Peptides are molecules composed of relatively short chains of amino acids connected by peptide bonds. An amino acid contains an amino group, a carboxyl group and a characteristic side chain, and peptide formation occurs when amino acids are joined through condensation reactions involving their functional groups. The resulting peptide chain has directionality, with an amino terminal end and a carboxyl terminal end.
The number of amino acids in a peptide can vary substantially, and there is no universally accepted boundary separating a peptide from a protein purely by chain length. The distinction is often based on convention, function or context rather than a strict chemical rule. Peptides can exist as biological signalling molecules, hormones, antimicrobial agents, toxins, structural components, metabolic intermediates and fragments generated during protein processing. Their functions depend on amino acid sequence, chemical modifications, three dimensional structure and interactions with other molecules.
Peptide bonds have partial double bond character because of resonance involving the carbonyl group and nitrogen atom. This gives the peptide backbone restricted geometry and contributes to the predictable structural behaviour of peptide chains. Rotation is possible around some adjacent bonds, allowing peptides to adopt different conformations, but steric and electronic constraints limit the available conformational space. Side chains influence whether particular regions prefer certain structural arrangements.
Hydrogen bonding, ionic interactions, hydrophobic interactions and other forces can stabilise folded peptide structures. Some peptides remain relatively flexible, while others form well defined helices, sheets, turns or compact structures. Disulfide bonds can form between cysteine residues in suitable oxidising environments and can greatly stabilise extracellular peptides and proteins. Peptide structure is therefore determined by both covalent sequence and noncovalent physical chemistry.
Many peptides act as signalling molecules. Peptide hormones can be released by specialised cells and bind receptors on target cells, initiating intracellular signalling pathways. Examples include insulin, glucagon and many other regulatory peptides. Neuropeptides participate in communication between neurons and between neurons and other cell types, influencing processes such as pain perception, appetite and physiological regulation.
Peptides can also serve as antimicrobial molecules, particularly in innate immune systems where their physicochemical properties allow them to interact with microbial membranes or intracellular targets. Venoms produced by animals can contain complex mixtures of peptides that affect ion channels, receptors, enzymes or other physiological systems. The biological activity of a peptide depends strongly on its precise sequence and chemical state, so even a small sequence alteration can change receptor affinity, stability, localisation or biological effect.
Peptides are produced through several biological mechanisms. Ribosomes synthesise polypeptide chains by translating messenger RNA, and these chains can subsequently be processed into functional peptides. Some signalling peptides are initially produced as larger precursor molecules that undergo enzymatic cleavage to release mature active forms. Non ribosomal peptide synthesis provides another mechanism in which specialised enzyme complexes assemble peptides without direct translation from messenger RNA.
This mechanism can produce unusual amino acids and chemical structures that are difficult to generate through ordinary ribosomal synthesis. Post translational modifications can further diversify peptide structures through processes such as phosphorylation, glycosylation, amidation or disulfide formation. These modifications can influence activity, stability and localisation. Peptide biosynthesis therefore includes both template directed and specialised enzymatic pathways.
Peptides have important pharmaceutical applications because their structures can be designed to interact selectively with biological targets. Therapeutic peptides can replace deficient hormones, activate or block receptors, influence metabolic pathways or deliver biological signals. However, many peptides are vulnerable to enzymatic degradation and may have limited ability to cross cell membranes. Researchers therefore investigate sequence modifications, chemical stabilisation, controlled release systems and alternative delivery methods to improve their pharmacological properties.
Some peptide medicines are administered by injection because oral delivery can be difficult when digestive enzymes degrade the peptide or when intestinal absorption is poor. Advances in peptide engineering have produced molecules with altered stability, receptor selectivity and duration of action. The usefulness of a therapeutic peptide therefore depends not only on biological potency but also on pharmacokinetics, manufacturing, stability, delivery and safety.
Peptides are related to proteins but should not automatically be treated as identical. Proteins are generally larger folded biological macromolecules composed of one or more polypeptide chains, whereas peptides are often shorter and may function without forming the complex architecture characteristic of many proteins. The terms oligopeptide, polypeptide and protein are used with overlapping boundaries, and terminology can vary between biochemical contexts.
A peptide sequence is written from the amino terminal residue toward the carboxyl terminal residue, and substitutions can be described by identifying the original and replacement residues. Peptide fragments can also be produced when proteins are digested by proteolytic enzymes. Such fragments may have biological activity even when the intact parent protein performs a different primary function. Consequently, peptide biology includes both independently synthesised signalling molecules and fragments generated from larger proteins.
Peptide chemistry is also important in laboratory research and biotechnology. Synthetic peptide chemistry permits researchers to manufacture defined sequences for studying receptor interactions, antibody recognition, enzyme activity and protein structure. Peptides can be attached to surfaces or other molecules to provide recognition sites, purification handles or experimental probes. Mass spectrometry can identify peptides generated by protein digestion and is widely used in proteomics to infer the proteins present in a sample.
Peptide sequencing and quantitative analysis can reveal changes in biological pathways and disease associated molecular patterns. Computational methods can predict peptide structure, activity and potential interactions, although experimental validation remains essential because biological environments can produce effects that are difficult to predict from sequence alone. Peptides therefore provide a useful intermediate scale between small molecules and large proteins, combining relatively manageable chemical synthesis with highly specific biological interactions.
The diversity of peptides arises from the enormous number of possible amino acid sequences and from chemical modifications that can occur before, during or after synthesis. Two peptides containing the same amino acids in different orders can have completely different structures and functions. Changes in length, charge, hydrophobicity and conformational preference can alter membrane interactions, receptor binding and degradation rates. Some peptides form stable complexes with metal ions or nucleic acids, while others bind proteins with high specificity.
Their biological effects can be local or systemic depending on secretion, transport and receptor distribution. Peptides also participate in evolution because gene duplication, mutation and processing can create new signalling molecules, while natural selection can preserve sequences that produce useful biological effects. Understanding peptides therefore requires integration of organic chemistry, molecular biology, structural biology, physiology, pharmacology and evolutionary biology.
