Peptide Science Dictionary

Addition of an acetyl group (CH₃CO–) to a peptide’s N-terminus or lysine side chain; regulates activity and stability

Attachment of a fatty-acid chain to a peptide to extend its half-life in circulation (e.g., semaglutide, liraglutide)

Strength of binding between a peptide ligand and its receptor; measured as dissociation constant (K_d)

Coiled secondary structure where the backbone spirals with 3.6 residues per turn, stabilized by i → i+4 hydrogen bonds

Organic molecule with NH₂ and COOH groups; 20 standard types build all peptides and proteins

Having both hydrophobic and hydrophilic faces; many antimicrobial peptides adopt amphipathic helical structures to disrupt membranes

Insoluble peptide aggregate with cross-β structure; linked to Alzheimer’s (Aβ), Parkinson’s (α-synuclein), and type 2 diabetes (amylin)

Peptide hormone system (I, II, III) that regulates blood pressure and fluid balance via the renin–angiotensin system

Host defense peptide that kills bacteria, viruses, and fungi by disrupting membranes (e.g., defensins, LL-37, magainins)

Modified fragment (amino acids 176–191) of human growth hormone studied for fat metabolism

Dipeptide methyl ester (Asp-Phe) used as an artificial sweetener; approximately 200× sweeter than sucrose

28-amino-acid hormone released by heart atria in response to stretch; promotes sodium excretion and lowers blood pressure

Repeating N–Cα–C(=O) chain shared by all peptides; side chains branch off at each Cα carbon

Secondary structure where peptide strands align side-by-side (parallel or antiparallel), stabilized by inter-strand hydrogen bonds

Tight 4-residue loop that reverses the direction of a peptide chain; common in compact peptide structures

A peptide with biological effects beyond basic nutrition — hormones, antimicrobials, neurotransmitters, or signaling molecules

Fraction of an administered peptide that reaches systemic circulation in active form; a major challenge for oral peptide drugs

Acid-labile protecting group for the N-terminus used in the original Merrifield SPPS method; largely replaced by Fmoc

14-amino-acid peptide originally from frog skin; mammalian analog GRP regulates gut function and is studied in cancer diagnostics

Synthetic 15-amino-acid peptide derived from a gastric juice protein; studied for tissue repair, gut healing, and anti-inflammatory effects

9-amino-acid peptide that dilates blood vessels and mediates inflammation, pain, and vascular permeability

End of a peptide chain bearing a free carboxyl group (–COOH); opposite of N-terminus

32-amino-acid peptide hormone from the thyroid that lowers blood calcium; salmon calcitonin is used to treat osteoporosis

Dipeptide (β-Ala-His) concentrated in muscle and brain; acts as intracellular pH buffer, antioxidant, and metal chelator

Opioid peptide derived from casein during dairy digestion; example of a food-derived bioactive peptide

Short peptide (<30 residues) that crosses cell membranes; used to deliver drugs, nucleic acids, and cargo into cells (e.g., TAT peptide)

Gut peptide hormone that stimulates bile release, pancreatic enzyme secretion, and satiety signaling after a meal

Synthetic 29-amino-acid GHRH analog studied for sustained growth hormone release; often paired with ipamorelin

Most abundant protein in the body (~30%); triple-helix structure in skin, bones, tendons, and cartilage; 28 types identified

Hydrolyzed collagen fragments (~2–3 kDa) used in supplements; also called hydrolyzed collagen; ~90% bioavailable

Bond-forming reaction that releases water; the mechanism by which peptide bonds are created during translation

Chemical that activates the carboxyl group of one amino acid to form a peptide bond with the next during SPPS (e.g., HATU, HBTU, DIC)

Peptide whose backbone or side chains form a ring; often more stable, cell-permeable, and protease-resistant than linear peptides

Sulfur-containing amino acid whose thiol side chain (–SH) forms disulfide bonds; critical for peptide structure and redox biology

Mirror-image (right-handed) form of an amino acid; rare in nature but used in drug design for protease resistance

Unit of molecular mass equal to ~1.66 × 10⁻²⁴ g; peptides are typically measured in daltons or kilodaltons (kDa)

Family of small cysteine-rich antimicrobial peptides in innate immunity (α-defensins in neutrophils, β-defensins in epithelia)

Loss of a peptide or protein’s 3D structure due to heat, pH, or chemical stress; unfolds but doesn’t break covalent peptide bonds

Peptide with exactly 2 amino acids joined by one peptide bond (e.g., carnosine, aspartame, anserine)

Covalent S–S bond between two cysteine residues that stabilizes peptide and protein tertiary structure

Opioid neuropeptide that binds kappa receptors; involved in pain modulation, stress response, and addiction neurobiology

Sequencing method that removes one amino acid at a time from the N-terminus using PITC to determine peptide sequence

Elastic structural protein in skin, lungs, and arteries; rich in glycine and proline; elastin-derived peptides signal tissue repair

Opioid neuropeptide produced by the pituitary gland; reduces pain and produces euphoria during exercise, stress, and laughter

21-amino-acid vasoconstrictor peptide produced by endothelial cells; most potent known vasoconstrictor; implicated in pulmonary hypertension

Pentapeptide neurotransmitter (Met-enkephalin, Leu-enkephalin) that binds delta opioid receptors to modulate pain

Protein (or sometimes RNA) that catalyzes a specific biochemical reaction by lowering activation energy

Specific peptide sequence on an antigen recognized by an antibody or T-cell receptor; basis of vaccine design

One of 9 amino acids the body cannot synthesize: His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Val

39-amino-acid GLP-1 receptor agonist (Byetta) originally derived from Gila monster venom; first GLP-1 drug approved (2005)

Opioid peptide derived from food proteins during digestion (e.g., gluten exorphins from wheat, casomorphins from dairy)

Small peptide cleaved from fibrinogen by thrombin during blood clot formation; a marker of coagulation activation

Base-labile protecting group for the N-terminus in modern SPPS; removed by piperidine at each coupling step

Process by which a peptide or protein adopts its functional 3D shape, driven by hydrophobic interactions, H-bonds, and disulfide bridges

Fluorescence technique used to measure peptide–protein interactions and conformational changes at the nanometer scale

29–30 amino acid neuropeptide that modulates pain, feeding, mood, and cognition; co-expressed with other neurotransmitters

Peptide hormone secreted by G-cells in the stomach that stimulates acid secretion and gastric motility

Partially hydrolyzed collagen that dissolves in warm water and gels on cooling; precursor to collagen peptides

Copper-binding tripeptide (Gly-His-Lys) studied for wound healing, skin remodeling, and anti-aging effects

28-amino-acid “hunger hormone” secreted by the stomach that stimulates appetite and growth hormone release

30-amino-acid incretin hormone that stimulates insulin; basis for semaglutide (Ozempic) and tirzepatide (Mounjaro)

29-amino-acid peptide hormone from pancreatic alpha cells that raises blood glucose; counterpart to insulin

Tripeptide (Glu-Cys-Gly) that serves as the body’s master antioxidant; protects cells from oxidative damage and detoxifies xenobiotics

Smallest amino acid (R group = H); highly flexible; makes up every third residue in collagen (Gly-X-Y repeat)

Regulatory quality standard for pharmaceutical production; required for peptides used in human compounding and clinical applications

Time for half of a peptide dose to be cleared from the body; native peptides often have very short half-lives (minutes)

9-amino-acid peptide derived from hemoglobin; acts as an inverse agonist at cannabinoid CB1 receptors; studied for appetite suppression

Signaling molecule secreted into the bloodstream to regulate distant organs; many hormones (insulin, oxytocin, GLP-1) are peptides

Analytical technique used to measure peptide purity; the gold standard for quality testing reported on Certificates of Analysis

24-amino-acid mitochondria-derived peptide with neuroprotective and cytoprotective effects; studied in Alzheimer’s and aging research

Bond-breaking reaction that adds water across a peptide bond; reverse of condensation; fundamental mechanism of digestion

Collagen broken into small peptide fragments by enzymatic hydrolysis; synonym for collagen peptides

Modified amino acid unique to collagen; formed by post-translational hydroxylation of proline; vitamin C is required as cofactor

Half-maximal inhibitory concentration; the drug concentration needed to inhibit a biological process by 50%

Gut peptide hormone (GLP-1, GIP) released after eating that amplifies insulin secretion in a glucose-dependent manner

51-amino-acid peptide hormone (A-chain + B-chain linked by disulfide bonds) that regulates blood glucose; first peptide drug (1921)

Self-splicing protein segment that excises itself from a precursor; used in biotechnology for tag-free recombinant peptide production

Selective growth hormone secretagogue pentapeptide studied for GH release without cortisol or prolactin elevation

pH at which a peptide carries zero net charge; affects solubility, purification, and behavior in electrophoresis

15 kDa peptide that links IgA or IgM antibody subunits into dimers or pentamers; essential for mucosal immunity

Spin–spin coupling between nearby nuclei observed in NMR spectroscopy; used to determine peptide backbone geometry and structure

Fibrous structural protein rich in cysteine disulfide bonds; main component of hair, nails, and the outer layer of skin

Enzyme that transfers a phosphate group to a peptide or protein, activating or deactivating it (phosphorylation)

Peptide hormone (encoded by KISS1 gene) that triggers puberty onset and regulates reproductive function via GnRH neurons

Anti-inflammatory tripeptide (Lys-Pro-Val) derived from alpha-MSH C-terminus; studied for gut inflammation and colitis

Left-handed stereoisomer of an amino acid; the form used almost exclusively in biology to build peptides and proteins

167-amino-acid peptide hormone secreted by adipose tissue that signals satiety and regulates long-term energy balance

Decapeptide (also called GnRH) from the hypothalamus that controls reproductive hormones; analogs treat prostate cancer and endometriosis

Molecule (often a peptide) that binds to a receptor to trigger or block a biological response

GLP-1 receptor agonist (Victoza, Saxenda) acylated with a C16 fatty acid for once-daily dosing; approved for diabetes and obesity

37-amino-acid human cathelicidin antimicrobial peptide; part of innate immunity against bacteria, viruses, fungi, and biofilms

Freeze-drying; standard method for preserving peptides as stable powder for long-term storage and reconstitution before use

23-amino-acid antimicrobial peptide from African clawed frog skin; model for understanding how AMPs disrupt bacterial membranes

Analytical method that measures molecular mass to identify and characterize peptides; paired with HPLC (LC-MS) for quality control

Peptide fragment released from extracellular matrix proteins during tissue remodeling; signals wound repair, angiogenesis, and inflammation

Family of peptide hormones (ACTH, α/β/γ-MSH) derived from POMC that regulate pigmentation, inflammation, appetite, and sexual function

26-amino-acid amphipathic peptide from bee venom; potent antimicrobial and hemolytic agent studied in cancer and inflammation research

Mass of a peptide in daltons; peptides are typically <5 kDa, proteins >5 kDa; determines behavior in HPLC, filtration, and dosing

16-amino-acid mitochondria-derived peptide that regulates metabolic homeostasis, insulin sensitivity, and exercise response

End of a peptide chain bearing a free amino group (–NH₂); convention is to write sequences from N- to C-terminus

Coenzyme central to metabolism and DNA repair; not a peptide itself but frequently co-administered with peptide therapy protocols

Family of peptide hormones (ANP, BNP, CNP) that lower blood pressure by promoting sodium excretion; BNP is a heart failure biomarker

Peptide functioning as a neurotransmitter or neuromodulator in the nervous system (e.g., substance P, endorphins, NPY)

36-amino-acid peptide that is one of the most abundant neuropeptides in the brain; stimulates appetite and regulates stress and anxiety

13-amino-acid peptide in the gut and brain that modulates dopamine signaling, pain perception, and body temperature

Peptide with exactly 9 amino acids (e.g., oxytocin, vasopressin)

Peptide containing approximately 4–20 amino acid residues; between dipeptide and polypeptide in size

Peptide that binds opioid receptors to modulate pain and mood (endorphins, enkephalins, dynorphins, endomorphins)

Neuropeptide pair (orexin-A, orexin-B) produced by the hypothalamus that regulates wakefulness, appetite, and arousal; loss causes narcolepsy

49-amino-acid peptide hormone secreted by osteoblasts; regulates bone mineralization and acts as an endocrine signal to muscle, brain, and testes

9-amino-acid cyclic peptide hormone governing bonding, trust, lactation, and uterine contraction; structurally related to vasopressin

84-amino-acid peptide hormone that raises blood calcium by stimulating bone resorption and kidney reabsorption; fragment (1–34) treats osteoporosis

Attachment of polyethylene glycol chains to a peptide to increase half-life, reduce immunogenicity, and improve solubility

Peptide with exactly 5 amino acids (e.g., enkephalins, thymopentin, ipamorelin)

Short chain of 2–50 amino acids linked by peptide bonds; smaller than proteins; serves as hormones, signals, and drugs

Covalent bond (–CO–NH–) joining two amino acids via a condensation reaction; the fundamental link in all peptides and proteins

Collection of thousands to millions of peptide variants screened to identify sequences with desired binding or activity

Complete set of peptides present in a cell, tissue, or organism at a given time; studied by mass spectrometry-based peptidomics

Synthetic molecule designed to mimic a peptide’s structure and activity with improved stability, oral bioavailability, or selectivity

Peptide-like oligomer where side chains attach to nitrogen instead of carbon; highly protease-resistant; used in antimicrobial and drug design

Technique where peptides are displayed on bacteriophage surfaces and screened for binding to targets; Nobel Prize 2018 (Smith & Winter)

Addition of a phosphate group to Ser, Thr, or Tyr residues by a kinase; a key on/off switch regulating peptide and protein activity

Peptide chain of approximately 21–50+ amino acids; the term bridges short peptides and full proteins

Chemical change to a peptide after ribosomal synthesis (phosphorylation, glycosylation, acetylation, methylation, etc.)

Inactive peptide precursor that requires enzymatic cleavage to become active (e.g., proinsulin → insulin + C-peptide)

Amino acid with a cyclic side chain that introduces rigid kinks in peptide chains; abundant in collagen (Gly-Pro-Hyp repeat)

Enzyme that catalyzes the hydrolysis of peptide bonds; also called peptidase or proteinase; major barrier to oral peptide drugs

Macromolecule of 50+ amino acids with complex 3D folding and defined biological function

Breakdown of proteins into peptides and amino acids by proteases; fundamental to digestion, immune defense, and cell regulation

Arrangement of multiple polypeptide subunits into a functional complex (e.g., hemoglobin has 4 subunits, insulin forms hexamers)

Variable chemical group attached to each amino acid’s Cα; determines charge, size, hydrophobicity, and reactivity

Graph of backbone dihedral angles (φ and ψ) showing allowed conformations for amino acid residues in a peptide chain

Protein on or in a cell that binds a specific peptide ligand to trigger a biological response (e.g., GLP-1 receptor, opioid receptors)

Peptide produced by genetically engineered cells (E. coli, yeast, CHO cells) rather than chemical synthesis; scalable for large peptides

A single amino acid unit within a peptide chain after the water molecule has been lost during peptide bond formation

Molecular machine that reads mRNA and assembles amino acids into peptide chains; the site of biological peptide synthesis (translation)

Local folding patterns in a peptide chain — alpha helices, beta sheets, and turns — stabilized by backbone hydrogen bonds

27-amino-acid gut hormone that stimulates bicarbonate secretion from the pancreas; the first hormone ever discovered (1902)

Synthetic heptapeptide analog of tuftsin developed in Russia; studied for anxiolytic and nootropic effects

Peptide designed to spontaneously form nanostructures (fibers, hydrogels, nanotubes); used in tissue engineering and drug delivery scaffolds

GLP-1 receptor agonist (Ozempic, Wegovy, Rybelsus) for type 2 diabetes and obesity; acylated with C18 fatty diacid for weekly dosing

Synthetic heptapeptide analog of ACTH(4–10) developed in Russia; studied for neuroprotection and cognitive enhancement

Ordered list of amino acids in a peptide, written from N-terminus to C-terminus using one-letter (ACDE…) or three-letter (Ala-Cys…) codes

Short N-terminal sequence (15–30 residues) that directs a newly synthesized peptide to the correct cellular compartment; cleaved after delivery

14-amino-acid cyclic peptide hormone that inhibits growth hormone, insulin, glucagon, and gastrin release

Lab method for building peptides one amino acid at a time on a solid resin bead (Merrifield method, Nobel Prize 1984)

Peptide with a hydrocarbon cross-link (“staple”) that locks its alpha-helical shape, improving cell penetration, stability, and potency

11-amino-acid neuropeptide of the tachykinin family involved in pain transmission, inflammation, nausea, and mood regulation

Family of neuropeptides sharing a C-terminal sequence (Phe-X-Gly-Leu-Met-NH₂); includes substance P and neurokinin A/B

43-amino-acid peptide involved in actin regulation, cell migration, wound healing, and tissue repair

Overall 3D shape of a single polypeptide chain, determined by interactions between R groups (hydrophobic, ionic, disulfide, H-bonds)

Peptide with exactly 4 amino acids (e.g., tuftsin, FMRF-amide, rigin)

28-amino-acid peptide that modulates T-cell and dendritic cell function; approved as an immunotherapy in over 35 countries

Dual GIP/GLP-1 receptor agonist peptide (Mounjaro, Zepbound) for type 2 diabetes and obesity; the first twincretin drug

Tripeptide (pGlu-His-Pro-NH₂) from the hypothalamus that stimulates TSH and prolactin release; one of the smallest bioactive peptides

Peptide with exactly 3 amino acids joined by 2 peptide bonds (e.g., glutathione, GHK-Cu, TRH)

Serine protease in the small intestine that cleaves peptide bonds after lysine and arginine; widely used in proteomics sample preparation

76-amino-acid peptide that tags damaged or unneeded proteins for destruction by the proteasome (Nobel Prize 2004)

Any amino acid not among the 20 standard ones; used in peptide drug design for improved stability, selectivity, and novel functions

Cyclic peptide (11 amino acids in humans) that is the most potent vasoconstrictor identified; studied in cardiovascular disease

Cyclic depsipeptide (12 residues, alternating amino acids and esters) that selectively transports potassium ions across membranes

28-amino-acid neuropeptide that relaxes smooth muscle, stimulates water and electrolyte secretion, and modulates immune responses

9-amino-acid cyclic peptide hormone that regulates water retention, blood pressure, and social behavior; differs from oxytocin by 2 amino acids

Bioactive peptide found in animal venoms (snakes, scorpions, spiders, cone snails); rich source of drug leads (e.g., ziconotide, exenatide, captopril)

Laboratory technique that separates proteins/peptides by size on a gel, transfers them to a membrane, and detects specific targets with antibodies

Bioactive peptide fragments derived from whey protein hydrolysis; studied for satiety, muscle protein synthesis, and blood pressure reduction

Technique that reveals atomic-level 3D structures of peptides and proteins by diffracting X-rays through a crystallized sample

Synthetic or foreign peptide not found in the host organism; includes therapeutic peptides, peptide vaccines, and research tools

C-terminal fragment ion produced during tandem mass spectrometry (MS/MS); used alongside b-ions to determine peptide amino acid sequences

Percentage of target peptide obtained from a synthesis run; decreases with chain length in SPPS due to cumulative coupling inefficiencies

25-amino-acid peptide from cone snail venom (Prialt); FDA-approved for severe chronic pain via intrathecal delivery; a venom-to-drug success story

Amino acid in its doubly charged form (NH₃⁺ and COO⁻) carrying both positive and negative charges simultaneously; dominant state at physiological pH