10.5 Peptides

Peptide structure and composition

Peptides are chains of amino acids linked by peptide bonds. They serve as signaling molecules, enzyme substrates, and drug targets throughout the body. In medicinal chemistry, understanding how peptides are built and how they fold is the foundation for designing peptide-based drugs.

Amino acid building blocks

Amino acids are organic compounds that each contain an amino group ($-NH_2$), a carboxyl group ($-COOH$), and a unique side chain (R group). The R group is what gives each amino acid its distinct chemical personality: some are hydrophobic, some are charged, some are polar.

There are 20 standard amino acids encoded by the genome. Peptide bonds form through a condensation reaction between the carboxyl group of one amino acid and the amino group of the next, releasing a molecule of water.

Primary structure of peptides

The primary structure is simply the linear sequence of amino acids in the chain, read from the N-terminus (free amino group) to the C-terminus (free carboxyl group). This sequence is dictated by the genetic code and determines everything about how the peptide will fold and function. Even a single amino acid substitution can dramatically alter a peptide's activity.

Secondary structures: α-helices and β-sheets

Secondary structures are repeating local folding patterns stabilized by hydrogen bonds between backbone atoms (not the side chains).

• α-helix: A right-handed spiral with 3.6 amino acids per turn. Hydrogen bonds form between the carbonyl oxygen of residue n and the amino hydrogen of residue n+4, running parallel to the helix axis.
• β-sheets: Extended peptide strands aligned side by side, connected by hydrogen bonds between backbone atoms of adjacent strands. Strands can run in the same direction (parallel) or opposite directions (antiparallel), creating a pleated sheet-like structure.

Tertiary structure and folding

Tertiary structure is the overall three-dimensional shape of the peptide, formed when secondary structure elements fold and pack together. This folding is driven by non-covalent interactions between amino acid side chains:

• Hydrogen bonds
• Van der Waals forces
• Hydrophobic interactions (nonpolar side chains cluster away from water)
• Electrostatic interactions (salt bridges between charged residues)

The tertiary structure determines binding sites, catalytic activity, and molecular recognition. When peptides misfold, they can aggregate into insoluble fibrils, a hallmark of diseases like Alzheimer's (amyloid-β plaques) and Parkinson's (α-synuclein aggregates).

Peptide synthesis

Solid-phase peptide synthesis (SPPS)

SPPS, developed by Bruce Merrifield, is the standard method for making peptides in the lab. The key idea is anchoring the growing chain to an insoluble resin bead, which makes purification between steps as simple as washing.

The general SPPS cycle:

1. Attach the first (C-terminal) amino acid to the resin
2. Remove the protecting group from the N-terminus of the resin-bound amino acid
3. Activate the carboxyl group of the next amino acid using a coupling reagent
4. Couple the activated amino acid to the free N-terminus on the resin
5. Wash away excess reagents and byproducts
6. Repeat steps 2–5 for each amino acid in the sequence
7. Cleave the finished peptide from the resin and remove all side-chain protecting groups

SPPS produces longer peptides with higher purity than solution-phase methods because unreacted reagents and byproducts are simply washed away after each step.

Protecting groups and coupling reagents

Protecting groups temporarily block reactive functional groups to prevent unwanted side reactions during synthesis. Two widely used protection strategies:

• Fmoc (9-fluorenylmethoxycarbonyl): Protects the N-terminus; removed with a mild base (piperidine). This is the most common strategy in modern SPPS.
• tBu (tert-butyl): Protects reactive side chains; removed under acidic conditions (TFA).

The orthogonality of these groups is critical: you can remove Fmoc without disturbing tBu, and vice versa.

Coupling reagents activate the incoming amino acid's carboxyl group so it reacts efficiently with the free amine on the resin. Common examples:

• Carbodiimides: DCC, EDC
• Phosphonium salts: PyBOP
• Uronium/aminium salts: HBTU, HATU (HATU generally gives the best coupling efficiency)

Purification and characterization techniques

After cleavage from the resin, the crude peptide contains truncated sequences, deletion products, and other impurities that must be removed.

• RP-HPLC (reversed-phase high-performance liquid chromatography) is the workhorse for peptide purification, separating species based on hydrophobicity.
• Mass spectrometry (MALDI-TOF, ESI-MS) confirms molecular weight and can verify the sequence.
• Circular dichroism (CD) spectroscopy provides information about secondary structure content (α-helix vs. β-sheet vs. random coil).
• NMR spectroscopy gives detailed structural and conformational data, especially useful for smaller peptides.

Challenges in peptide synthesis

• Racemization: Loss of stereochemical integrity at the α-carbon during activation and coupling
• Aspartimide formation: A common side reaction at aspartate residues, especially in Asp-Gly sequences
• Aggregation: Hydrophobic or β-branched sequences can cause the growing chain to aggregate on the resin, leading to incomplete coupling
• Length limitations: Peptides longer than ~50 amino acids become increasingly difficult to synthesize with acceptable purity. For longer sequences, fragment condensation or native chemical ligation may be needed.
• Special modifications: Cyclization, stapling, or incorporation of non-natural amino acids often require specialized reagents and optimized protocols.

Peptide-based drug discovery

Therapeutic targets for peptide drugs

Peptides can be designed to modulate a wide range of biological targets:

• GPCRs are the most common targets for peptide drugs. These receptors mediate signaling in nearly every organ system and are implicated in diabetes, cancer, and cardiovascular disease.
• Ion channels and transporters can be modulated by peptide toxins and analogs (e.g., conotoxins from cone snails).
• Protein-protein interactions (PPIs), traditionally considered "undruggable," are increasingly accessible to peptide-based inhibitors because peptides can cover the large, flat interfaces involved.

Notable examples of peptide drug targets:

• GLP-1 receptor for type 2 diabetes
• μ-opioid receptor for pain management
• VEGF pathway for anti-cancer therapy

Peptide libraries and screening methods

Peptide libraries are large collections of diverse sequences screened against a target to find hits.

Library generation methods:

• Chemical synthesis (combinatorial libraries, one-bead-one-compound)
• Biological display methods: phage display, mRNA display, ribosome display

Screening techniques:

• Fluorescence polarization
• Surface plasmon resonance (SPR)
• ELISA

These methods are often used in iterative cycles: screen → select → amplify → re-screen. Each round enriches for peptides with higher affinity and selectivity.

Peptidomimetics and peptide analogs

Natural peptides often make poor drugs on their own because they're rapidly degraded and poorly absorbed. Peptidomimetics are compounds designed to mimic peptide structure and function while overcoming these pharmacological limitations.

Common peptidomimetic strategies:

• D-amino acid substitution: Protease-resistant mirror-image residues
• β-amino acids: Extra carbon in the backbone alters geometry and resists degradation
• Peptoids: N-substituted glycines where the side chain is on nitrogen instead of the α-carbon
• Small molecule mimics: Non-peptide scaffolds that reproduce key pharmacophoric features

Peptide analogs are modified versions of natural peptides. Modifications can improve receptor selectivity, reduce off-target effects, and increase metabolic stability.

Peptide drug delivery systems

Peptide drugs face significant delivery challenges: poor oral bioavailability, rapid renal clearance, and limited membrane permeability. Several strategies address these problems:

• Liposomes and nanoparticles: Encapsulate peptides, protecting them from degradation and facilitating transport across biological barriers
• Cell-penetrating peptides (CPPs): Short, often cationic peptides (e.g., Tat, penetratin) conjugated to therapeutic peptides to enhance cellular uptake
• Sustained-release formulations: Hydrogels, microspheres, and depot injections provide controlled release over days to months, reducing injection frequency

Peptide-protein interactions

Peptide binding sites on proteins

Peptides bind to specific sites on target proteins through complementary shape, electrostatic interactions, and hydrophobic contacts. These binding sites include:

• Active sites of enzymes (substrate-binding pockets)
• Allosteric sites that regulate protein function from a distance
• PPI interfaces where two proteins normally contact each other
• MHC grooves that present peptide fragments to T cells for immune recognition

The affinity and specificity of binding depend on the amino acid composition, sequence, and three-dimensional conformation of both the peptide and the binding site.

Peptide-mediated signaling pathways

Peptides act as ligands for cell surface receptors, triggering intracellular signaling cascades:

• Peptide hormones and neuropeptides bind GPCRs, activating G proteins and downstream effectors like adenylyl cyclase (cAMP pathway) and phospholipase C ($IP_3$/DAG pathway).
• Peptide growth factors such as IGF and EGF bind receptor tyrosine kinases (RTKs), initiating phosphorylation cascades (e.g., Ras/MAPK, PI3K/Akt) that control gene expression and cell fate.
• Peptides can also disrupt signaling by interfering with adaptor protein recruitment or signaling complex assembly.

Peptide inhibitors and agonists

Peptides can be designed as either inhibitors or agonists of specific targets:

Peptide inhibitors block the binding of natural ligands, substrates, or protein partners:

• Enfuvirtide: An HIV-1 fusion inhibitor that blocks interaction between the viral gp41 envelope protein and the host cell membrane, preventing viral entry
• Cilengitide: Inhibits $\alpha_v\beta_3$ and $\alpha_v\beta_5$ integrins involved in tumor angiogenesis

Peptide agonists mimic natural ligands to activate their targets:

• Exenatide: A GLP-1 receptor agonist for type 2 diabetes, originally derived from Gila monster venom
• Teriparatide: A PTH(1-34) analog that stimulates bone formation for osteoporosis treatment

Peptide-based protein-protein interaction modulators

PPIs are attractive but challenging therapeutic targets because their interfaces are typically large and flat. Peptides are well-suited to disrupt these surfaces because they can cover more contact area than typical small molecules.

• PPI inhibitors compete with one protein partner by mimicking its binding interface. Example: PMI peptide disrupts the p53-MDM2 interaction, reactivating p53 tumor suppressor function.
• PPI stabilizers promote formation of specific protein complexes, enhancing their biological activity.

Peptide stability and metabolism

Enzymatic degradation of peptides

Peptides are vulnerable to proteolytic enzymes throughout the body:

• Endopeptidases (trypsin, chymotrypsin) cleave bonds within the peptide chain at specific recognition sites
• Exopeptidases trim residues from the ends: aminopeptidases from the N-terminus, carboxypeptidases from the C-terminus

Stability depends on amino acid composition and structure. Peptides containing proline, glycine, or D-amino acids tend to resist proteolysis because these residues either constrain the backbone conformation or aren't recognized by proteases.

Strategies for improving peptide stability

Several approaches can protect peptides from degradation:

1. Non-natural amino acid incorporation: D-amino acids, β-amino acids, or modified side chains reduce protease recognition
2. Cyclization: Head-to-tail cyclization or disulfide bridges constrain the peptide's conformation and shield cleavage sites
3. Peptide stapling: Covalent cross-links between side chains (typically at i, i+4 or i, i+7 positions) lock α-helical conformations and increase protease resistance
4. PEGylation: Attaching polyethylene glycol (PEG) chains increases hydrodynamic size and steric shielding, reducing both enzymatic degradation and renal clearance

Peptide half-life and clearance

Half-life is the time for a peptide's plasma concentration to drop by 50%. Most unmodified peptides have very short half-lives (minutes) due to:

• Rapid renal filtration (peptides below ~5 kDa are quickly filtered by the kidneys)
• Enzymatic degradation in blood and tissues
• Hepatic clearance

Strategies to extend half-life:

• Increase size: PEGylation, fusion to albumin or Fc domains (e.g., dulaglutide fuses GLP-1 to an Fc fragment)
• Reduce renal filtration: Charge modification, glycosylation
• Enhance plasma protein binding: Fatty acid conjugation (e.g., semaglutide has a C18 fatty acid chain that binds albumin, extending its half-life to ~1 week)

Peptide prodrugs and bioconjugation

Peptide prodrugs are inactive precursors converted to the active drug in vivo by enzymatic or chemical transformation. Prodrug strategies can improve stability, bioavailability, and tissue targeting. For example, esterification of carboxyl groups increases lipophilicity and oral absorption.

Bioconjugation involves covalently attaching peptides to other molecules to modify their pharmacokinetic or pharmacodynamic properties:

• Peptide-drug conjugates (PDCs): Therapeutic peptides linked to cytotoxic agents for targeted delivery to tumor cells
• Peptide-oligonucleotide conjugates (POCs): Improve cellular uptake of oligonucleotides for gene silencing applications
• Antibody or aptamer conjugation: Directs peptide drugs to specific tissues or cell types

Peptide-based therapies

Peptide hormones and neurotransmitters

Many approved peptide drugs are synthetic versions of endogenous hormones or neurotransmitters:

Peptide hormones:

• Insulin: Regulates glucose metabolism; the first peptide drug (approved 1922, recombinant form 1982)
• Glucagon: Raises blood sugar; used for severe hypoglycemia
• Oxytocin: Induces labor and promotes lactation
• Vasopressin (ADH): Regulates water balance and blood pressure

Peptide neurotransmitters:

• Endorphins: Modulate pain and reward pathways
• Substance P: Mediates pain sensation and inflammation
• Neuropeptide Y: Regulates appetite and stress response

Synthetic analogs of these molecules can treat disorders caused by their dysregulation, including diabetes, obesity, and various neurological conditions.

Antimicrobial and anticancer peptides

Antimicrobial peptides (AMPs) are part of the innate immune system across many organisms. They typically share two features: a net positive (cationic) charge and an amphipathic structure. This combination allows them to bind and disrupt the negatively charged membranes of bacteria, fungi, and enveloped viruses.

Examples include defensins, cathelicidins, and magainins (originally isolated from frog skin). AMPs are of particular interest because they may help address antibiotic resistance.

Anticancer peptides (ACPs) selectively target cancer cells, which often have more negatively charged membranes than healthy cells. ACPs act through diverse mechanisms:

• Membrane disruption
• Apoptosis induction (e.g., the proapoptotic peptide $(KLAKLAK)_2$)
• Inhibition of angiogenesis (e.g., anginex)
• Intracellular delivery of toxic cargo via cell-penetrating sequences

Peptide vaccines and immunotherapies

Peptide vaccines present specific epitopes (short peptide fragments of antigens) to the immune system to generate targeted immune responses. They can be designed for:

• Infectious disease prevention (e.g., peptide-based approaches in influenza vaccine development)
• Cancer treatment (e.g., gp100 peptide vaccine for melanoma)
• Autoimmune disease modulation

Peptide immunotherapies modulate the immune system more broadly:

• TCR mimic peptides for cancer treatment
• Tolerogenic peptides to suppress autoimmune reactions
• Checkpoint modulator peptides to enhance anti-tumor immunity

Clinical applications and challenges

Peptide-based therapies have achieved significant clinical success:

• Diabetes: Insulin, GLP-1 receptor agonists (exenatide, liraglutide, semaglutide)
• Cancer: Goserelin (GnRH agonist for prostate/breast cancer), octreotide (somatostatin analog for neuroendocrine tumors)
• HIV: Enfuvirtide (fusion inhibitor)
• Osteoporosis: Teriparatide

Advantages of peptide drugs include high target specificity, low off-target toxicity, and the ability to address targets that small molecules cannot easily reach (especially PPIs).

Persistent challenges:

• Poor oral bioavailability (most peptide drugs require injection)
• Rapid clearance and short half-lives
• Potential immunogenicity
• High manufacturing costs compared to small molecules

These challenges are being addressed through the stability and delivery strategies covered earlier: PEGylation, fatty acid conjugation, cyclization, non-natural amino acid incorporation, and advanced formulation technologies.