What Are Amino Acids?

Amino acids are small molecules that your body links together into chains to build proteins and peptides. They are the building blocks, not the finished object. A protein in your muscle, an enzyme in your gut, a hormone traveling in your blood — each one is a string of amino acids joined end to end in a particular order. Change the order and you get a different molecule that does a different job. There are only twenty of these blocks in the standard set your body builds from, and nearly everything alive is assembled out of them.

What does one actually look like?

Every amino acid is built around a single carbon atom. Picture that carbon as a small hub with four things attached to it.

Two of those four are the same in every amino acid, and they are where the name comes from. On one side of the hub sits an amino group — a small cluster of one nitrogen atom with hydrogen atoms attached. On the other side sits an acid group, a cluster of carbon, oxygen, and hydrogen that behaves like a mild acid. Amino, then acid. The name is a plain description of the two ends 2.

The third thing attached to the hub is a single hydrogen atom. It is small and it does nothing interesting.

The fourth is where all the variety lives. It is called the side chain, and it is different in every amino acid. In glycine, the smallest of the twenty, the side chain is just one more hydrogen atom. In tryptophan it is a bulky structure of two joined rings. Some side chains carry an electrical charge. Some mix happily with water and some repel it.

Here is a way to hold all of that in your head at once. Imagine a bag of beads that all clip together the same way — the same socket on the left of each bead, the same plug on the right. Because those clips are identical, any bead can join to any other bead, in any order you like. Now imagine that each bead has a different shape fixed to its top. One has a tiny nub. One has a wide flat paddle. One has a hook.

String them together and the finished chain has a form that depends entirely on which beads you chose and what order you used. The clips did the joining. The shapes on top decided what you ended up with.

Amino acids work close to that. The shared amino and acid ends are the clips, and they let any amino acid link to any other. The side chain is the shape on top. Glycine and tryptophan connect to their neighbors in exactly the same way. Everything that makes them behave differently — how much room they take up, whether they attract water or push it away, whether they pull on the amino acids near them — comes down to that one variable part.

There are twenty, and nine of them are called essential

Your body builds proteins from a standard set of twenty amino acids. Many more amino acids exist in nature — hundreds of them — but these twenty are the ones your cells reach for when they follow the instructions written in your genes.

Nine of the twenty are described as essential. This is one of the most misread words in the whole subject, and it is worth slowing down here.

Essential does not mean more important. It does not mean the other eleven are optional, or lesser, or that a body could manage without them. All twenty are needed. Running short of any single one halts protein building just as firmly as running short of any other.

What essential means is narrower and much more specific. It means your body has no way to manufacture that particular amino acid from scratch, so the only way it can arrive is in food 34. The other eleven your body can assemble internally out of materials it already has. Those eleven are often labeled non-essential, which is an even more misleading name — it sounds like they are not needed, when in truth they are needed exactly as much and you simply happen to own a factory that makes them.

Some scientists prefer the pair indispensable and dispensable for the same two groups 3. Those words are not perfect either, but they at least point at the real question, which is where an amino acid has to come from rather than how much it matters.

• What people usually assume "essential" means: this amino acid is more important than the others.
• What it actually means: your body cannot build this one, so it has to arrive ready-made from outside.
• Both groups are required in full. The label describes the supply route, not the value.

What happens when you eat protein

This part catches people out, and it quietly explains a great deal of what you will read about peptides later.

The protein in a meal does not become the protein in your body. It is taken apart first.

When food reaches your stomach and then your small intestine, it meets enzymes — proteins whose job is to speed up one particular chemical reaction. Several of these enzymes do nothing but cut protein chains. They work their way along a chain, snipping the links between one amino acid and the next, until what remains is mostly single amino acids and very short fragments.

Only then does anything get absorbed. The lining of your small intestine carries transporters — molecular doorways, each shaped to let particular things through. They carry single amino acids and short fragments across into the bloodstream. A long, intact protein chain does not fit through that door in any meaningful quantity.

So the sequence runs: dismantle, absorb, rebuild. Your cells take the loose amino acids that arrive in the blood and assemble them into whatever they currently need, following the instructions in your own genes rather than anything about the food they came from.

An analogy that holds up well: imagine a delivery of secondhand furniture that you take apart completely, down to the individual screws, planks, and hinges. Then you build what you actually want out of the pile. The bookshelf you end up with is not the chair that arrived. It is made of the chair's materials, arranged to your plan, not to the plan of whoever built the chair.

Hold on to that picture. It is the reason a chain of amino acids swallowed whole rarely survives the trip. Your digestive system is extremely good at taking such chains apart, and it does not pause to check whether a particular chain was one you hoped to keep intact. That has consequences worth an article of its own, but the groundwork is right here: chains of amino acids get dismantled on the way in.

Amino acids do more than build things

Building proteins is the headline job, but it is not the only one. Amino acids are also raw material — starting points that the body converts into a range of other molecules it needs 1.

Some of what gets made this way are signaling chemicals: molecules that carry a message from one cell to another. Several of the chemicals your nerve cells use to pass signals between themselves begin as ordinary amino acids and are reshaped into their final form by enzymes. Other amino acids feed into the making of pigments, into molecules the immune system uses, and into the process by which the body handles waste nitrogen 1.

Amino acids can also be broken down for energy when a cell needs it, though that is a side role rather than their main purpose.

So it is more accurate to think of amino acids as a general-purpose currency than as bricks for one kind of wall. Most of the supply gets spent on proteins. Some of it gets spent elsewhere.

Left hands and right hands

There is one more feature of amino acids worth knowing, and it becomes important the moment anyone makes a peptide in a laboratory.

Hold up your two hands. Same parts, same arrangement — a thumb, four fingers, a palm — and yet they are not the same object. Lay one on top of the other and they refuse to match. They are mirror images of each other.

Nineteen of the twenty amino acids have exactly this property. The same atoms can sit around that central carbon hub in two arrangements that mirror each other, and the two versions are not interchangeable. Chemists label them L and D 2. Glycine is the one exception: its side chain is a single hydrogen atom, which makes both arrangements identical, rather as a mitten looks the same in a mirror when a glove does not.

Living things use almost exclusively the L form. Your proteins are built from L amino acids. The enzymes that cut protein chains apart grew up alongside L amino acids and are shaped to grip them.

That last point is the interesting one. An enzyme recognizes what it works on by shape, the way a glove fits one hand and not the other. Swap an L amino acid in a chain for its D mirror image, and an enzyme that would ordinarily cut at that spot may no longer be able to get hold of it.

Chemists making peptides in a laboratory sometimes do this on purpose. Placing a D form at a point where an enzyme would normally cut can make the chain last longer before it is broken down. It is one of the reasons a peptide made in a lab can behave quite differently from a natural one, even when the two look nearly identical written out on paper.

So what is a peptide?

Now the payoff for all of the above.

A peptide is a short chain of amino acids joined end to end. That is the whole definition.

The link between two neighboring amino acids even has its own name — the peptide bond — and it forms when the acid group of one amino acid joins to the amino group of the next 2. Two amino acids joined together make a dipeptide. A handful joined together is an oligopeptide. Longer chains are called polypeptides, and chains long enough to fold into a stable three-dimensional shape are generally called proteins.

Where exactly the line falls between a long peptide and a small protein is a matter of convention rather than a fact about matter. Different fields draw it in different places.

The idea itself never gets harder than that one sentence. Once you know what an amino acid is, a peptide is simply several of them in a row, in a particular order, and the order is what decides what the chain does. Everything that follows — how peptides are made, why some fall apart within minutes, why chains of amino acids are so easily taken to pieces — is built on the parts you now have in hand.