Covers how skeletal muscle handles amino acids during fasting and exercise, including the glucose-alanine cycle, glutamine export as a nitrogen carrier, and preferential catabolism of branched-chain amino acids such as leucine. It then turns to the brain, describing the glutamine-glutamate cycle, GABA synthesis with vitamin B6, and biogenic amine production from tyrosine and tryptophan, with links to disorders. A closing section ties muscle and brain nitrogen handling together, and two pictures accompany the text.
Topic 53 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 3 sections, about 444 words (3 min of reading), with 10 flashcards.
In this note
Amino Acid Metabolism in Muscle and CNS.
Amino Acid Metabolism in Muscle and CNS
Interconnection Between Muscle and Brain
Amino Acid Metabolism in Muscle and CNS
1) Amino Acid Metabolism in Muscle
Muscle uses amino acids mainly as fuel during fasting or intense exercise
Muscle proteins are continuously broken down and resynthesized.
During fasting → protein breakdown dominates → amino acids released.
Muscle cannot perform the urea cycle
So it cannot detoxify ammonia directly.
Therefore, it uses special nitrogen transport mechanisms.
The Glucose-Alanine Cycle (Muscle → Liver)
Pyruvate (from glycolysis) accepts amino group → forms Alanine.
Alanine is transported via blood to liver.
In liver: Alanine → Pyruvate → used for gluconeogenesis.
The amino group enters urea cycle → urea excreted by kidney.
This cycle allows muscle to remove nitrogen safely and obtain glucose back during fasting.
Glutamine Release by Muscle
Muscle converts Glutamate → Glutamine using Glutamine synthetase.
Glutamine is exported to kidney and gut.
Glutamine acts as a nitrogen carrier
Particularly important when ammonia load is high (exercise, fasting).
Muscle uses Branched Chain Amino Acids (BCAAs) preferentially for energy
Leucine, Isoleucine, Valine are catabolized mainly in muscle (not liver).
These provide fuel during exercise and fasting.
Leucine is purely ketogenic
Important for ATP production during carbohydrate deficiency.
Also stimulates mTOR → muscle protein synthesis (anabolic signal).
During exercise:
Proteolysis increases → Alanine and Glutamine export rises.
Provides both energy and nitrogen transport.
2) Amino Acid Metabolism in CNS (Brain)
The brain cannot store energy and has high metabolic demand
Requires constant supply of glucose (or ketone bodies in starvation).
Uses amino acids mainly for neurotransmitter synthesis, not energy.
Glutamate is the major excitatory neurotransmitter
Formed from Glutamine → Glutamate in neurons.
Released into synapse → reuptaken by astrocytes → converted back to Glutamine → returned to neurons.
This is the Glutamine-Glutamate Cycle.
GABA (Gamma-Aminobutyric Acid) is the major inhibitory neurotransmitter
Synthesized from Glutamate via Glutamate decarboxylase (requires Vitamin B6).
Low B6 → seizures (due to reduced GABA).
Brain also uses Amino Acids to synthesize biogenic amines
