Biomolecules · Lesson 7 of 12
Proteins: Peptide Bonds, Folding and Denaturation
“Build peptides in a specified direction and relate the four levels of protein organisation to shape, interactions and biological activity.”
• Construct and name simple dipeptides from their amino-acid order. • Count peptide bonds and hydrolysis water molecules. • Distinguish primary, secondary, tertiary and quaternary structures. • Explain α-helices and β-sheets using backbone hydrogen bonds. • Distinguish denaturation from peptide hydrolysis.
The peptide bond joins carboxyl carbon to nitrogen
An amino acid’s carboxyl group can form an amide linkage with another amino acid’s amino group. The bond region is –C(=O)–NH–, called a peptide linkage. A formal condensation equation removes OH from a carboxyl group and H from an amino group as H₂O. In biological protein synthesis, activated amino acids and cellular machinery drive the process; simply mixing free amino acids in water does not efficiently build a protein.
The order matters. Joining the carboxyl group of glycine to the amino group of alanine gives glycylalanine, Gly–Ala. The left end conventionally retains a free amino group, called the N terminus; the right retains a free carboxyl group, called the C terminus. Reverse the sequence and you obtain alanylglycine, Ala–Gly, a different peptide. A sequence is read N to C unless another direction is explicitly given.
The peptide nitrogen lone pair interacts with the adjacent carbonyl system, giving the C–N bond partial double-bond character and helping make the peptide group approximately planar. This constrains backbone geometry. Side chains project from the α-carbons and retain their own chemical features; they are not replaced by peptide bonds. Proline-linked peptide nitrogen may have no N–H, an important exception to a universal –CONH– shorthand.
Problem
Construct Gly–Ala and Ala–Gly in neutral shorthand.
- 1.Gly–Ala is H₂N–CH₂–C(=O)–NH–CH(CH₃)–COOH. The glycine carboxyl carbon bonds to alanine nitrogen.
- 2.Ala–Gly is H₂N–CH(CH₃)–C(=O)–NH–CH₂–COOH. The methyl-bearing residue now occurs first.
- 3.Both have one peptide bond and the same overall molecular formula, but different sequence and connectivity relative to the termini. Equal composition does not imply equal primary structure.
From dipeptides to proteins
Two residues make a dipeptide; three a tripeptide; longer chains are oligopeptides or polypeptides. An ordinary linear n-residue peptide has n−1 peptide bonds. Complete hydrolysis breaks those bonds and consumes one water per bond. A tripeptide therefore has two links, not three. Cyclic peptides and covalent crosslinks require separate counting, so the simple rule should be applied to the stated linear chain.
Proteins are functional biological molecules consisting of one or more folded polypeptide chains. Length and molecular mass are helpful descriptors but no sharp universal boundary separates a peptide from a protein. Insulin has 51 amino-acid residues in two chains and is a protein despite being shorter than an arbitrary hundred-residue cutoff. Simple proteins yield only amino-acid components on complete hydrolysis; some other proteins also contain non-protein groups.
Primary structure: the sequence
Primary structure specifies the amino-acid order in each chain. Knowing the total counts of Gly, Ala and Ser would not tell us whether the sequence was Gly–Ala–Ser or Ser–Gly–Ala. Sequence determines which side chains can meet during folding, so a substitution can change stability or function. In multi-chain proteins, chain identity and covalent connections also need to be described.
Secondary structure: local backbone patterns
An α-helix is a common right-handed backbone coil. Hydrogen bonds form between backbone carbonyl oxygen and backbone N–H groups along the same chain, typically linking residue i to residue i+4. Side chains project outward rather than forming the helix’s repeated hydrogen-bond spine. The helix is a conformation of one chain; hydrogen bonds stabilise it but do not replace the covalent peptide backbone.
In a β-sheet, extended backbone segments lie alongside one another. Hydrogen bonds connect backbone C=O and N–H groups between adjacent strands. Strands may run in the same or opposite N-to-C directions and may belong to different chains or different stretches of the same folded chain. Therefore β-sheet hydrogen bonding need not always be intermolecular. The pleated appearance follows alternating backbone geometry.
Tertiary and quaternary structure
Tertiary structure is the overall three-dimensional fold of one polypeptide chain, including its helices, sheets, turns and side-chain arrangements. Hydrogen bonding, electrostatic attractions, van der Waals interactions and the hydrophobic effect contribute. Some proteins contain covalent disulfide links, –S–S–, formed by oxidation of cysteine side chains. These interactions need appropriate geometry; a list of possible forces does not mean every protein contains all of them.
Quaternary structure describes how two or more polypeptide subunits assemble. A single-chain protein has no quaternary level simply because it is folded. Haemoglobin provides a useful example: its four globular subunits assemble to support oxygen transport, and each contains a haem group. The chain sequences, local helices, each subunit’s fold and their assembly describe different structural levels of the same functional molecule.
Fibrous and globular shapes
| Feature | Fibrous proteins | Globular proteins |
|---|---|---|
| General shape | Extended or fibre-like assemblies | Compact folded structures |
| Typical role | Mechanical support or force generation | Transport, regulation and catalysis |
| Water behaviour | Often insoluble | Often water soluble, with exceptions |
| Examples | Keratin in hair/wool; fibroin in silk; myosin in muscle | Albumin, many enzymes, insulin |
Silk’s major structural protein is fibroin, whereas hair and wool contain keratin. Grouping all three materials under the single protein name keratin would lose a real molecular distinction. Solubility is also a tendency, not a defining law: protein surfaces, aggregation and surrounding conditions matter.
Denaturation changes higher structure and activity
A native protein has an organisation associated with its biological activity. Heating, extreme pH or certain chemicals can disrupt interactions stabilising that organisation. The protein may unfold, misfold or aggregate and lose activity: this is denaturation. Secondary, tertiary and, where present, quaternary organisation can be affected. The primary peptide sequence ordinarily remains intact unless conditions also cause chemical cleavage.
Boiling egg white unfolds and aggregates proteins into a network. Much of the water remains trapped in that network; solidification does not mean every water molecule was consumed to form bonds or evaporated. Acidification of milk reduces the stability of casein dispersions and causes curd formation. This is a useful coagulation example, but casein aggregation need not be described as the uncoiling of a typical compact globular protein.
Problem
After heating, an enzyme loses activity but complete amino-acid sequence analysis gives the same sequence. What changed?
- 1.The unchanged sequence indicates that peptide-bond hydrolysis is not the main event.
- 2.Heat can disrupt the native fold and active-site arrangement, so substrates are no longer positioned appropriately.
- 3.The observation supports denaturation, perhaps followed by aggregation. It does not prove that every possible secondary element disappeared or that all denaturation is reversible.
Quick Quiz
Quiz
What is the ordinary peptide-bond region?
Explanation: A peptide linkage is an amide between carboxyl carbon and amino nitrogen.
How many peptide bonds are in a linear pentapeptide?
Explanation: An n-residue linear chain has n−1 connecting peptide bonds.
What stabilises the regular α-helix pattern?
Explanation: The repeating backbone hydrogen bonds support the helical conformation.
Which feature defines quaternary structure?
Explanation: Quaternary organisation concerns the relative arrangement of separate chains.
What is ordinarily preserved during denaturation?
Explanation: Denaturation disrupts higher organisation without normally hydrolysing the backbone.
Practice Problems
- Write glycylalanine and explain why alanylglycine is different.
- A linear peptide contains eight residues. Count peptide bonds and water consumed in complete hydrolysis of 0.020 mol peptide.
- Compare α-helix and β-sheet hydrogen bonding.
- Describe the four structural levels using haemoglobin as a reference.
- Explain denaturation and where water goes when egg white sets on boiling.
- Contrast fibrous and globular proteins with correct material examples.
Glycylalanine is H₂N–CH₂–C(=O)–NH–CH(CH₃)–COOH in neutral shorthand. The sequence runs glycine N terminus to alanine C terminus. Alanylglycine reverses the residues: H₂N–CH(CH₃)–C(=O)–NH–CH₂–COOH. Composition is identical but primary structure differs.
Key Takeaways
• Peptide bonds are amide links with a defined N-to-C sequence direction. • Gly–Ala and Ala–Gly have different primary structures. • A linear n-residue chain has n−1 peptide bonds. • Secondary structure arises from repeated backbone hydrogen bonding. • Tertiary structure describes one chain’s overall fold. • Quaternary structure describes assembly of multiple chains. • Denaturation ordinarily changes higher structure while preserving peptide sequence.