Primary & Secondary Structure
Biology: Cambridge International A Level, topic 2.3.2
Primary Structure of Proteins
The order of amino acids in a polypeptide determines the primary structure of a protein.
Amino acid sequence
- Amino acids in a polypeptide chain are arranged in a specific sequence.
- The sequence of amino acids is the primary structure.
- The primary structure is determined by the gene encoding the protein.
Importance
- A change in the nucleotide sequence of the gene's coding region may lead to a different amino acid being added to the growing polypeptide chain.
- A change in the amino acids in a protein could change the protein's structure and function.
Secondary Structure of Proteins
The primary polypeptide chain folds to form a secondary structure.
Hydrogen bonds
- Amino acids in a polypeptide chain can form hydrogen bonds between other amino acids within the chain.
- The hydrogen bonds cause the protein to fold into specific structures.
- Folding of the polypeptide determines its secondary structure.
Common structures
- The most common secondary structures are the alpha-helix and beta-pleated sheet structures.
- E.g. Silk has a β-pleated sheet structure.
Stability
- The secondary structure of a protein is stable.
- Individual hydrogen bonds are quite weak, but there are many bonds throughout the molecule which leads to overall high stability.
- The stability of the secondary structure is decreased by environmental factors that weaken the hydrogen bonds (e.g. temperature, pH).
Test yourself on Primary & Secondary Structure
What determines the sequence of amino acids in the primary structure?
Which of the following statements about primary structure are true?
Which bonds form between the amino acids to hold the secondary structure in place?