Protein Purification
Hydrophobic Amino Acid Side Chains
Made mostly of carbon and hydrogen, so they are nonpolar.
Do not form hydrogen bonds with water.
Water pushes them away, causing them to group together.
Their main function is to create a stable core that holds the protein’s shape.
Without hydrophobic amino acids, proteins would not fold correctly.
Folding in a protein or pool noodle
These should end up on the inside of the folded structure.
When folding the noodle, bring hydrophobic pieces together toward the center.
Think of them being buried, not exposed.
Hydrophilic Amino Acid Side Chains
Polar side chains that can form hydrogen bonds with water.
Comfortable interacting with water.
Help proteins stay dissolved and interact with other molecules.
Often involved in binding and recognition.
Folding in a protein or pool noodle
These belong on the outside of the protein.
When folding, let these face outward.
They should be more exposed than hydrophobic ones.
Basic Amino Acid Side Chains
Carry a positive charge.
Hydrophilic because charged molecules interact well with water.
Can form ionic bonds with acidic amino acids.
Important for stabilizing structure and binding negatively charged molecules like DNA.
Folding in a protein or pool noodle
Usually located on the outside.
Try to place them near acidic side chains when folding.
They should not be buried in the center.
Acidic Amino Acid Side Chains
Carry a negative charge.
Hydrophilic and interact easily with water.
Form ionic bonds with basic amino acids.
Help stabilize proteins and allow them to respond to changes in pH.
Folding in a protein or pool noodle
Also belong on the outside.
Place them near basic side chains when possible.
Keep them exposed rather than folded inward.
Cysteine Amino Acid Side Chains
Contain sulfur, which allows special bonding.
Two cysteines can form a covalent disulfide bond.
These bonds act like staples that lock parts of the protein together.
Add strength and long-term stability to proteins.
Folding in a protein or pool noodle
The two cysteines should be positioned so they can come close together.
They may pull distant parts of the noodle into the same area.
Think of them as ties that hold the folded shape in place.
Big Picture Folding Idea
Proteins fold because different side chains want different environments.
Hydrophobic side chains hide inside.
Hydrophilic, basic, and acidic side chains face outward.
Cysteines lock the structure together.
“Inside” just means buried and surrounded, not hollow.
Folding a rope or noodle into a tight bundle is the best comparison.
Amino Acid Bridges
Ionic Bridges (Salt Bridges)
Formed between basic (positively charged) and acidic (negatively charged) amino acids.
Think of them as magnets: opposite charges attract.
Help stabilize the folded protein, especially on the outside where water is present.
Example: Lysine (basic) + Aspartic acid (acidic) can form an ionic bond.
Folding Tip
Place basic and acidic amino acids near each other on the outside.
They act like tiny “ties” holding parts of the noodle close without being inside.
Disulfide Bridges
Formed between two cysteine amino acids.
A covalent bond, which is much stronger than ionic or hydrogen bonds.
Acts like a staple or lock that can hold distant parts of the protein chain together.
Especially important in proteins that go outside the cell (like antibodies), where extra stability is needed.
Folding Tip
Position the two cysteine pieces so they can touch.
They may pull parts of your noodle that are far apart closer together.
Think of them as tying two loops of your noodle bundle together.
Hydrogen Bridges (Hydrogen Bonds)
Formed when hydrophilic side chains or backbone atoms interact through hydrogen atoms.
Not as strong as covalent or ionic bonds, but very common.
Help fine-tune and stabilize protein folds.
Folding Tip
Hydrophilic side chains on the outside can form small attractions with each other or with water.
These are smaller “connectors” in your noodle model.