Peptide Sequence
| Category | Glossary |
|---|---|
| Also known as | Amino Acid Sequence, Primary Structure, Peptide Chain |
| Last updated | 2026-04-13 |
| Reading time | 4 min read |
| Tags | biochemistrypeptide-structurenomenclatureglossary |
Overview
A peptide sequence (also called the primary structure) is the specific linear order in which amino acid residues are linked together by peptide bonds in a peptide or protein chain. The sequence is universally read and written from the N-terminus (the free amino group end) to the C-terminus (the free carboxyl group end). This directionality convention is analogous to reading text from left to right and is critical for unambiguously identifying any peptide.
The peptide sequence is the most fundamental level of structural information — it dictates how the molecule folds, which receptors it can bind, and ultimately what biological effects it produces.
Detailed Explanation
Nomenclature and Notation
Peptide sequences are written using either the three-letter code or the single-letter code for each amino acid:
- Three-letter example: Gly-His-Lys (the peptide GHK)
- Single-letter example: GHK
By convention, the N-terminal residue is written first (on the left) and the C-terminal residue is written last (on the right). Hyphens between residues in three-letter notation indicate peptide bonds.
N-Terminus and C-Terminus
Every linear peptide has two distinct ends:
- N-terminus (amino terminus): The end of the chain with a free alpha-amino group (-NH2). This is where ribosomal synthesis begins and where the sequence is read from.
- C-terminus (carboxyl terminus): The end with a free alpha-carboxyl group (-COOH). This is where synthesis terminates.
These terminal positions are particularly susceptible to enzymatic degradation by exopeptidases — aminopeptidases cleave from the N-terminus, while carboxypeptidases cleave from the C-terminus. This vulnerability is why terminal modifications (acetylation, amidation) are common strategies for extending peptide half-life.
Sequence Determines Function
The biological activity of a peptide is entirely determined by its sequence. Even small changes can have dramatic effects:
- Single residue substitution can abolish activity, alter receptor selectivity, or change a peptide from an agonist to an antagonist.
- Truncation (removing residues from either end) may retain partial activity if the critical binding epitope remains intact, or may eliminate activity entirely.
- Extension (adding residues) can improve stability or alter pharmacokinetics without necessarily changing the core biological activity.
Sequence Modifications
Several modifications to the natural sequence are commonly employed in peptide research:
| Modification | Purpose | Example |
|---|---|---|
| N-terminal acetylation | Protects against aminopeptidases | Ac-peptide |
| C-terminal amidation | Protects against carboxypeptidases, improves receptor binding | Peptide-NH2 |
| D-amino acid substitution | Resists all proteases at substituted positions | Uses lowercase letters (e.g., dAla) |
| Cyclization | Eliminates terminal vulnerability, constrains conformation | Head-to-tail or side-chain cyclics |
| PEGylation | Extends half-life, reduces immunogenicity | PEG-peptide conjugates |
Sequence Analysis Methods
The sequence of a peptide can be determined or confirmed through several analytical techniques:
- Edman degradation: Sequentially removes and identifies one amino acid at a time from the N-terminus. Limited to shorter sequences.
- Mass spectrometry (MS/MS): Fragments the peptide and analyzes the mass differences between fragments to reconstruct the sequence. The standard method for peptide identification.
- Amino acid analysis: Hydrolyzes the peptide completely and quantifies each amino acid present, confirming composition (but not order).
Relevance to Peptide Research
Identity Verification
The sequence is the definitive identifier of any peptide. A certificate of analysis confirms that the synthesized product matches the intended sequence through mass spectrometry and other analytical methods. Sequence errors during synthesis — deletions, insertions, or substitutions — produce impurities that must be separated during purification.
Structure-Activity Relationships (SAR)
Systematic modification of individual positions within a peptide sequence — known as an alanine scan or positional scanning — is a standard approach for identifying which residues are critical for biological activity. This information guides the design of more potent, selective, or stable analogs.
Database Resources
Peptide sequences are cataloged in public databases such as UniProt (for natural proteins and peptides), PDB (for structures), and specialized peptide databases. These resources enable researchers to compare sequences across species and identify conserved functional domains.
Examples
BPC-157 has the sequence Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val (15 residues). The high proportion of proline residues (4 out of 15) confers structural rigidity and contributes to its unusual stability in gastric environments.
GHK-Cu is one of the simplest biologically active peptide sequences — just three residues (Gly-His-Lys) complexed with a copper(II) ion. The histidine residue is essential for copper coordination, and substitution of any residue significantly diminishes biological activity.
Related Terms
Peptide sequences are composed of individual amino acids connected by peptide bonds. The sequence determines molecular weight and susceptibility to proteolysis. Sequence identity is confirmed through a certificate of analysis.
Related entries
- Amino Acid— The fundamental building blocks of peptides and proteins, consisting of 20 standard types encoded by DNA, each with distinct chemical properties that determine peptide structure and function.
- Certificate of Analysis (COA)— A quality assurance document issued by a laboratory that verifies the identity, purity, and composition of a peptide product through standardized analytical testing methods.
- Molecular Weight— The total mass of a peptide molecule measured in Daltons (Da), determined by the sum of its constituent amino acid residues, which influences bioavailability, half-life, and pharmacological behavior.
- Peptide Bond— A covalent chemical bond formed between the carboxyl group of one amino acid and the amino group of another through a condensation reaction, serving as the fundamental linkage in all peptides and proteins.