2.2.11 Protein: Strucutre Flashcards
Levels of Protein Structure
There are 4 levels of structure in proteins
- 3 of them are related to a single polypeptide
- the 4th level relates to a protein that has 2 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 Structure
The sequence of amino acids bonded by covalent peptide bonds is the primary structure of a protein
The DNA of a cell determines the primary structure of a protein by instructing the cell to add certain amino acids in specific quantities in a certain sequence
This affects the shape and therefore the function of the protein
The primary structure is specific for each protein (one alteration in the sequence of amino acids can affect the function of the protein)
Secondary Structure
The secondary structure of a protein occurs when the weak negatively charged nitrogen and oxygen atom interact with the weak positively charges hydrogen atoms to from hydrogen blonds
There are 2 shapes that can form within proteins due to the hydrogen bonds:
- alpha-helix
- beta-pleated sheet
The alpha-helix shape occurs when the hydrogen bonds form between every 4th peptide bond (between the oxygen of the carboxyl group and the hydrogen of the amine group)
The beta-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 relayed 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
Tertiary Structure
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 interaction (between non-polar R groups)
This structure is common in globular proteins
Disulphide
Disulphide bonds are strong covalent bonds that form between two cysteine R groups (as this is the only amino acid with a sulphur atom)
These bonds are the strongest within a protein, but occur less frequently, and help stabilise the proteins
These are also known as disulphide bridges
Disulphide bonds can be broken by oxidation
This type of bond is common in proteins secreted from cells
- e.g. insulin
Ionic
Ionic bonds form between positively charged (amine group - NH 3+) and negatively charged (carboxylic acid - COO -) R GROUPS
Ionic bonds are stronger than hydrogen bonds but they are not common
These bonds are broken by pH changes
Hydrogen
Hydrogen bonds form between strongly polar R groups
These are the weakest bonds that form but the most common as they from between a wide variety of R groups
Hydrophobic Interactions
Hydrophobic interactions form between non-polar (hydrophobic) R groups within the interior of proteins
Tertiary Structure Determines Function
A polypeptide chain will fold differently due to the interactions (and hence the bonds that form) between the R groups
Each of the 20 amino acids that make up proteins has a unique R group and therefore many different interactions can occur creating a vast range of protein configurations and therefore functions
Quaternary Structure
Quaternary structure exists in proteins that have moe than one polypeptide chain, working together as a functional macromolecule
- e.g. haemoglobin
Each polypeptide chain in the quaternary structure is referred to as a subunit of the protein
Summary of Bonds in Proteins
Peptide
- primary
- secondary
- tertiary
Hydrogen
- secondary (only between the amino and carboxyl groups)
- tertiary (R groups + amino and carboxyl groups)
Disulphide
- tertiary
Ionic
- tertiary
Hydrophobic interactions
- tertiary
