Understand how amino acids link together to form peptides and proteins. Explore each amino acid, calculate molecular weights, and test your knowledge with our interactive tools.
The peptide bond is one of the most important chemical bonds in all of biology. It is the molecular glue that connects amino acids into the peptides, polypeptides, and proteins that run virtually every process in your body.
Every protein in your body — from the hemoglobin carrying oxygen in your blood to the collagen giving structure to your skin — is held together by peptide bonds. A single antibody molecule contains over 1,300 peptide bonds. Understanding this bond is the foundation of peptide science, drug design, and modern biochemistry.
Peptide bond formation is an elegant three-step process that happens millions of times per second in every living cell, catalyzed by the ribosome.
The atoms highlighted in rose are removed to form water. The new C–N bond (the peptide bond) links the two residues. Each additional amino acid added releases one more water molecule.
The condensation diagram above shows *what* happens. Here's *how* it happens at the electron level — the kind of detail tested on organic chemistry exams and the MCAT Chem/Phys section.
Every textbook diagram shows amino acids in their neutral form (NH₂ and COOH). But at physiological pH (7.4), amino acids almost never look like that. They exist as zwitterions — from the German zwitter meaning "hybrid."
What happens: At physiological pH, the amino group (–NH₂) is basic enough to pick up a proton from solution, becoming –NH₃⁺ (positive). The carboxyl group (–COOH) is acidic enough to donate its proton to solution, becoming –COO⁻ (negative). The molecule has charges on both ends but is net neutral overall. This is the zwitterion.
Why this matters for peptide bond formation: The zwitterion form creates a problem — the amino group is protonated (NH₃⁺), so its lone pair is already being used to hold that extra proton. It can't perform a nucleophilic attack. And the carboxylate (COO⁻) is resonance-stabilized and not electrophilic. For bond formation to proceed, the amino group must lose a proton (reverting to NH₂) and the carboxyl must be activated (in vivo, by aminoacyl-tRNA synthetase using ATP). The zwitterion form is in equilibrium with the neutral form — the ribosome shifts this equilibrium during translation.
Proteins have four levels of structural organization, and the peptide bond is the foundation of all of them. Understanding this hierarchy is essential for biochemistry courses and the MCAT.
Type or click amino acids to build a peptide sequence. Instantly see molecular weight, residue count, and classification.
There are 20 standard amino acids. You must know every one — its name, one-letter code, three-letter code, side chain classification, charge at physiological pH, and any special properties. No shortcuts. Here they are, grouped by how the MCAT categorizes them.
These have hydrocarbon or aromatic side chains that avoid water. They're typically buried in the interior of folded proteins, driving the hydrophobic effect that is the primary force in protein folding.
These can form hydrogen bonds with water but carry no charge at pH 7.4. They're often found on protein surfaces and in active sites where H-bonding matters.
Side chains have carboxyl groups (–COOH) with pKa values below 7.4 → deprotonated → carry −1 charge at physiological pH.
Side chains have amino or guanidinium groups with pKa values above 7.4 → protonated → carry +1 charge at physiological pH.
Glycine (G) — R group is just H, so the α-carbon has two identical substituents. Not optically active. Smallest AA → fits in tight spaces (collagen interior, β-turns).
Proline (P) — Side chain bonds back to backbone N, forming a ring. Restricts φ angle rotation. Breaks α-helices. Enables β-turns. Only AA with a secondary amine (imino acid).
Cysteine (C) — Thiol (–SH) group can form covalent disulfide bonds (Cys–S–S–Cys) between two cysteines. Only covalent bond in tertiary structure besides peptide bonds. Critical for protein stability (e.g., insulin has 3 disulfide bonds).
Trp (W), Tyr (Y), Phe (F) — Aromatic rings absorb UV light. Trp dominates at 280nm. This is how protein concentration is measured by UV spectrophotometry. Know that Trp has the highest molar extinction coefficient.
Ser (S), Thr (T), Tyr (Y) — Hydroxyl groups (–OH) can be phosphorylated by kinases, adding a −2 charge. This is the most common post-translational modification and the basis of cell signaling cascades (e.g., MAPK pathway).
PVT TIM HALL — Phe, Val, Thr, Trp, Ile, Met, His, Arg*, Leu, Lys. These 10 cannot be synthesized by humans and must come from diet. (*Arg is conditionally essential — needed during growth but synthesized in adults.)
Click any amino acid to explore its properties, molecular weight, and role in biology. Colors indicate chemical character.
Click the card to flip. Test yourself on all 20 amino acids — their one-letter codes, three-letter codes, properties, and molecular weights. Track your progress below.
The basics are simple — two amino acids join, water leaves. But exams and real biochemistry demand much deeper understanding. These are the concepts that separate surface-level knowledge from genuine mastery.
Formation builds the bond. Hydrolysis breaks it — by adding water back across the C–N bond. This is how your body digests proteins, how researchers analyze peptide sequences, and how proteases regulate cellular processes. Two fundamentally different approaches exist.
Different proteases recognize different amino acids at the cleavage site. This specificity comes from the shape of the enzyme's S1 binding pocket — the pocket that accommodates the side chain of the residue being recognized. Know these for exams.
| Protease | Cleaves After | Why (S1 Pocket) | Type | Where Found |
|---|---|---|---|---|
| Trypsin | Arg, Lys (+ charged) | Asp at pocket bottom — electrostatic match with positive side chains | Serine protease | Pancreas → small intestine |
| Chymotrypsin | Phe, Trp, Tyr (aromatic) | Large hydrophobic pocket — accommodates bulky aromatic rings | Serine protease | Pancreas → small intestine |
| Pepsin | Phe, Tyr, Leu (hydrophobic) | Broad hydrophobic pocket; works at very low pH | Aspartyl protease | Stomach (pH 1.5–2.5) |
| Elastase | Ala, Gly, Ser (small) | Shallow pocket — only small side chains fit | Serine protease | Pancreas → small intestine |
| Carboxypeptidase A | C-terminal residue (not Arg/Lys/Pro) | Uses Zn²⁺ cofactor; exopeptidase (works from the end) | Metalloprotease | Pancreas → small intestine |
| Thrombin | Arg (specifically in clotting factors) | Highly specific — recognizes extended sequence, not just one residue | Serine protease | Blood (coagulation cascade) |
Given the peptide Ala–Gly–Arg–Phe–Leu–Lys–Trp, trypsin would cleave after Arg (position 3) and Lys (position 6), producing three fragments: Ala–Gly–Arg, Phe–Leu–Lys, and Trp. Chymotrypsin would cleave after Phe (position 4) and Trp (position 7), producing different fragments. Using both enzymes together and comparing the fragments is how researchers determine peptide sequences — a technique called peptide mapping.
The MCAT will give you a peptide sequence and a pH, and expect you to determine the net charge. This requires understanding which ionizable groups are protonated vs deprotonated at the given pH. Master this and you'll handle every charge/pI question they throw at you.
Every peptide has at least two ionizable groups — the α-amino (N-terminus) and α-carboxyl (C-terminus). Additionally, 7 of the 20 amino acids have ionizable side chains. Here are all the pKa values you need:
| Group | pKa | Protonated Form | Deprotonated Form | Charge Change |
|---|---|---|---|---|
| α-Carboxyl (C-term) | 2.0 | –COOH (0) | –COO⁻ (−1) | 0 → −1 |
| Asp (D) | 3.65 | –COOH (0) | –COO⁻ (−1) | 0 → −1 |
| Glu (E) | 4.25 | –COOH (0) | –COO⁻ (−1) | 0 → −1 |
| His (H) | 6.0 | –ImH⁺ (+1) | –Im (0) | +1 → 0 |
| Cys (C) | 8.18 | –SH (0) | –S⁻ (−1) | 0 → −1 |
| α-Amino (N-term) | 9.0 | –NH₃⁺ (+1) | –NH₂ (0) | +1 → 0 |
| Tyr (Y) | 10.07 | –OH (0) | –O⁻ (−1) | 0 → −1 |
| Lys (K) | 10.53 | –NH₃⁺ (+1) | –NH₂ (0) | +1 → 0 |
| Arg (R) | 12.48 | –C(NH₂)₂⁺ (+1) | –C(NH)(NH₂) (0) | +1 → 0 |
Step 1: Identify all ionizable groups and their pKa values:
• α-amino (N-term): pKa = 9.0
• α-carboxyl (C-term): pKa = 2.0
• Glu side chain: pKa = 4.25
• Lys side chain: pKa = 10.53
Step 2: For each group, ask: is pH > pKa or pH < pKa?
• α-amino: pH 7.4 < pKa 9.0 → PROTONATED → +1
• α-carboxyl: pH 7.4 > pKa 2.0 → DEPROTONATED → −1
• Glu: pH 7.4 > pKa 4.25 → DEPROTONATED → −1
• Lys: pH 7.4 < pKa 10.53 → PROTONATED → +1
Step 3: Sum: (+1) + (−1) + (−1) + (+1) = 0
The net charge of Ala-Glu-Lys at pH 7.4 is 0. This also means pH 7.4 is approximately the isoelectric point (pI) of this peptide.
The isoelectric point is the pH where net charge = 0. To find it, average the two pKa values that bracket the zero-charge state — meaning the pKa of the last group to gain a proton (going from negative to zero) and the pKa of the first group to lose a proton (going from zero to positive).
Neutral peptide (no charged side chains): pI = average of α-amino and α-carboxyl pKa values = (9.0 + 2.0)/2 = 5.5
Acidic peptide (has Asp or Glu, no Lys/Arg/His): pI = average of the two lowest pKa values (the two carboxyl groups)
Basic peptide (has Lys, Arg, or His, no Asp/Glu): pI = average of the two highest pKa values (the two amino groups)
Both acidic and basic: Walk through each pKa from low to high, tracking the charge. Find where the charge crosses zero and average the two flanking pKa values.
At pH = pI, a peptide has zero net charge and will not migrate in an electric field. At pH < pI, the peptide is positively charged and migrates toward the cathode (−). At pH > pI, the peptide is negatively charged and migrates toward the anode (+). This is the principle behind isoelectric focusing (IEF) — proteins migrate through a pH gradient until they reach their pI, where they stop.
The MCAT tests resonance in the peptide bond more than almost any other single concept in amino acid/protein biochemistry. Here's everything you need to know, condensed.
"The peptide bond is planar because of hydrogen bonding" — WRONG. Hydrogen bonds stabilize secondary structure (α-helices, β-sheets) but have nothing to do with the planarity of the peptide bond itself. Planarity comes from resonance/partial double-bond character. Don't confuse these two concepts.
25 questions across four categories. Filter by topic or take the full test.
Understanding peptide bond chemistry is critical for modern pharmaceutical development. Here's how researchers exploit — and work around — the properties of peptide bonds to create life-saving therapies.
The stability problem. Peptide bonds are kinetically stable but vulnerable to proteases — enzymes that hydrolyze them. This means that peptide drugs are rapidly degraded in the bloodstream and gut, giving them short half-lives. A natural peptide like GLP-1 (glucagon-like peptide-1) is degraded within 2 minutes by the enzyme DPP-4. This creates the central challenge of peptide drug design: how do you make a peptide that lasts long enough to be therapeutic?
Strategies to overcome degradation include replacing natural amino acids with D-amino acids (which proteases can't recognize), N-methylation of the peptide bond backbone (blocking protease access), cyclization (connecting the ends to eliminate vulnerable termini), PEGylation (attaching polyethylene glycol to extend circulation time), and fatty acid conjugation (binding to albumin for slow release). Semaglutide uses the fatty acid strategy — a C18 fatty acid chain allows it to bind serum albumin, extending its half-life from 2 minutes to 7 days.
Passage-based questions modeled after real MCAT biochemistry items. Read the passage, then answer each question.
Researchers studied the hydrolysis of peptide bonds under various conditions. In Experiment 1, a synthetic hexapeptide (Ala-Gly-Pro-Phe-Leu-Lys) was incubated at pH 7.4 and 37°C without enzymes. After 30 days, less than 0.1% hydrolysis was observed. In Experiment 2, the same hexapeptide was incubated with the protease trypsin, which cleaves peptide bonds on the C-terminal side of positively charged residues (Lys, Arg). Complete cleavage at the Leu-Lys bond was observed within 15 minutes.
In Experiment 3, the hexapeptide was modified by replacing the L-Phe at position 4 with D-Phe. When incubated with trypsin, cleavage at the Leu-Lys bond occurred normally, but a second protease (chymotrypsin, which cleaves after aromatic residues) could no longer cleave at the D-Phe position.