Proteins
- Each amino acid contains an amine (-NH2) and carboxylic acid (-COOH) group
- The -NH2 group of one amino acid can react with the -COOH group of another amino acid in a condensation reaction to form a dipeptide
- The new amide bond between two amino acids is also called a peptide link or peptide bond
- Since this is a condensation reaction, a small molecule (in this case H2O) is eliminated
- The dipeptide still contains an -NH2 and -COOH group at each end of the molecule which can again participate in a condensation reaction to form a tripeptide
A peptide bond is an amide bond between two amino acids
- A polypeptide is formed when many amino acids join together to form a long chain of molecules
A polypeptide is a long chain of amino acid molecules joined together
The structure of proteins
- There are four levels of structure in proteins, three are related to a single polypeptide chain and the fourth level relates to a protein that has two or more polypeptide chains
- Polypeptide or protein molecules can have anywhere from 3 amino acids (Glutathione) to more than 34,000 amino acids (Titan) bonded together in chains
Primary
- The sequence of amino acids bonded by covalent peptide bonds is the primary structure of a protein
- The primary structure is specific for each protein (one alteration in the sequence of amino acids can affect the function of the protein)
The primary structure of a protein. The three-letter abbreviations indicate the specific amino acid (there are 20 commonly found in cells of living organisms)
Secondary
- The secondary structure of a protein occurs when the weak negatively charged nitrogen and oxygen atoms interact with the weak positively charged hydrogen atoms to form hydrogen bonds
- There are two shapes that can form within proteins due to the hydrogen bonds:
- α-helix
- β-pleated sheet
- The α-helix shape occurs when the hydrogen bonds form between every fourth peptide bond (between the oxygen of the carboxyl group and the hydrogen of the amine group)
- The β-pleated sheet shape forms when the protein folds so that two parts of the polypeptide chain are parallel to each other enabling hydrogen bonds to form between parallel peptide bonds
- Most fibrous proteins have secondary structures (e.g. collagen and keratin)
- The secondary structure only relates to hydrogen bonds forming between the amino group and the carboxyl group (the ‘protein backbone’)
- The hydrogen bonds can be broken by high temperatures and pH changes
The secondary structure of a protein with the α-helix and β-pleated sheet shapes highlighted. The magnified regions illustrate how the hydrogen bonds form between the peptide bonds
Tertiary
- Further conformational change of the secondary structure leads to additional bonds forming between the R groups (side chains)
- The additional bonds are:
- Hydrogen (these are between R groups)
- Disulphide (only occurs between cysteine amino acids)
- Ionic (occurs between charged R groups)
- Weak hydrophobic interactions (between non-polar R groups)
- This structure is common in globular proteins
The tertiary structure of a protein with hydrogen bonds, ionic bonds, disulphide bonds and hydrophobic interactions formed between the R groups of the amino acids
Summary of the types of bonding in peptides Table
Examiner Tip
You should be able to draw the peptide formed by the joining of up to three amino acids.