43. Metabolism of Carbon Skeletons of Аmino Acids – Interaction with Crebs Cycle. Glucogenic and Ketogenic Amino Аcids. Nitrogen Balance.
Covers how the carbon skeletons left after amino group removal feed into Krebs cycle intermediates, then sorts amino acids into glucogenic and ketogenic groups with named examples. It lists which amino acids enter at pyruvate, oxaloacetate, α-ketoglutarate, succinyl-CoA, fumarate and acetyl-CoA, and notes a vitamin B12 requirement for some. It closes with nitrogen balance, covering positive and negative states and their causes, and why linking protein breakdown to the Krebs cycle matters during fasting. Three pictures accompany the text.
Topic 43 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 6 sections, about 330 words (3 min of reading), with 20 flashcards.
In this note
Metabolism of Carbon Skeletons of Amino Acids – Interaction with Crebs Cycle.
Metabolism of Carbon Skeletons of Amino Acids
Glucogenic vs Ketogenic Amino Acids
Entry of Amino Acids into Krebs Cycle Intermediates
Nitrogen Balance
Importance of Linking Amino Acid Metabolism to Krebs Cycle
Metabolism of Carbon Skeletons of Amino Acids
After removal of the amino group (via transamination or deamination), each amino acid leaves behind a carbon skeleton.
These carbon skeletons are converted into intermediates of the Citric Acid (Krebs) Cycle.
The final entry molecules into metabolism are typically:
Pyruvate
Acetyl-CoA
Acetoacetyl-CoA
α-Ketoglutarate
Succinyl-CoA
Fumarate
Oxaloacetate
These intermediates determine whether an amino acid can produce glucose (gluconeogenesis) or ketone bodies (ketogenesis).
Glucogenic vs Ketogenic Amino Acids
Glucogenic Amino Acids:
Their carbon skeletons are converted to pyruvate or TCA cycle intermediates that can form oxaloacetate → glucose.
Used during fasting to maintain blood glucose.
Most amino acids are glucogenic.
Ketogenic Amino Acids:
Their carbon skeletons form Acetyl-CoA or Acetoacetyl-CoA, which cannot be converted to glucose.
They instead form ketone bodies (acetoacetate, β-hydroxybutyrate).
Used in starvation or low carbohydrate intake.
Amino acids that are strictly ketogenic:
Leucine
Lysine
Amino acids that are both glucogenic and ketogenic:
Isoleucine
Phenylalanine
Tyrosine
Tryptophan
Threonine
Entry of Amino Acids into Krebs Cycle Intermediates
To Pyruvate:
Alanine, Serine, Glycine, Cysteine, Tryptophan
These → pyruvate → can become oxaloacetate → glucose.
To Oxaloacetate:
Asparagine and Aspartate
Directly replenish Krebs cycle (anion gap closure, anaplerosis).
