The tricarboxylic acid cycle (TCA cycle also known as the citric acid cycle or Krebs cycle) is a cyclic metabolic path- way in the mitochondrial matrix. In eight-steps, it oxidizes acetyl residues (CH3–CO–) to carbon dioxide (CO2).
Reactions of Tricarboxylic Acid Cycle
The Krebs cycle starts with reaction between the acetyl moiety of acetyl-CoA and the four-carbon dicarboxylic acid oxaloacetate, forming a six-carbon tricarboxylic acid (citrate). In the subsequent reactions, two molecules of CO2 are released and oxaloacetate is regenerated (Fig. 6.1).
Significance
• Complete oxidation of acetyl-CoA • Adenosine triphosphate (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 skeletons of amino acids finally enter citric acid cycle
• Amphibolic pathway • Anaplerotic role.
Complete Oxidation of Acetyl-CoA
Carbon Dioxide Removal Steps
During the citric acid cycle, two carbon dioxide molecules are removed in the following reactions (Table 6.1):
• The two carbon atoms of acetyl-CoA are removed as CO2 in steps 3 and 4
• Net result is that acetyl-CoA is completely oxidized dur- ing one turn of cycle.
ATP Generating Steps in TCA Cycle
Importance
Alpha-ketoglutarate dehydrogenase reaction is the only one irreversible step in the cycle. Free energy changes of the reactions are such that the cycle will operate spontane- ously in the clockwise direction (Table 6.2).
Final Common Oxidative Pathway
Citric acid cycle may be considered as the final common oxidative pathway of all foodstuffs and all the major ingre- dient of foodstuffs are finally oxidized through TCA cycle.
Integration of Major Metabolic Pathways
1. Carbohydrates are metabolized through glycolytic pathway to pyruvate and then converted to acetyl- CoA, which enters the citric cycle.
2. Fatty acids through beta-oxidation are broken down to acetyl-CoA and then enter this cycle.
3. Glucogenic amino acids after transamination enter at some points in this cycle. Ketogenic amino acids are converted into acetyl-CoA.
4. The integration of metabolism is achieved at junction points by key metabolites. Ketogenic amino acids are converted into acetyl-CoA. Several pathways can con- verge at this point with the result that carbon atoms from one source can be used for synthesis of another. Impor- tant intermediates are acetyl-CoA and oxaloacetate.
Carbohydrates are Required for Oxidation of Fats
In the body, oxidation of fat (acetyl-CoA) needs the help of oxaloacetate and oxidizes acetyl-CoA to two CO2 mol- ecules. Here, oxaloacetate acts as a true catalyst. The major source of oxaloacetate is pyruvate (carbohydrate). Hence, carbohydrates are absolutely required for oxidation of fats.
Excess Carbohydrates are Converted to Neutral Fat
Excess calories are deposited as fat in adipose tissue. The pathway is glucose to pyruvate to acetyl-CoA to fatty acid. However, fat cannot be converted to glucose because pyru- vate dehydrogenase reaction (pyruvate to acetyl-CoA) is an absolutely irreversible step.
Table 6.1: Carbon dioxide removal steps
Step No Reaction Enzyme involved
3 Isocitrate to a-ketoglutarate Isocitrate dehydrogenase 4 a-ketoglutarate to succinyl-CoA a-ketoglutarate dehydrogenase
Table 6.2: ATP generating steps
Step No Reactions Coenzyme ATPs
3 Isocitrate to a-ketoglutarate NADH* 2.5
4 a-ketoglutarate to succinyl-CoA NADH 2.5 5 Succinyl-CoA to succinate GTP† (substrate level phosphorylation) 1
6 Succinate to fumarate FADH2‡ 1.5
8 Malate to oxaloacetate NADH 2.5 Total 10
*NADH, nicotinamide adenine dinucleotide; †GTP, guanosine triphosphate; ‡FADH
Chapter 6: Cellular Energetics 67
Amino Acids Finally Enter the TCA
The acetyl-CoA molecules either enter the TCA cycle and are completely oxidized or are channeled to ketone body formation. Hence, they are called ketogenic amino acids. Glucogenic amino acids also get converted to intermedi- ates of TCA cycle.
Amphibolic Pathway
The tricarboxylic acid cycle has both catabolic and ana- bolic functions—it is amphibolic.
As a Catabolic pathway, it initiates the ‘terminal oxida-
tion’ of energy substrates. Many catabolic pathways lead to
intermediates of the tricarboxylic acid cycle or supply me- tabolites such as pyruvate and acetyl-CoA that can enter the cycle where their C atoms are oxidized to CO2.
The tricarboxylic acid cycle also supplies important precursors for anabolic pathways (Fig. 6.2).
Intermediates in the cycle are converted into:
• Glucose (gluconeogenesis, precursors oxaloacetate and malate)
• Porphyrins • Amino acids
• Fatty acids and isoprenoids.
After the oxidation of acetyl-CoA to CO2, they are con- stantly regenerated and their concentrations therefore re- main constant, averaged over time.
Anabolic pathways, which remove intermediates of the cycle (e.g. gluconeogenesis) would quickly use up the
Fig. 6.2: Anabolic pathway
small quantities present in the mitochondria, if metabo- lites did not re-enter the cycle at other sites to replace the compounds consumed.
Processes that replenish the cycle in this way are called
anaplerotic reactions. The degradation of most amino ac-
ids is anaplerotic because it produces either intermediates of the cycle or pyruvate (glucogenic amino acids). A par- ticularly important anaplerotic step in animal metabolism leads from pyruvate to oxaloacetic acid. This ATP-depen- dent reaction is catalyzed by pyruvate.
SUMMARY
• Composed of eight reactions
• Four carbon intermediates are regenerated • Two molecules of CO2 released (6C 4C)
• Most of energy stored as nicotinamide adenine di- nudeotide (NADH) and QH2.
Net reaction for citric acid cycle
Acetyl-CoA + 3NAD+ + Q + GDP (ADP) + P
1 + 2H2O
HS-CoA + 3NADH + QH2 + GTP (ATP) + 2CO2 + 2H+