Amino acid pool

• Describes all free amino acids in the body
• Amino acids: body’s primary source of nitrogen

AMINO ACID STRUCTURE

• Central carbon bound to: -H, -COOH, -NH3, R-group
• R-group: acidic (-), basic (+), branched, etc.

AMINO ACID POOL: INPUTS AND OUTPUTS

Input

• Dietary proteins: absorbed as amino acids/peptides in small intestine
  – Pass through hepatic portal vein and immediately enter liver
  – Liver plays central role in fates of free amino acids
• De novo amino acid synthesis: use C-skeletons & alpha-amino group of other amino acids to synthesize new ones
• Protein turnover: breakdown of cellular proteins

Output

• Oxidation: excess amino acids degraded to alpha amino group & C-skeleton
  – Urea cycle: liver rids body of nitrogen waste
  – Energy production: C-skeletons enter glycolysis or citric acid cycle
  – Gluconeogenesis: occurs in liver
  – Storage: amino acids can’t be stored –> C-skeletons stored as glycogen/fats
• Synthesis of nitrogen-containing molecules
  – Amino acids
  – Neurotransmitters
  – Nucleotides (purines and pyrimidines)
  – Creatine, etc.
• Protein turnover: synthesis of new proteins

Protein turnover

Cyclic synthesis and breakdown of proteins in the body

• Produces and consumes free amino acids
• Protein synthesis: depletes pool of free amino acids
• Protein breakdown: replenishes pool

ESSENTIAL AMINO ACIDS

Must be obtained from diet

• “My Tall Vegan Friend Is Watering Kale Leaves” – M, T, V, F, I, W, K, L
• Glucogenic: Methionine, threonine, valine
• Glucogenic & ketogenic: Phenylalanine, isoleucine, tryptophan
• Ketogenic: lysine & leucine

Branched chain amino acids (BCAA)

• Valine, leucine, isoleucine
• Immediately shunt from liver to circulation
• Liver lacks an aminotransferase that muscle, adipose, kidney & brain have

Aminotransferases: family of enzymes involved in amino acid breakdown

NONESSENTIAL AMINO ACIDS

Synthesized endogenously (The rest of the amino acids)

• Glucogenic: alanine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, histidine*, proline & serine
• Glucogenic and ketogenic: tyrosine
• Histidine is often considered essential because it is essential during the first 5 years of life but later becomes nonessential.

Note that there are many ways to categorize essential/nonessential amino acids

CLINICAL CORRELATION

Conditionally essential amino acids

• Cannot be synthesized under certain physiologic conditions
• Infants < 5 years old can’t synthesize: arginine, cysteine, histidine, tyrosine
• Immunosuppressed patients/postoperative infections: treat w/ arginine & glutamine supplements