The Krebs Cycle (Citric Acid Cycle)
The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a central metabolic pathway occurring in the mitochondrial matrix that oxidizes acetyl-CoA to generate ATP, NADH, FADH2, and CO2 while serving as a metabolic hub connecting carbohydrate, fat, and protein metabolism. 1
Core Function and Location
The Krebs cycle takes place in the mitochondrial matrix of eukaryotic cells and represents the final common pathway for aerobic metabolism of carbohydrates, fatty acids, and amino acids 1. This cycle is central to cellular energy homeostasis and serves multiple metabolic functions beyond simple energy production 1, 2.
Key Metabolic Reactions
The cycle operates through a series of enzymatic reactions that:
- Oxidize acetyl-CoA (2-carbon unit) completely to CO2 1
- Generate reduction equivalents (NADH and FADH2) that feed into the electron transport chain for ATP synthesis 3, 4
- Produce GTP/ATP directly through substrate-level phosphorylation 4
- Generate biosynthetic precursors for amino acids, nucleotides, and other cellular components 1
Metabolic Intermediates and Their Roles
The cycle involves several key intermediates including citrate, α-ketoglutarate (AKG), succinate, fumarate, malate, and oxaloacetate 1. Citrate is a tricarboxylic acid intermediate that can be metabolized to yield energy and bicarbonate (0.59 kcal/mmol and 3 mmol of bicarbonate/mmol of citrate) 1. Importantly, citrate does not require insulin to enter cells, distinguishing it from glucose metabolism 1.
Integration with Other Metabolic Pathways
Connection to Glycolysis
The cycle receives acetyl-CoA primarily from pyruvate (the end product of glycolysis) through pyruvate dehydrogenase (PDH) 1. This represents the major link between glucose metabolism and the TCA cycle 1.
Glutamine Metabolism
Glutamine enters the cycle as α-ketoglutarate through glutaminolysis, providing an alternative carbon source 1. The cycle can operate in both oxidative (forward) and reductive (reverse) directions depending on cellular needs 1:
- Oxidative direction: Glutamine → AKG → succinate → fumarate → malate → oxaloacetate, generating NADH 1
- Reductive carboxylation: AKG → citrate (reverse direction), important for lipogenesis in cancer cells 1
Anaplerotic Reactions
Pyruvate carboxylase converts pyruvate to oxaloacetate, replenishing cycle intermediates that are withdrawn for biosynthesis 1. This anaplerotic reaction is critical for maintaining cycle function 1.
Energy Production
The complete oxidation of one acetyl-CoA through the Krebs cycle generates approximately 10 ATP equivalents when coupled to oxidative phosphorylation 4:
- 3 NADH (yielding ~7.5 ATP)
- 1 FADH2 (yielding ~1.5 ATP)
- 1 GTP/ATP (direct)
The cycle is tightly coupled to the electron transport chain, with the pH gradient across the mitochondrial membrane serving as an essential regulator of kinetics 4.
Regulation
The Krebs cycle is regulated at multiple levels 5, 4:
- Allosteric regulation of key enzymes (citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase) 4
- Product inhibition by NADH and ATP 4
- Substrate availability (acetyl-CoA, NAD+, ADP) 4
- Post-translational modifications including phosphorylation/dephosphorylation by protein kinases and phosphatases 5
Clinical Relevance
Mitochondrial Disorders
Defects in the Krebs cycle enzymes cause mitochondrial disorders affecting high-energy-demand organs (brain, muscle, liver, heart, kidney) 1. The severity depends on the percentage of mutant mitochondrial DNA (heteroplasmy vs. homoplasmy) 1.
Insulin Deficiency Effects
In type 1 diabetes with insulin deficiency, dysfunction of mitochondria leads to impaired Krebs cycle enzyme activity, resulting in general cellular metabolic disorder and susceptibility to ketone body generation 1. The more severe the insulin deficiency, the greater the metabolic disorder 1.
Kidney Replacement Therapy
During continuous renal replacement therapy, citrate used for anticoagulation provides energy substrates (0.59 kcal/mmol) as it enters the Krebs cycle directly without requiring insulin 1. This can contribute 100-1300 kcal/day depending on the protocol used 1.
Compartmentalization
The cycle operates in the mitochondrial matrix, but several intermediates (pyruvate, acetyl-CoA, citrate, malate, oxaloacetate, AKG) exist in both cytosolic and mitochondrial compartments 1. Transport reactions allow specific metabolites to move between compartments, with compartment-specific isozymes (e.g., cytosolic vs. mitochondrial isocitrate dehydrogenase) operating independently 1.
Evolutionary Significance
The Krebs cycle represents the best possible chemical design for acetate oxidation, having the least possible number of steps and the greatest ATP yield 3. Evidence suggests it evolved from amino acid biosynthetic pathways through molecular opportunism, and a reverse version operates in some photosynthetic bacteria for CO2 fixation 6, 3.