Proteins: Tertiary and Quaternary Structure | A-level Biology | OCR, AQA, Edexcel

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The key points covered in this video include:

1. Tertiary Structure
2. Quaternary Structure
3. The Importance of Protein Structure

Tertiary Structure of Proteins

The α-helices and β-pleated sheets also twist and turn to form a protein with a unique 3-D structure. This structure is held in place by bonds formed between the R groups of amino acids. Hydrogen bonds form between polar R groups, Ionic bonds form between positive and negatively charged R groups, Disulphide links form between sulphur atoms in R groups. The specific 3-D structure is also determined by hydrophilic and hydrophobic R groups. Amino acids with hydrophobic R groups tend to be found in the centre of the protein, Amino acids with hydrophilic R groups tend to be found on the outside of the protein. The overall 3-D structure is therefore a result of R group properties and interactions - the primary structure determines the tertiary structure. The tertiary structure of a protein is the overall specific 3-D shape of a protein. This is determined by interactions between R groups and properties of R groups.

Quaternary Structure of Proteins

Some proteins are only composed of one polypeptide chain. However, many proteins are made up of multiple polypeptide chains. Just like tertiary structure bonding, the polypeptide chains are held together by hydrogen bonds, ionic bonds and disulphide links. Non-protein groups (prosthetic groups) may also be associated with the polypeptide chains forming conjugated proteins. The bonding of different chains and prosthetic groups results in the formation of proteins with a unique quaternary structure. The quaternary structure of a protein is the specific 3-D shape of a protein that is determined by the multiple polypeptide chains and/or prosthetic groups bonded together.

The Importance of Protein Structure

Overall, the primary structure determines the secondary, tertiary and quaternary structures which gives proteins their unique shapes. This allows every protein to carry out a specific function. However, the bonds that maintain the structure of proteins can be broken by changes in temperature and pH. This results in a change of shape, preventing the protein from being able to carry out its function - we say that the protein is denatured.

Summary

The primary structure of a protein is the sequence of amino acids
The secondary structure of a protein is the curling or folding of the polypeptide chain into α- helices and β-pleated sheets
The secondary structure of a protein is stabilised by hydrogen bonds
The tertiary structure of a protein is the overall specific 3-D shape of a protein
The quaternary structure of a protein is the specific 3-D shape of a protein that is determined by multiple polypeptide chains and/or prosthetic groups bonded together
Both the tertiary and quaternary structure are held in shape by hydrogen bonds, ionic bonds and disulphide links
These bonds can be broken by changes in temperature or pH resulting in the protein becoming denatured