Protein Structure — Primary, Secondary and Tertiary

Spec mapping — OCR H420 Module 2.1.2 — Biological molecules. This lesson covers the first three levels of protein structure: primary (amino-acid sequence held by peptide bonds), secondary (α-helix and β-pleated sheet held by backbone hydrogen bonds), and tertiary (overall 3D fold held by R-group interactions including ionic bonds, hydrogen bonds, hydrophobic interactions and disulfide bridges). Quaternary structure is treated in Lesson 10 (refer to the official OCR H420 specification document for exact wording).

A polypeptide chain is not a functional protein until it has folded into a precise three-dimensional shape. Protein structure is described at four hierarchical levels — primary, secondary, tertiary and quaternary. This lesson examines the first three, showing how amino acid sequence gives rise to function.

The α-helix and β-pleated sheet were predicted by Linus Pauling, Robert Corey and Herman Branson in 1951 from purely theoretical considerations of peptide-bond planarity and hydrogen-bonding geometry — before any high-resolution protein structure was known. Both motifs were subsequently confirmed by X-ray crystallography (myoglobin, Kendrew 1958; haemoglobin, Perutz 1959). Christian Anfinsen demonstrated experimentally in the 1950s and 1960s that protein folding is a thermodynamically deterministic process: a denatured polypeptide can refold spontaneously to its native conformation, indicating that the amino-acid sequence alone contains all the information necessary to specify the folded structure (the "Anfinsen thermodynamic hypothesis", paraphrased).

1. Overview of the Four Levels

graph TD
  A["Primary structure<br/>Sequence of amino acids<br/>Peptide bonds only"] --> B["Secondary structure<br/>α-helix and β-pleated sheet<br/>Hydrogen bonds between backbone"]
  B --> C["Tertiary structure<br/>Overall 3D fold of one polypeptide<br/>R group interactions"]
  C --> D["Quaternary structure<br/>Two or more polypeptide subunits<br/>Same R group interactions"]

Key Principle: Each higher level of structure depends on the lower levels. The sequence of amino acids (primary structure) ultimately determines everything else about how a protein folds and functions.

2. Primary Structure

The primary structure of a protein is the specific sequence of amino acids in the polypeptide chain, held together by peptide bonds.

Features of primary structure:

Unique to each protein — determined by the order of codons in the gene.
Written from N-terminus (left) to C-terminus (right) using three-letter or one-letter abbreviations (e.g., Met-Val-Leu-Ser-Pro... or MVLSP...).
The only bond involved at this level is the peptide bond — no folding or interactions yet.
Even a single amino acid change can dramatically alter function — sickle cell anaemia results from a single glutamic acid (Glu) → valine (Val) substitution at position 6 of the β-chain of haemoglobin.

3. Secondary Structure

Secondary structure refers to the regular, repeating folding patterns that arise from hydrogen bonding between atoms in the polypeptide backbone — specifically between the carbonyl oxygen (C=O) of one peptide group and the amide hydrogen (N–H) of another. R groups are not involved at this level.

There are two main secondary structures.

3.1 The α-Helix

An α-helix is a right-handed helical coil, resembling a spiral staircase.

Each turn of the helix contains 3.6 amino acids.
The pitch (vertical distance per turn) is 0.54 nm.
Hydrogen bonds form between the C=O of one amino acid and the N–H of the amino acid four residues further along the chain (i and i + 4).
R groups project outward from the helix.
The helix is held together entirely by hydrogen bonds along the backbone.

Proteins rich in α-helices include keratin (hair, wool, nails) and myoglobin.

3.2 The β-Pleated Sheet

A β-pleated sheet consists of polypeptide chains (called β-strands) running either in the same direction (parallel) or in opposite directions (antiparallel), laid side by side and connected by hydrogen bonds.

The strands zigzag to form a pleated, sheet-like structure.
Hydrogen bonds form between strands — C=O of one strand to N–H of the next.
R groups alternate above and below the plane of the sheet.
Sheets may be flat or twisted.

Proteins rich in β-pleated sheets include silk fibroin and amyloid deposits.

3.3 Other Secondary Features

β-turns and loops — short regions connecting α-helices and β-strands, allowing the chain to reverse direction.
Random coil — regions with no regular pattern, but still precisely folded.

4. Tertiary Structure

Tertiary structure is the overall three-dimensional shape of a single polypeptide chain, produced by interactions between the R groups (side chains). Tertiary structure gives the protein its functional shape, including the active site of an enzyme.

Tertiary structure arises from five types of R group interactions:

4.1 Hydrogen Bonds

Formed between polar R groups (e.g., serine, threonine, asparagine, glutamine, tyrosine). Individually weak but collectively important.

4.2 Ionic Bonds (Salt Bridges)

Formed between positively charged R groups (e.g., lysine, arginine) and negatively charged R groups (e.g., aspartate, glutamate). Stronger than hydrogen bonds. Sensitive to pH — at extreme pH, charged R groups are protonated or deprotonated, breaking ionic bonds.

4.3 Disulfide Bridges (Disulfide Bonds)

Formed between two cysteine residues. The –SH (thiol) groups of two cysteines are oxidised to form a covalent S–S bond. Disulfide bridges are strong covalent bonds — among the most stable interactions in a protein.

Disulfide bridge equation: $\text{Cys-SH} + \text{HS-Cys} \rightarrow \text{Cys-S-S-Cys} + 2H^+ + 2e^-$

Proteins with many disulfide bridges (e.g., keratin in hair, insulin) are especially stable.

4.4 Hydrophobic Interactions

Non-polar R groups (e.g., valine, leucine, isoleucine, phenylalanine) cluster together in the interior of the protein, away from the aqueous environment. The surrounding water is "forced" to maintain structure around hydrophobic groups, which is entropically unfavourable; burying these groups releases the water and increases entropy. This is called the hydrophobic effect and is a major driving force in protein folding.

4.5 Van der Waals Forces

Weak, transient attractions between closely packed atoms. Individually weak but collectively significant, especially in the tightly packed protein core.

Summary of R Group Interactions

Interaction	R groups involved	Strength	Disrupted by
Hydrogen bond	Polar	Weak	Heat, extreme pH
Ionic bond	Charged (+/−)	Moderate	Extreme pH (ionisation changes)
Disulfide bridge	Cysteine	Strong (covalent)	Reducing agents (e.g., mercaptoethanol)
Hydrophobic interaction	Non-polar	Weak individually, collectively strong	Detergents, organic solvents

5. Denaturation

Denaturation is the loss of a protein's three-dimensional shape due to disruption of the bonds that maintain secondary and tertiary structure. The primary structure (peptide bonds) is not broken by typical denaturing conditions.

Causes of denaturation:

Heat — increased kinetic energy breaks hydrogen bonds, ionic bonds and hydrophobic interactions.
Extreme pH — changes the charges on R groups, disrupting ionic bonds and hydrogen bonds.
Heavy metal ions (e.g., Hg²⁺, Pb²⁺) — bind to –SH groups and disrupt disulfide bridges.
Organic solvents — disrupt hydrophobic interactions and hydrogen bonds.
Mechanical agitation (e.g., whipping egg whites).
Radiation — ionising radiation disrupts bonds.

Denaturation generally causes loss of function — an enzyme no longer fits its substrate, haemoglobin no longer binds oxygen, antibodies no longer recognise antigens.

6. Why Shape Matters: Enzyme Example

An enzyme's active site is formed by specific R groups brought into precise positions by tertiary folding. The active site is complementary in shape and chemistry to the substrate. If the tertiary structure is disturbed — by denaturation or by a mutation altering the primary structure — the active site loses its shape and the enzyme no longer functions. This explains why even small changes in amino acid sequence can be catastrophic.

7. Putting It Together: The Folding Pathway

graph LR
  A[DNA sequence] --> B[mRNA codon sequence]
  B --> C["Amino acid sequence<br/>Primary structure"]
  C --> D["Local folds<br/>Secondary structure"]
  D --> E["Full 3D shape<br/>Tertiary structure"]
  E --> F["Assembly of subunits<br/>Quaternary structure if applicable"]
  F --> G[Functional protein]

Key insight: a protein's sequence determines its shape, and its shape determines its function.

Exam Tips

Always distinguish clearly which bonds operate at each level:
- Primary: peptide bonds
- Secondary: hydrogen bonds (backbone only)
- Tertiary: all R group interactions
Mention right-handed helix and 3.6 residues per turn for the α-helix.
For β-pleated sheets, state that hydrogen bonds are between strands.
Use "hydrogen bonds along the backbone" for secondary, and "between R groups" for tertiary.

Common Exam Mistakes

Saying secondary structure involves R groups — it involves only the backbone.
Confusing α-helix and β-pleated sheet bond positions.
Forgetting that disulfide bridges are covalent.
Writing "denaturation breaks peptide bonds" — it does not; it breaks bonds stabilising higher-order structure.
Saying "the enzyme dies when it's denatured" — enzymes are not alive; they lose function.
Describing hydrophobic interactions as "bonds" — they are interactions driven by entropy.

8. Synoptic Links

This lesson connects across the OCR H420 specification:

ocr-alevel-biology-biological-molecules Lesson 10 — quaternary structure. Multiple folded polypeptides assemble into multimeric proteins (haemoglobin tetramer, antibody, RuBisCO L8S8 hexadecamer).
ocr-alevel-biology-nucleic-acids-enzymes — enzyme catalysis. The active site is a 3D arrangement of R groups generated by tertiary structure; lock-and-key (Fischer 1894) and induced fit (Koshland 1958) models depend on this 3D shape.
ocr-alevel-biology-diseases-immunity — antibody specificity. The variable region of an immunoglobulin is a precisely folded β-sandwich whose hypervariable loops bind antigen by R-group complementarity.
ocr-alevel-biology-genetics-inheritance — mutation consequences. A single nucleotide substitution can change a single R group in the protein (HbA → HbS, sickle-cell anaemia), altering tertiary structure and producing a clinically devastating phenotype.

9. Specimen Question (Modelled on the OCR H420 Paper Format)

Q (9 marks): Describe and explain the structure of a globular protein at primary, secondary and tertiary levels. Discuss how each level contributes to the protein's biological function.

Lesson 9: Protein Structure — Primary, Secondary and Tertiary