Tertiary and Quaternary Structure
Introduction
Introduction: Protein Structure Bonding at the 3-D Level
Protein Structure at the 3-D shape or conformational level is mostly stabilized by weak, non-covalent bonds. The energy difference between the Denatured and Native Structure of Proteins is small. This is because most of the weak, non-covalent bonds of a protein are made with water molecules and this is true in both the native shape and denatured form.
Figure 1. Illustration of Native versus Denatured State of a Protein.
The driving force for protein folding is provided by water. The aqueous environment drives hydrophobic side-chains to interior of the protein during folding. Hydrophobic side chains pack very tightly in clusters excluding water. Hydrophobic groups tend to structure water so they are driven by entropy to the interior of the protein. Thus, protein folding is a property of the system, which is more disordered when the protein is folded and so has a lower level of entropy as compared to the denatured protein fully extended in its linear form where water molecules must align themselves around the hydrophobic side chains. So protein folding is a bit like a drop of oil in water - where the oil stays together because its dispersion in water would result in a higher energy state and consequently the oil remains as a drop due to its hydrophobic nature.
This lecture discusses the 3rd and 4th levels of protein structure.
Remember that we have divided protein structure into these 4 levels:
1. Primary Structure- Polypeptide backbone
2. Secondary Structure- Local Hydrogen bonds along the backbone
3. Tertiary structure- Long distance bonding involving the AA side chains
4. Quaternary structure- Protein-Protein interactions leading to formation of dimers, tetramers, etc.
We have already discussed the covalent structure of the protein polypeptide backbone and the hydrogen bonding that forms locally to stabilize secondary structure. This lecture will describe the bonding which stabilizes tertiary and quaternary structure of proteins.
©Wilbur H. Campbell, 1995; wcampbel@mtu.edu