39. Overview of Metabolism of Amino Acids. Nitrogen Balance. Transamination Reactions. Role of Vitamin B6. Importance of Aminotransferases for Diagnostics.
Explains why amino acids are not stored and how their nitrogen and carbon parts are handled, including glucogenic and ketogenic classification. Covers nitrogen balance in positive, negative and zero forms with example conditions, then transamination reactions catalysed by ALT and AST with the amino acids that do not undergo it. Describes pyridoxal phosphate as the vitamin B6 coenzyme and its deficiency effects, and closes with how ALT and AST levels, including their AST/ALT ratio, help in diagnosing liver and muscle disorders.
Topic 39 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 6 sections, about 396 words (3 min of reading), with 10 flashcards.
In this note
Overview of Metabolism of Amino Acids. Nitrogen Balance. Transamination Reactions.
Overview of Metabolism of Amino Acids
Nitrogen Balance
Transamination Reactions
Role of Vitamin B6 (Pyridoxal Phosphate)
Importance of Aminotransferases in Diagnostics
Overview of Metabolism of Amino Acids
Amino acids are not stored in the body, unlike fats and carbohydrates.
Therefore, excess amino acids must be metabolized → their carbon skeleton and nitrogen group are handled differently.
Two components of amino acid metabolism:
Nitrogen metabolism → Removal and excretion of amino group.
Carbon skeleton metabolism → Used for:
Energy (ATP production)
Glucose (gluconeogenesis)
Ketone bodies (ketogenesis)
Fatty acids
Non-essential amino acids
First major step in amino acid metabolism is removal of the α-amino group → mainly through transamination.
After removal of nitrogen:
Carbon skeletons are converted into intermediates of TCA cycle (e.g., pyruvate, oxaloacetate, α-ketoglutarate, succinyl-CoA, fumarate).
These determine whether the amino acid is:
Glucogenic (forms glucose precursors)
Ketogenic (forms acetyl-CoA/ketone bodies)
Both
Nitrogen Balance
Nitrogen balance refers to the balance between nitrogen intake and nitrogen loss.
Positive nitrogen balance:
Intake > excretion
Seen in:
Growth (children)
Pregnancy
Bodybuilding
Tissue repair after illness
Negative nitrogen balance:
Excretion > intake
Seen in:
Starvation
Severe illness/infection
Burns
Trauma
Protein deficiency (e.g., Kwashiorkor)
Zero nitrogen balance:
Intake = excretion → normal healthy adults.
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