The amino acid that welds collagen fibres together — and the one most commonly missing from plant-based diets, explaining why food combining became a nutrition concept.
Symbol
Lys · K
Discovered
1889
Mol. Weight
146.19 g/mol
Essential
Yes
K
Discovery: From Milk Protein
L-Lysine
In 1889, German chemist Edmund Drechsel isolated lysine from casein — the main protein in milk — by hydrolyzing it with sulfuric acid and carefully fractionating the products. He recognized a new basic amino acid with an unusually long side chain ending in a second amino group. Unlike arginine, with its guanidinium cap, lysine's side chain ends simply with –NH₂: a plain amine group, flexible and highly reactive. The structure was elucidated in 1902 by Emil Fischer and Fritz Weigert, who confirmed it by synthesis.
In 1928, H.B. Vickery and C.S. Leavenworth prepared crystalline lysine, establishing cleaner physical reference material. Synthesis was later accomplished by Emil Fischer and Weigert through a reaction involving 7-cyanopropylmalonic ester treated with nitrous acid. That terminal amine group turned out to be one of the most chemically versatile functional groups in all of biochemistry — it can be acetylated, methylated, ubiquitinated, and cross-linked — making lysine residues the primary targets of protein modification chemistry and among the most important sites in epigenetic regulation.
🏗️ How Lysine Builds Collagen
Collagen is the most abundant protein in the human body — the structural scaffold of skin, bones, tendons, and cartilage. Its extraordinary strength comes from covalent crosslinks between adjacent collagen chains. These crosslinks are formed between lysine residues: the enzyme lysyl oxidase oxidizes the terminal amine of lysine to an aldehyde, which then reacts spontaneously with a neighboring lysine or hydroxylysine residue to form a stable covalent bond. No lysine — no crosslinks. No crosslinks — no strong collagen. Scurvy, the disease caused by vitamin C deficiency, manifests partly because vitamin C is required for the hydroxylation step that precedes crosslinking, causing collagen to lose structural integrity.
Identifiers and Properties of Lysine
Identity
IUPAC Name(2S)-2,6-Diaminohexanoic acid
FormulaC₆H₁₄N₂O₂
Mol. Weight146.19 g/mol
CAS Number56-87-1
MDL NumberMFCD00064433
Physical
Melting point215 °C
Solubility~740 g/L (25 °C)
pKa₁ (COOH)2.16
pKa₂ (NH₃⁺)9.06
pKa₃ (side chain)10.54
pI9.80
Rf (BuOH/AcOH/H₂O = 12:3:5)0.14
Identifiers
Canonical SMILESC(CCN)CC(C(=O)O)N
Isomeric SMILESC(CCN)C[C@@H](C(=O)O)N
InChIKeyKDXKERNSBIXSRK-YFKPBYRVSA-N
CategoryBasic
EssentialYes
The Limiting Amino Acid Problem
Proteins are only as complete as their most limiting amino acid. Cereal grains — wheat, rice, corn — are low in lysine. This makes lysine the "first limiting amino acid" in grain-based diets: even if total protein intake seems adequate, the lack of lysine means the body cannot synthesize proteins at full efficiency.
This is the biochemical basis of food combining — the practice of pairing grains with legumes (beans, lentils, chickpeas), which are rich in lysine but low in methionine. Together, they provide a more complete amino acid profile. It is also why lysine fortification of cereal crops has been a significant goal in agricultural biotechnology since the 1970s, and why high-lysine corn varieties were developed for use in regions where grain is the dietary staple.
Lysine and the Histone Code
Histones are the proteins that DNA wraps around in the nucleus. The pattern of chemical modifications on histone lysine residues — acetylation, methylation, ubiquitination — constitutes what is called the histone code: a layer of regulatory information that sits on top of the DNA sequence itself and controls which genes are expressed. Modifying a lysine residue on a histone can silence a gene or activate it, depending on which modification is applied and where.
This is one of the central mechanisms of epigenetics — heritable changes in gene expression that don't involve changes to the DNA sequence itself. The enzymes that write, read, and erase these lysine modifications are among the most intensively studied drug targets in cancer biology.
Functions of L-Lysine in the Body
Lysine is an essential amino acid that must be obtained from food. Beyond its structural roles in collagen and histones, it participates in several other important physiological processes.
Carnitine synthesis
Lysine, together with methionine, is a biosynthetic precursor of carnitine — a compound essential for the transport of long-chain fatty acids into the mitochondria, where they are oxidized for energy. Without adequate carnitine, the body cannot efficiently use fat as a fuel source. Lysine provides the carbon backbone of carnitine; this metabolic connection makes adequate lysine intake important not only for protein synthesis but also for fat metabolism.
Calcium absorption and bone health
Lysine promotes the intestinal absorption of calcium and reduces its renal excretion, increasing the amount of calcium retained by the body. Combined with its essential role in collagen crosslinking, this makes lysine doubly important for bone health. Research has shown that adequate lysine intake may contribute to the prevention of bone loss and reduce the risk of osteoporosis, particularly when combined with sufficient calcium and vitamin D.
Hormone, enzyme, and antibody production
As a building block for all proteins, lysine is required for the synthesis of hormones, digestive enzymes, and antibodies that form the humoral immune response. Its positively charged side chain is also important in active site architecture for many enzymes that need a protonatable residue at physiological pH.
Wound healing and recovery
Lysine's role in collagen synthesis makes it directly relevant to wound healing. Adequate lysine availability is necessary for the production of new collagen at injury sites, the repair of connective tissue, and recovery from surgery. Clinical nutrition protocols for post-surgical patients commonly ensure adequate protein and lysine intake to support tissue reconstruction.
Did You Know?
Lysine crosslinks are what make collagen strong. The enzyme lysyl oxidase converts lysine's terminal amine to an aldehyde, which then bonds spontaneously to a neighbouring lysine or hydroxylysine residue. Without this step, collagen fibres cannot form the covalent crosslinks that give tendons, skin, and bone their tensile strength.
Interesting Facts
🔤
Why K, not L? Lysine's single-letter code is K — L was already taken by leucine, and K was chosen partly for its phonetic proximity to the German spelling Lysin. It remains one of the more puzzling assignments in the system, since K has no obvious connection to "lysine" in English.
🐄
Added to animal feed worldwide. Lysine is the second most commercially produced amino acid after glutamic acid, with millions of tons manufactured annually by bacterial fermentation. Most of this lysine goes into animal feed — poultry and pig diets in particular — because grain-based feeds are lysine-deficient. Adding pure lysine allows farmers to use less total protein in feed without compromising animal growth.
🎨
The Maillard reaction's favourite target. When food browns during cooking, the Maillard reaction is responsible. It starts with a reaction between a reducing sugar and an amino acid — and lysine's free epsilon-amine group is particularly reactive. Lysine residues in food proteins are often the first to react in Maillard browning, which is why heavily heat-processed foods can be lower in bioavailable lysine than the same foods consumed raw or lightly cooked.
🦠
Viruses need lysine too. Several viruses require the host cell's lysine supply for replication. The antiviral strategy of adjusting the lysine-to-arginine ratio has been studied in the context of herpesvirus replication, which depends on arginine and is competitively inhibited by lysine at the cellular uptake level. The lysine-arginine ratio affects how efficiently certain viruses can replicate inside cells.
Where Lysine Is Found
Lysine is most abundant in animal proteins and legumes. Cereal grains are characteristically low in lysine, making dietary diversity important for adequate intake. Values below are approximate per 100g of food:
