AgonistA molecule that binds a receptor and activates it, producing a biological response (e.g., a peptide hormone acting on its receptor).Amino acidThe building blocks of peptides and proteins. There are 20 standard amino acids in humans, each with a unique side chain (R group) that determines chemical properties.AntagonistA molecule that binds a receptor but does not activate it, blocking or reducing the effect of an agonist.Antimicrobial peptide (AMP)A naturally occurring or synthetic peptide that can kill or inhibit bacteria, fungi, or viruses, often by disrupting microbial membranes.BioavailabilityThe fraction of administered peptide that reaches systemic circulation in active form. Most peptides have low oral bioavailability.Boctert-Butyloxycarbonyl protecting group used in peptide synthesis; removed under acidic conditions (historical SPPS method).C-terminusThe carboxyl (COOH) end of a peptide chain. Often amidated in synthetic peptides for stability.CyclizationConnecting the N- and C-termini or side chains of a peptide to form a ring structure, increasing resistance to proteases.Cyclic peptideA peptide whose backbone or side chains form a closed ring, often conferring greater enzymatic stability and defined conformation.Disulfide bondA covalent bond between the thiol groups of two cysteine residues that stabilizes peptide and protein tertiary or quaternary structure.Fmoc9-Fluorenylmethoxycarbonyl protecting group widely used in modern solid-phase peptide synthesis; removed under basic conditions.Half-lifeThe time required for half of a peptide to be cleared or degraded in vivo. A critical parameter in therapeutic peptide design.HydrophobicityThe tendency of amino acid side chains to avoid water. Used to predict membrane interaction, solubility, and folding behavior.IC50The concentration of a peptide required to inhibit 50% of a biological response or binding; measure of potency.Lactam bridgeA side-chain cyclization (amide bond) between residues such as Lys and Glu or Asp to constrain peptide conformation.LigandA molecule (such as a peptide) that binds specifically to a receptor or other target protein.LipidationCovalent attachment of fatty acid chains (e.g., palmitoyl) to peptides to improve membrane permeability and serum half-life.N-terminusThe amino (NH2) end of a peptide chain. Often acetylated in synthetic peptides to improve stability.Native Chemical Ligation (NCL)A chemoselective method for joining two unprotected peptide fragments to form a native peptide bond under mild aqueous conditions.NeuropeptideA peptide that acts as a neurotransmitter or neuromodulator in the nervous system, influencing pain, mood, appetite, memory, and other functions.OligopeptideA short peptide, conventionally containing 2–20 amino acids.pKaThe pH at which half of a given ionizable group (carboxyl, amino, or side chain) is dissociated. Used to calculate net charge and isoelectric point.PEGylationAttachment of polyethylene glycol (PEG) polymer chains to a peptide to increase solubility, reduce immunogenicity, and extend circulation time.Peptide bondThe covalent amide bond linking the carboxyl group of one amino acid to the amino group of the next, formed by condensation (loss of water).Peptide hormoneA signaling peptide secreted into the bloodstream to act on distant target cells (e.g., insulin, oxytocin, GLP-1).PeptidomimeticA synthetic non-peptide or modified-peptide molecule designed to mimic the binding and function of a natural peptide while improving drug-like properties.Post-translational modification (PTM)Enzymatic or chemical modifications (phosphorylation, glycosylation, cleavage, etc.) applied to peptides/proteins after ribosomal synthesis.Primary structureThe linear amino acid sequence of a peptide or protein.Protease (peptidase)An enzyme that hydrolyzes peptide bonds, breaking down peptides and proteins. Major factor in peptide degradation in vivo.PolypeptideA chain of amino acids, generally longer than ~20 residues; often used interchangeably with peptide in literature.ProteinA folded macromolecule composed of one or more polypeptide chains, typically >50 amino acids, with complex 3D structure and diverse functions.ReceptorA protein (membrane-bound or intracellular) that specifically binds a peptide ligand and transduces a signal into a cellular response.ResidueAn individual amino acid unit within a peptide or protein chain after incorporation (minus the elements of water lost in bond formation).Secondary structureLocal folding patterns in peptides/proteins such as alpha-helices, beta-sheets, or turns stabilized by hydrogen bonds.Side chainThe variable R-group attached to the alpha-carbon of an amino acid that confers its unique physicochemical properties.Solid-phase peptide synthesis (SPPS)Stepwise chemical assembly of peptides on an insoluble resin support. The dominant method for laboratory and industrial peptide production.Stapled peptideA synthetic peptide constrained into an alpha-helical conformation by a covalent hydrocarbon 'staple' bridge between side chains; improves binding and stability.SequenceThe specific order of amino acids in a peptide, conventionally written N-to-C terminus using one- or three-letter codes.Fluorescent labelingCovalent attachment of fluorophores (FITC, Cy5, etc.) for visualization, tracking, or fluorescence-based assays.Bioactive peptideA peptide that elicits a measurable biological response through receptor binding or other mechanisms.Recombinant peptidePeptide or protein produced in genetically engineered host cells (E. coli, yeast, mammalian) rather than chemical synthesis.Oral bioavailabilityAbility of a peptide to survive digestion and be absorbed when taken by mouth; usually very low without special formulation.Structure-activity relationship (SAR)Systematic study of how changes in peptide sequence or structure affect biological activity and potency. 