Protein Structure & Denaturation — Quick Review

Protein structure overview

• Proteins fold from their primary structure to a defined 3D structure essential for function.
• Myoglobin example: length is $153$ amino acids in the primary sequence.

Primary structure

• Primary structure = amino acid sequence; determines overall folding.
• Some proteins have non-amino-acid components bound to them, called prosthetic groups (also known as cofactors or coenzymes).
• In myoglobin, the prosthetic group is the heme group, which binds oxygen.

Prosthetic groups

• Prosthetic group = non-amino-acid component tightly bound to a protein.
• Heme is a prosthetic group that enables oxygen binding in myoglobin.

Myoglobin function

• Oxygen is $ext{O}_2$, a small molecule.
• Myoglobin binds O2 at the heme site and facilitates storage/release as needed.

Residues and bonding

• Polar residues are typically on the exterior; nonpolar residues tend to be buried inside.
• Polar residues can form hydrogen bonds or participate in catalysis; hydrophobic residues contribute to core packing.

Visualizing protein structures

• There are 3D visualization tools available online that let you rotate structures to inspect the backbone and heme group.
• Visualization color conventions (example): Nitrogen is blue, Carbon is black, Oxygen is red.

Peptide backbone and terminus terminology

• A protein chain runs from the N-terminus to the C-terminus; each repeating unit is a residue (amino acid).
• The backbone atoms are the main chain components; side chains extend off the backbone.
• Example illustration often uses a simplified motif like NCCNCCNCC to convey backbone connectivity.

Denaturation and chemical denaturants

• Denaturation via heat: increasing temperature increases vibrational motions until the protein unfolds.
• Chemical denaturants that disrupt peptide interactions:
  • Urea
  • Guanidine hydrochloride (guanidinium chloride in solution)
• These agents interact with peptide backbone to promote unfolding.

Folding fraction terminology

• Fraction folded, denoted as $F\in[0,1]$:
  • $F=0$ means totally unfolded.
  • $F=1$ means fully folded (100%).

Quick takeaways

• Primary structure dictates folding; prosthetic groups extend function beyond amino acids.
• Myoglobin demonstrates oxygen binding via a heme prosthetic group.
• Denaturation can be physical (heat) or chemical (urea, guanidinium chloride) via backbone interactions and stability disruption.