10. Steps 4 & 5 Formation of -ketoglutarate
• Isocitrate dehydrogenase (ICDH) catalyses the
conversion of (oxidative decarboxylation) of isocitrate
to oxalosuccinate & then to -ketoglutarate.
• The formation of NADH & the liberation of CO2
occure at this stage.
• Stimulated (cooperative) by isocitrate, NAD+, Mg2+,
ADP, Ca2+ (links with contraction).
• Inhibited by NADH & ATP

11. Step: 6 Conversion of -ketoglutarate to
succinyl CoA
•Occurs through oxidative decarboxylation,
catalysed by -ketoglutarate dehydrogenase
complex.
•-ketoglutarate dehydrogenase is an multienzyme
complex.
•At this stage of TCA cycle, second NADH is
produced & the second CO2 is liberated.

12. Step: 7 Formation of succinate
•Succinyl CoA is converted to succinate by succinate
thiokinase.
•This reaction is coupled with the phosphorylation
of GDP to GTP.
•This is a substrate level phosphorylation.
•GTP is converted to ATP by the enzyme nucleoside
diphosphate kinase.

13. Step: 8 Conversion of succinate to fumarate
•Succinate is oxidized by succinate dehydrogenase to
fumarate.
•This reaction results in the production of FADH2.
•Step: 9 Formation of malate: The enzyme fumarase
catalyses the conversion of fumarate to malate with
the addition of H2O.

14. Step:10 Conversion of malate to
oxaloacetate
•Malate is then oxidized to oxaloacetate by malate
dehydrogenase.
•The third & final synthesis of NADH occurs at this
stage.
•The oxaloacetate is regenerated which can combine
with another molecule of acetyl CoA & continue the
cycle.

15. Pyruvate
Acetyl CoA
Citrate
Cis-Aconitase
Iso-citrate
Oxalosuccinate
ɑ-Ketoglutarate
Succinyl CoA
Succinate
Fumarate
Malate
Oxaloacatete
PDH
CO2, NADH + H+
NAD+
NADH + H+
NAD+
CO2, NADH + H+
NAD+
GDP+Pi
GTP
FADH2
FAD
- H2O
NADH + H+
NAD+
Citrate
synthase
Aconitase
Aconitase
SDH
Fumarase
TCA

16. From: Summerlin LR (1981) Chemistry for the Life Sciences. New York: Random House p 550.

17. Regeneration of oxaloacetate
•The TCA cycle basically involves the oxidation of
acetyl CoA to CO2 with the simultaneous
regeneration of oxaloacetate.
•There is no net consumption of oxaloacetate or any
other intermediate in the cycle.

18. Significance of TCA cycle
• Complete oxidation of acetyl CoA.
• ATP generation.
• Final common oxidative pathway.
• Integration of major metabolic pathways.
• Fat is burned on the wick of carbohydrates.
• Excess carbohydrates are converted as neutral fat
• No net synthesis of carbohydrates from fat.
• Carbon skeleton of amino acids finally enter the TCA cycle.

19. Requirement of O2 by TCA cycle
•There is no direct participation of O2 in TCA cycle.
•Operates only under aerobic conditions.
•This is due to, NAD+ & FAD required for the
operation of the cycle can be regenerated in the
respiratory chain only in presence of O2.
•Therefore, citric acid cycle is strictly aerobic.

20. Energetics of TCA Cycle
•Oxidation of 3 NADH by ETC coupled with
oxidative phosphorylation results in the synthesis of
9ATP.
•FADH2 leads to the formation of 2ATP.
•One substrate level phosphorylation.
•Thus, a total of 12 ATP are produced from one acetyl
CoA.

23. NADH, ATP, succinyl
CoA, citrate
-
Regulation of the citric acid cycle

24. Regulation of TCA Cycle
•Three regulatory enzymes
1. Citrate synthase
2. Isocitrate dehydrogenase
3.α-ketoglutarate dehydrogenase

25. •Citrate synthase is inhibited by ATP, NADH, acyl
CoA & succinyl CoA.
•Isocitrate dehydrogenase is activated by ADP &
inhibited by ATP and NADH
•α-ketoglutarate dehydrogenase is inhibited by
succinyl CoA & NADH.
•Availability of ADP is very important for TCA cycle
to proceed.

26. Inhibitors of TCA Cycle
•Aconitase is inhibited by fluoro-acetate.
•This is a non-competitive inhibition.
•Alpha ketoglutarate is inhibited by Arsenite.
•This is also a non-competitive.
•Succinate dehydrogenase is inhibited by malonate.
•This is competitive inhibition.

27. Amphibolic nature of the TCA cycle
•TCA cycle is both catabolic & anabolic in nature,
called as amphibolic.
•Since various compounds enter into or leave from
TCA cycle, it is sometimes called as metabolic traffic
circle.

28. Important anabolic reactions of TCA cycle
•Oxaloacetate is precursor for aspartate.
•α-ketoglutarate can be transaminated to
glutamate.
•Succinyl CoA is used for synthesis of heme.
•Mitochondrial citrate is transported to cytoplasm
& it is cleaved into acetyl CoA to provide substrate
for fatty acid synthesis.

29. Anaplerosis or anaplerotic reactions
•The reactions concerned to replenish or to fill up
the intermediates of citric acid cycle are called
anaplerotic reactions or Anaplerosis

30. Krebs Cycle is a Source of Biosynthetic Precursors
Phosphoenol-
pyruvate
Glucose
The citric acid cycle
provides
intermediates for
biosyntheses

31. Important anaplerotic reactions
•Pyruvate carboxylase catalyses conversion of
pyruvate to oxaloacetate.
•This is an ATP dependent carboxylation reaction.
Pyruvate+CO2+ATP Oxaloacetate + ADP + Pi

32. •Pyruvate is converted to malate by NADP+
dependent malate dehydrogenase (malic enzyme).
Pyruvate + CO2 + NADPH + H+ malate
+ NADPH + H2O

33. •α- ketoglutarate can also be synthesized from
glutamate by glutamate dehydrogenase.
Glutamate + NAD(P) + H2O α-
ketoglutarate +NAD(P)H + H+ + NH4
+

34. Transamination
•Transamination is a process where an amino acid
transfers its amino group to a keto group and
itself gets converted to a keto acid.
•The formation of Alpha ketoglutarate &
oxaloacetate occures by this mechanism.

35. The Glyoxylate Cycle
A variant of TCA for plants and bacteria
•Acetate-based growth - net synthesis of
carbohydrates and other intermediates from
acetate - is not possible with TCA
•Glyoxylate cycle offers a solution for plants and
some bacteria and algae
•The CO2-evolving steps are bypassed and an extra
acetate is utilized
•Isocitrate lyase and malate synthase are the
short-circuiting enzymes

36. Glyoxylate cycle
shares some
enzymes with
citric acid cycle
p.624

38. Glyoxylate Cycle
•Isocitrate lyase produces glyoxylate and
succinate
•Malate synthase does a Claisen condensation of
acetyl-CoA and the aldehyde group of
glyoxylate - classic CoA chemistry!
•The glyoxylate cycle helps plants grow in the
dark!
•Glyoxysomes borrow three reactions from
mitochondria: succinate to oxaloacetate