Essential and nonessential amino acids are sorted by whether an organism can synthesize them. Human cells build eleven of the twenty standard amino acids from common metabolites and lack the enzymes for the other nine, so those nine are essential: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. The label belongs to the organism, never to the molecule.
- LabFirst Peptides certificates report identity by LC-MS, purity by RP-HPLC on a C18 column (most at 214 nm) and net content as a separate line, each by lot number, with a link to the issuing laboratory's site.
- Human cells can synthesize eleven of the twenty standard amino acids and lack the pathways for nine: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
- The twenty standard amino acids fall into six biosynthetic families by carbon precursor: alpha-ketoglutarate, oxaloacetate, pyruvate, 3-phosphoglycerate, phosphoenolpyruvate with erythrose 4-phosphate, and glucose 6-phosphate (histidine, through a phosphorylated ribose).
- Animals lack the shikimate pathway, which builds chorismate in seven steps and supplies phenylalanine, tyrosine and tryptophan in bacteria, fungi and plants.
- Tyrosine is conditionally essential because human cells make it only from phenylalanine by phenylalanine hydroxylase, and cysteine's sulfur comes from methionine through the transsulfuration pathway.
- By Reeds' metabolic definition only lysine, threonine and tryptophan are strictly indispensable for humans and only glutamate and serine strictly dispensable.
Two cells, one sugar solution
Put a microbe in a flask of sugar, a nitrogen salt and a few minerals, and whether it grows depends on which enzymes it carries. Microbiologists call an organism fastidious when it cannot make certain nutrients and needs them added to the medium. Human cells are fastidious in exactly that sense for amino acids. They can build eleven of the twenty standard ones. For the other nine they have no working pathway, so those nine have to arrive from outside the cell, ready-made.
That is the whole difference between an essential amino acid and a nonessential one. The molecule is the same in every flask. What differs is the enzyme set of the organism trying to make it. This guide lays out which amino acids fall on which side for humans, where the carbon skeleton of each nonessential one comes from, why a handful are called conditionally essential, and why the lists shift from one organism to the next. For what the twenty are and how they are coded, start with the amino acid chart.
- Essential (9): His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Val
- Nonessential (11): Ala, Arg, Asn, Asp, Cys, Glu, Gln, Gly, Pro, Ser, Tyr
- Conditional: Arg, Tyr and Cys, made from a limited supply
- Six precursors: alpha-ketoglutarate, oxaloacetate, pyruvate, 3-phosphoglycerate, PEP plus erythrose 4-phosphate, ribose 5-phosphate
- Missing in animals: the seven-step shikimate pathway to chorismate
What makes an amino acid essential or nonessential?
An amino acid is nonessential for an organism when that organism has the enzymes to synthesize it, and essential when it does not. The Fundamentals of Biochemistry chapter on amino acid biosynthesis puts it in one line: the distinction reflects which organisms possess the relevant biosynthetic enzymes. OpenStax gives the human count, eleven that the body can biosynthesize and nine that it cannot, and names phenylalanine as an example of the second group.
Nothing about the amino acid itself is essential. Lysine has the same formula whether it was built by a soil bacterium or arrived from outside a human cell. The word describes a gap in the organism's chemistry, so the same amino acid can be nonessential for one species and essential for another. In plain terms, essential means the cell has to be handed it, and nonessential means the cell can make it on its own.
The human list was pinned down experimentally by William C. Rose and his colleagues in a series of Journal of Biological Chemistry papers titled The amino acid requirements of man, the fifteenth of which, in 1955, carried the summary and final observations. The classification used today descends from that work.
Which amino acids are essential and which are nonessential in humans?
For humans, nine amino acids are essential and eleven are nonessential, with two of the eleven carrying a conditional flag. The table follows the lists in the Fundamentals of Biochemistry chapter.
| Group | Amino acids | Count |
|---|---|---|
| Essential | Histidine (His, H), isoleucine (Ile, I), leucine (Leu, L), lysine (Lys, K), methionine (Met, M), phenylalanine (Phe, F), threonine (Thr, T), tryptophan (Trp, W), valine (Val, V) | 9 |
| Nonessential | Alanine (Ala, A), asparagine (Asn, N), aspartate (Asp, D), cysteine (Cys, C), glutamate (Glu, E), glutamine (Gln, Q), glycine (Gly, G), proline (Pro, P), serine (Ser, S), tyrosine (Tyr, Y) | 10 |
| Conditionally essential | Arginine (Arg, R), tyrosine (Tyr, Y) | 2 |
Arginine is the eleventh nonessential amino acid and the reason the counts in the table add to twenty-one rather than twenty: it appears twice, once in the human synthesis set and once with its conditional flag. Tyrosine sits in both the nonessential row and the conditional row for a different reason, covered below. The three branched-chain residues, valine, leucine and isoleucine, all land on the essential side, as do both aromatic residues that a human cell cannot make, phenylalanine and tryptophan.
A useful way to hold the list in memory is by chemistry rather than by name. Every essential amino acid for humans has either a branched or aromatic side chain, a sulfur atom, a hydroxyl on a branched carbon, an imidazole ring, or a four-carbon amine tail. The nonessential set is dominated by the small and the simple: glycine, alanine, serine, and glutamate and aspartate, each one transamination from a citric acid cycle intermediate.
Where does each nonessential amino acid come from?
Every amino acid a cell builds starts from a handful of ordinary metabolites, and the Fundamentals of Biochemistry chapter sorts the twenty into families by that starting point. The precursor supplies the carbon skeleton; the amino group usually arrives by transamination from glutamate.
| Carbon precursor | Amino acids built from it | Status in humans |
|---|---|---|
| Alpha-ketoglutarate | Glutamate, glutamine, proline, arginine | All made; arginine conditional |
| Oxaloacetate | Aspartate, asparagine, methionine, threonine, lysine | Aspartate and asparagine made; methionine, threonine and lysine essential |
| Pyruvate | Alanine, valine, leucine, isoleucine | Alanine made; valine, leucine and isoleucine essential |
| 3-Phosphoglycerate | Serine, glycine, cysteine | All made; cysteine's sulfur comes from methionine |
| Phosphoenolpyruvate plus erythrose 4-phosphate | Phenylalanine, tyrosine, tryptophan (via chorismate) | Phenylalanine and tryptophan essential; tyrosine made from phenylalanine |
| Glucose 6-phosphate, via the pentose phosphate pathway and PRPP | Histidine | Essential |
Read the right-hand column and a pattern appears. Within one family, the amino acids closest to the precursor are the ones a human cell can make, and the ones that need a long dedicated pathway are the ones it cannot. Alanine comes from pyruvate in a single transamination with glutamate, and it is nonessential. Valine and leucine share a four-step pathway from two pyruvate molecules, and both are essential. Aspartate is one transamination from oxaloacetate and is the common precursor for asparagine; the longer routes from the same starting point, to methionine, threonine and lysine, are the ones humans lack.
The aromatic family is the sharpest case. Phenylalanine, tyrosine and tryptophan all branch from chorismate, which the shikimate pathway builds from phosphoenolpyruvate and erythrose 4-phosphate in seven steps, with tryptophan needing five more after that. Animals do not have the shikimate pathway at all. Bacteria, fungi and plants require it, which is why its enzymes are targets for antibiotics and herbicides, and why phenylalanine and tryptophan are essential for every animal.
What are conditionally essential amino acids?
A conditionally essential amino acid is one the organism can synthesize, but only when a precursor is available and the pathway is keeping up. Reeds, writing in the Journal of Nutrition in 2000, set this out as a third class beside dispensable and indispensable, and described the limit as one set by precursor supply and by the state of the individual organism. Two residues on the human list carry the flag in the Fundamentals of Biochemistry chapter, and a third behaves the same way.
Tyrosine is the clearest example. Human cells make it from phenylalanine using phenylalanine hydroxylase, an enzyme that depends on tetrahydrobiopterin, so tyrosine synthesis runs only as far as the phenylalanine supply allows. The chapter notes that when tyrosine is low, phenylalanine is spent to replace it, which is the sense in which tyrosine is conditionally essential: the cell can build it, from an essential starting material.
Cysteine works the same way with sulfur. Its carbon comes from serine, a nonessential amino acid, but its sulfur arrives from homocysteine through the transsulfuration pathway, and that sulfur traces back to methionine. Serine's hydroxyl group is replaced by sulfide to give cysteine. So cysteine is built in the cell, from one nonessential and one essential ingredient, and its supply tracks the methionine supply.
Arginine is made in the urea cycle, which is why it appears among the amino acids human cells can synthesize. The chapter flags it as conditionally essential without spelling out the limit, and that is as far as this page goes with it. The point shared by all three is that essential and nonessential are the ends of a scale with a middle, and the middle is defined by pathway capacity rather than by the presence or absence of an enzyme.
Why do the lists differ between organisms?
Because the lists are a catalog of missing enzymes, and different lineages have lost different pathways. The clearest divide is the shikimate pathway. Bacteria, fungi and plants carry it and use it to make chorismate, the branch point for all three aromatic amino acids. Animals do not, so the aromatic amino acids that bacteria make freely are essential for every animal. A pathway that is present in one kingdom and absent in another moves whole families of amino acids from one list to the other.
Even within humans the classification depends on what question is being asked. Reeds compared the classical definition, built on whether an amino acid is needed for normal growth, with definitions built on metabolism and on function, and found they give different answers. By the metabolic test, which asks whether the carbon skeleton can be made at all, only lysine, threonine and tryptophan came out as strictly indispensable and only glutamate and serine as strictly dispensable. By the functional test, every amino acid is needed, since each has a role in the organism. The nine-and-eleven split is the first definition, and it is the one most tables print.
So when a chart lists essential amino acids without saying for which organism, it almost always means humans, and it is using the growth-based definition. A chart for an organism that carries the full enzyme set would be empty. For the related question of how many amino acids exist beyond the twenty, including selenocysteine and pyrrolysine, see how many amino acids are there.
Does essential mean anything for a synthetic peptide?
No. A research peptide is assembled chemically, residue by residue, from protected amino acids on a solid support, and no cell built any of it. The solid-phase peptide synthesis guide covers how that works. In a synthetic chain, lysine and alanine are equals: each is one residue of known mass, coupled by the same chemistry, and the essential label never enters the record.
Where the biosynthesis story does touch a laboratory is in reading a sequence. The side chains that make an amino acid hard for a human cell to build, the branched and aromatic ones, the sulfur-bearing ones, are also the side chains that set how a peptide behaves on a reversed-phase column and in a mass spectrometer. The R group guide walks through those properties residue by residue, including which side chains are the sites where a peptide degrades. What a residue does for a cell is a separate question, and the what amino acids do guide takes that one.
In our experience checking a sequence against a certificate, the biosynthesis families earn their keep as a memory aid for which side chains carry a ring, a branch or a sulfur atom, and those are the residues we look at first when a chromatogram or a mass spectrum needs explaining.
FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION.
Bottom line
Essential and nonessential amino acids are defined by the organism, by which biosynthetic enzymes it carries. Human cells make eleven of the twenty and lack the pathways for histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. All twenty fall into six families by the common metabolite their carbon starts from, and the essential ones are the members of those families that need the longest dedicated routes, or a pathway animals never had. Tyrosine, cysteine and arginine sit in the middle, made in the cell from a limited supply. For a synthetic peptide the label is irrelevant; for reading a sequence it is a decent map of which side chains will matter.
Common questions
What are the nine essential amino acids?
What is the difference between essential amino acids and non essential amino acids?
Are nonessential amino acids less important than essential ones?
Why is tyrosine called conditionally essential?
Which amino acids come from the shikimate pathway, and why does it matter?
More science guides
Sources
- Fundamentals of Biochemistry (Jakubowski and Flatt), 22.2 Biosynthesis of Amino Acids, Biology LibreTexts. Essential, nonessential and conditionally essential lists for humans, the six precursor families, step counts for the shikimate and branched-chain pathways, and the phenylalanine, cysteine and arginine routes.
- OpenStax, Organic Chemistry, 26.1 Structures of Amino Acids. Eleven of the twenty amino acids can be biosynthesized by humans and nine cannot; phenylalanine named as essential.
- OpenStax, Microbiology, 9.6 Media Used for Bacterial Growth. Definition of a fastidious organism as one that cannot make certain nutrients and requires them added to the medium.
- Reeds PJ. Dispensable and indispensable amino acids for humans. Journal of Nutrition 2000;130(7):1835S-1840S doi:10.1093/jn/130.7.1835S. Growth-based, metabolic and functional definitions compared; the conditionally essential class and what limits it.
- Rose WC, Wixom RL, Lockhart HB, Lambert GF. The amino acid requirements of man. XV. The valine requirement; summary and final observations. Journal of Biological Chemistry 1955;217(2):987-995 doi:10.1016/s0021-9258(18)65962-9. The closing paper of the Rose series that established the human classification; record retrieved through the Europe PMC REST service.
