One to Three
Convert single-letter amino acid codes to three-letter codes
Input Protein

Paste a raw sequence or one or more FASTA sequences. Input limit: 100,000,000 characters.

💡 Quick Summary

One to Three converts single-letter amino acid codes to their three-letter equivalents. Paste any raw or FASTA protein sequence and each residue is expanded to its standard three-letter code — for example A → Ala, G → Gly, * → ***. Useful for preparing sequences for publication, annotation, or readability.

📋 How to Use
  1. Paste a raw protein sequence or one or more FASTA sequences into the input area.
  2. Click Convert. Each residue is replaced with its three-letter code. Multiple FASTA sequences are each converted separately.
  3. The Summary panel shows how many sequences were converted and the total number of residues processed.
  4. Use the Copy button to copy the result to your clipboard.
  5. Click Load Example to try with two short FASTA sequences covering all 20 standard amino acids plus ambiguity codes.
  6. Click Clear to reset.
🧮 Formulas & Logic
Conversion
Each single-letter code is replaced with its three-letter IUPAC name (e.g. A → Ala, G → Gly)
Ambiguity codes
B → Asx (Asp or Asn), X → Xaa (unknown), Z → Glx (Glu or Gln), * → ***
Case
Output codes are title-case (first letter uppercase, remaining lowercase): Ala, Cys, Asp…
Wrapping
Output is wrapped at 60 characters per line
📊 Result Interpretation
Sequences Converted

Number of FASTA entries (or bare sequences) processed.

Total Residues

Total number of amino acid characters converted across all sequences.

🔬 Applications
  • Preparing protein sequences for publication where three-letter notation is required
  • Converting sequences for use in structural biology tools that expect three-letter codes
  • Making a protein sequence more readable for manual inspection or teaching purposes
  • Checking that ambiguity codes (B, X, Z) are correctly placed in a sequence
⚠️ Common Mistakes & Warnings
Non-amino-acid characters are stripped before conversion

Digits, spaces, and other non-letter characters (except *) are removed before the sequence is converted. The residue count reflects only the characters that were actually converted.

Stop codons are shown as ***

The stop-codon symbol (*) is represented as "***" in the output to maintain alignment with the three-character width of amino acid codes. This matches the original SMS behaviour.

❓ Frequently Asked Questions

What is the difference between one-letter and three-letter amino acid codes?
Single-letter codes (e.g. A, G, P) are compact representations used in most sequence databases and bioinformatics tools. Three-letter codes (e.g. Ala, Gly, Pro) are the traditional biochemistry notation used in structural databases, publications, and lab protocols. Both systems encode the same 20 standard amino acids.
What does "Asx" mean?
"Asx" (code B) represents an ambiguous position that could be either Aspartate (Asp, D) or Asparagine (Asn, N). Similarly, "Glx" (code Z) covers Glutamate (Glu, E) or Glutamine (Gln, Q), and "Xaa" (code X) means any amino acid.
Can I paste multiple FASTA sequences?
Yes. Paste any number of FASTA sequences (each beginning with ">"). Each is converted independently and the output preserves the original FASTA headers with the residue count appended.
Why is the stop codon shown as "***" instead of "Ter"?
"***" is the representation used by the original Sequence Manipulation Suite. Some tools use "Ter" as the official IUPAC three-letter code for a stop signal; if you need "Ter" specifically, replace "***" in the copied output.
One to Three — conversion logic ported from the Sequence Manipulation Suite (Paul Stothard, University of Alberta).