Protein Structure
General Structure
- All composed of the same basic unites – amino acids
- 20 different amino acids
- Plants can make all these amino acids, animals can only make some
- Animals obtain the other amino acids through diet; essential amino acids
- Amine group; -NH2
- Carboxylic acid group; -COOH
- Hydrogen; -H
- All attached to a central carbon atom
- Each amino acid has a different, variable, side chain (residual group)
Primary Structure:
- The specific sequence of amino acids in the polypeptide chain
- Two amino acids join in a condensation reaction
- Forming a dipeptide with a peptide bond between the two subunits
- This process is repeated to form polypeptide chains
- A protein is made of one or more polypeptide chains
Secondary Structure:
- Interactions between the amino acids in the polypeptide chain cause the chain to twist into an α-helix or fold into a β-pleated sheet.
- Hydrogen bonds between the carboxylic and amine group
Tertiary and Quaternary Structure:
- Polypeptide chain bends and folds to produce and precise 3-D shape
- Chemical bonds and hydrophobic interactions between R groups maintain the tertiary structure
- R group is polar when sharing uneven electrons
- Polar R groups attract other polar molecules (water) and are therefore HYDROPHILIC
- Non-polar groups are HYDROPHOBIC and are arranged to face the inside of the protein
- Ionic bonds, hydrogen bonds, disulphide bonds
- Single chain proteins stop at tertiary level
Conjugated Proteins:
- These have another chemical group associated with their polypeptide chain
Globular Proteins:
- Polypeptide chain is folded into a compact spherical shape
- Soluble; due to hydrophilic side chains
- Enzymes are globular proteins
- 3-D shape is crucial to ability to form enzyme substrate complexes and catalyse reactions within cells & binding to other substances
- Antibodies are globular
Fibrous Proteins:
- Remain as long chains
- They can be cross-linked for additional strength
- Insoluble; important for structure