What Amino Acid Is at the Beginning of Every Polypeptide?

Methionine is the amino acid at the beginning of every polypeptide in eukaryotes, and N-formylmethionine fills that spot in bacteria. The first residue sits at the N-terminus, the end of the chain that carries a free amino group. The ribosome places it there by reading the start codon AUG, and enzymes often remove it later.

The Short Answer: Methionine Starts Almost Every Polypeptide

Every polypeptide chain is built from amino acids added one at a time, and the very first one is almost always methionine. The genetic code makes this predictable: AUG is both the start codon and the codon for methionine.

• Eukaryotes: methionine (Met) starts the chain in the cytoplasm.
• Bacteria: N-formylmethionine (fMet) starts the chain.
• Mitochondria: N-formylmethionine starts the chain in many organisms, including humans.
• Archaea: methionine starts the chain, though the machinery differs from eukaryotes.

Three conclusions are worth remembering. Methionine is the first amino acid in virtually every eukaryotic polypeptide chain. Bacteria use N-formylmethionine instead of methionine to start a polypeptide chain. The start codon AUG codes for methionine and also marks where translation begins.

Why the Ribosome Puts Methionine First

The ribosome is the machine that reads mRNA and links amino acids into a chain. If you want to know what is the function of the ribosome in polypeptide synthesis, the short version is that it binds mRNA, matches each codon to a transfer RNA, and forms the peptide bonds that hold the chain together.

Initiation is the step that fixes the first residue. A special initiator tRNA carrying methionine pairs with the AUG codon in the ribosome's P site, and only then does elongation begin. Because that initiator tRNA is the only one that can start the job, the first amino acid of a new polypeptide chain is methionine in eukaryotes and formylmethionine in bacteria.

Methionine is not chemically special. It is simply the amino acid that evolution tied to the AUG start signal, which is why the same answer holds across nearly all life.

The Polypeptide N Terminus, Explained

Every chain has two chemically distinct ends. The polypeptide n terminus, usually written as the N-terminus or amino terminus, is the end that keeps a free amino group. The opposite end is the C-terminus, which carries a free carboxyl group.

By convention, protein sequences are written from N-terminus to C-terminus, so the first letter in a written sequence is the starting methionine. The amino acid sequence of a polypeptide is called its primary structure, and it is read in that same direction.

Charged tRNAs deliver new amino acids to the C-terminal end of the growing chain, so the N-terminus stays put while the chain extends behind it. That is why the N-terminal residue is a stable marker of where translation started.

What Happens to the Starting Methionine After Translation

The starting residue does not always survive into the finished protein. Several processing steps act on it:

• Trimming: Enzymes called methionine aminopeptidases clip the first methionine off many proteins shortly after synthesis.
• Modification: Some proteins keep the methionine and add acetyl, methyl, or lipid groups to it.
• Targeting: N-terminal signal sequences direct proteins to the nucleus, mitochondria, or secretory pathway, and those signals are often cut away once the protein arrives.

The gene sequence still shows AUG at the start of the coding region even when the mature protein begins with a different residue. Methionine is the start, not necessarily the final first letter.

Methionine vs. N-Formylmethionine

System	Initiating amino acid	Start codon	Notes
Eukaryotic cytoplasm	Methionine	AUG	Standard initiator tRNAMet; the 5' cap helps position the ribosome.
Bacteria	N-formylmethionine	AUG, sometimes GUG or UUG	A formyl group is added to the charged tRNA before initiation.
Archaea	Methionine	AUG	Initiation machinery resembles the eukaryotic system.
Mitochondria	N-formylmethionine	AUG	Bacterial-style initiation inside a eukaryotic cell.

Both residues sit at the N-terminus, and both are removed from many mature proteins. The difference is a single formyl group on the amino group, which matters for how the initiator tRNA is recognized and how the protein is processed.

Why the Start Residue Matters in the Lab

Cloning a gene means keeping the start codon in frame, because a shifted reading frame changes every amino acid after it. Researchers who express a protein in E. coli often add a short tag to the N-terminus, then remove it with a protease after purification.

Analytical methods read the same end. Edman degradation and mass spectrometry both report the N-terminal sequence, which is a fast way to confirm that a protein was made as expected and that its start methionine was processed correctly.

Structure depends on the whole chain, not just the first residue. Polypeptide secondary structure is the result of hydrogen bonding between backbone atoms, while the overall fold emerges from the full sequence. To answer what is the monomer of a polypeptide, it is the amino acid, and each of those monomers is added to the growing chain in the order the mRNA dictates.

Common Misconceptions

• "Every protein starts with methionine." The polypeptide does, but the mature protein often does not after processing.
• "AUG is the only start codon." Bacteria can also initiate at GUG or UUG, still inserting N-formylmethionine.
• "The first amino acid determines the fold." Folding depends on the entire sequence and on the cellular environment.

Any questions about how this applies to a specific protein, a research project, or a clinical lab result are best directed to a qualified instructor, researcher, or healthcare professional.

COMMON QUESTIONS

Frequently Asked Questions

Does every polypeptide start with methionine?

Nearly every polypeptide chain starts with methionine in eukaryotes and with N-formylmethionine in bacteria and mitochondria, because translation begins at the AUG start codon with a dedicated initiator tRNA. The starting residue is frequently removed after translation, so a mature protein may begin with a different amino acid.

What is the N-terminus of a polypeptide?

The N-terminus is the end of the chain that carries a free amino group, and it corresponds to the first amino acid added during translation. Protein sequences are written from the N-terminus to the C-terminus, so the N-terminal residue appears first in a written sequence.

Can a protein start with an amino acid other than methionine?

Yes, after processing. Enzymes called methionine aminopeptidases remove the starting methionine from many proteins, and signal sequences are often clipped off as well. In bacteria, alternative start codons such as GUG or UUG can begin translation, but they still insert N-formylmethionine as the first residue.