Amino Acids and the Urea Cycle

Amino Acid Classification

• Essential and non-essential amino acids are classified based on whether they can be synthesized by cells or need to be obtained from the diet.
• This classification has little to do with the structure of amino acids.
• Essential amino acids vary from one organism to another and even differ in humans, depending on whether they are adults or children.

Amino Acid Chemistry

• Amino acids are categorized based on the chemistry of their R-groups.
• Different textbooks may have different names for the categories and may categorize the same amino acid differently.

Post-Translational Modifications

• After a protein is synthesized, amino acid side chains within it can be chemically modified.
• Common modifications include phosphorylation of hydroxyl groups of serine, threonine, or tyrosine.
• Other modifications include addition of fatty acids, isoprenoid groups, acetyl groups, methyl groups, iodine, carboxyl groups, or sulfates.

Amino Acid Functions

• Some amino acids are precursors of important compounds in the body, such as epinephrine, thyroid hormones, L-dopa, and dopamine (from tyrosine), serotonin (from tryptophan), and histamine (from histidine).
• Amino acids serve as constituents of proteins, which are polymers of amino acids.

Specific Amino Acids

• Ornithine is a metabolic precursor of arginine and citrulline.
• Citrulline is a metabolic byproduct and is sometimes used as a dietary supplement to reduce muscle fatigue.
• Lysine is a precursor of important compounds, including serotonin, niacin, and the auxin phytohormone.
• Tyrosine is a non-essential amino acid that is a target for phosphorylation in proteins and plays a role in signaling processes.
• Serine is a precursor of many important cellular compounds, including purines, pyrimidines, sphingolipids, folate, and the amino acids glycine, cysteine, and tryptophan.