Amino acid synthesis encompasses the metabolic pathways by which cells produce amino acids. Humans can synthesize eleven of the twenty standard amino acids, while the remaining nine — called essential amino acids — must be obtained from the diet.
Essential vs. Non-Essential Amino Acids
Essential Amino Acids
Humans cannot synthesize histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These must be supplied by the diet. The lack of any essential amino acid leads to protein deficiency, as protein synthesis requires all twenty amino acids simultaneously.
Non-Essential Amino Acids
The remaining eleven amino acids can be synthesized by the human body. They are produced from common metabolic intermediates such as alpha-keto acids, which are derived from glycolysis and the citric acid cycle.
Biosynthetic Pathways
Transamination
The first step in synthesizing many amino acids is transamination. An alpha-keto acid accepts an amino group from another amino acid (usually glutamate), catalyzed by aminotransferases. This reaction converts the keto acid into an amino acid.
Families of Amino Acids
Amino acids are grouped into families based on their biosynthetic precursors: from alpha-ketoglutarate come glutamate, glutamine, proline, and arginine; from oxaloacetate come aspartate, asparagine, methionine, threonine, and lysine; from pyruvate come alanine, valine, leucine, and isoleucine; from 3-phosphoglycerate come serine, glycine, and cysteine; from shikimate come phenylalanine, tyrosine, and tryptophan; and from ribose-5-phosphate comes histidine.
Regulation
Amino acid synthesis is tightly regulated by feedback inhibition. The end product of a pathway inhibits the first committed enzyme, preventing overproduction. This allows cells to match amino acid production to their needs.
Practical Amino Acid Analysis by HPLC
Prepare samples (e.g., plasma, cell lysate, or culture media) by deproteinizing with an equal volume of 10% trichloroacetic acid or sulfosalicylic acid. Centrifuge at 10,000 × g for 10 minutes and collect the supernatant. Derivatize amino acids for detection: for pre-column derivatization, mix 10 µL of sample with 70 µL of borate buffer (pH 8.8) and 20 µL of o-phthaldialdehyde (OPA) reagent containing 3-mercaptopropionic acid. Incubate for exactly 2 minutes at room temperature, then inject 10 µL onto a reverse-phase C18 column (4.6 × 150 mm, 5 µm) equilibrated with 40 mM sodium phosphate buffer (pH 7.8). Elute with a gradient of acetonitrile (0–60% over 30 minutes) at 1 mL/min. Detect OPA-derivatized amino acids by fluorescence (excitation 340 nm, emission 450 nm). Alternatively, separate underivatized amino acids by cation-exchange chromatography with post-column ninhydrin detection. Capillary zone electrophoresis (CZE) provides an alternative approach for amino acid analysis, enabling separation of both derivatized and underivatized amino acids with high efficiency and short analysis times — amino acids react with ninhydrin at 130°C to form a purple product (Ruhemann’s purple) detected at 570 nm, while proline and hydroxyproline give a yellow product at 440 nm. Quantify each amino acid by comparing peak areas to a standard mixture of 20 amino acids at known concentrations. For cell culture media formulation, ensure all essential amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine) are present at 0.5–12 mM depending on the cell type. Glutamine is typically added at 2–4 mM due to its instability in solution — it degrades to glutamate and ammonia at 37°C with a half-life of ~5 days.
Real-World Application
In fed-batch CHO cell culture for monoclonal antibody production, daily HPLC analysis of the spent medium reveals that glutamine, asparagine, and serine are depleted after day 5. A targeted feed containing these three amino acids is added on day 6, extending culture viability from day 10 to day 14 and increasing antibody titer by 40%.
