Module 2: Proteins
These free OCR A Level Biology Proteins revision notes have been written for specification points 2.1.2(k), 2.1.2(l), 2.1.2(m), 2.1.2(n) and 2.1.2(o).
Proteins
Proteins are important biological molecules with structural and metabolic roles determined by their structure. They can be enzymes, hormones, antibodies, structural scaffolds or transport molecules.
The table below outlines the structural levels of a protein:
| Level | Definition | Structural Bonds |
|---|---|---|
| Primary | Order of amino acids in a polypeptide chain | Peptide bonds |
| Secondary | Coiling and folding of the polypeptide chain into α-helix or β-pleated sheets (zig-zag) | Hydrogen bonds (between different amino acids’ –NH and –CO groups) |
| Tertiary | The 3D shape of the protein, stabilised by interactions between R-groups |
– Hydrogen bonds* – Ionic bonds* – Disulfide bridges* – Hydrophobic and hydrophilic interactions |
| Quaternary | Two or more polypeptide chains associating |
– Hydrogen bonds – Ionic bonds – Disulfide bridges – Hydrophobic and hydrophilic interactions |
*between different amino acids’ R groups.
Proteins are formed in condensation reactions between amino acids, forming peptide bonds between an amine group on one amino acid and a hydroxyl group on the other.
When proteins are hydrolysed (digested), enzymes (like protease) break the peptide bonds (with water) to break long chains into smaller chains, and may release individual amino acids from the end of a chain.
Fibrous and Globular Proteins
There are 2 types of protein: fibrous and globular.
What type of protein a polypeptide chain becomes depends on the properties of its tertiary and quaternary structure.
| Fibrous Proteins | Globular Proteins | |
|---|---|---|
| Structure | Long, thin, and strong fibres. | Compact spherical structures. |
| Solubility | Insoluble. | Soluble. |
| Function | Structural, providing tensile strength and mechanical support. | Metabolic, enzymes, hormones, and transport molecules. |
| Amino Acid Sequence | Regular, repetitive amino acid sequences. | Irregular amino acid sequences. |
| Examples | Collagen, elastin, keratin. | Haemoglobin, insulin, pepsin. |
Conjugated Proteins
A conjugated protein is a protein that is associated with a prosthetic group – a non-protein component that is permanently included in the final functioning protein.
