Protein Structure: Key Bonds and Denaturation
Bond Types Across Structure
Protein structure is stabilized by different types of interactions at each level, with covalent bonds providing the strongest anchors and noncovalent interactions guiding folding and assembly.
Primary structure: covalent peptide bonds linking amino acids in a chain. This sequence is the fundamental info for the protein and cannot be altered by noncovalent changes.
Secondary structure: backbone hydrogen bonds between amide and carbonyl groups create regular motifs such as alpha helices and beta sheets. These are largely governed by the backbone, not side chains.
Tertiary structure: the 3D fold of a single polypeptide, stabilized by a mix of interactions, including
Disulfide bridges (covalent): cross-links between cysteine residues that strongly stabilize the fold.
Hydrogen bonds: between side chains or between side chains and the backbone.
Electrostatic (ionic) interactions: salt bridges between charged residues.
Hydrophobic interactions: nonpolar residues cluster inside away from water, driving core formation; these are not true bonds but key stabilizing forces.
Quaternary structure: assembly of multiple polypeptide subunits into a functional complex; stabilized by the same interaction types as tertiary (including hydrophobic packing and salt bridges) at subunit interfaces.
Hydrophobic Effect and Environmental Effects
Nonpolar residues avoid water, driving folding into compact cores.
Environmental changes disrupt charges or water balance, altering electrostatic interactions and destabilizing folded structures.
Changes include: pH, salt concentration, or dehydration.
Salts can screen electrostatic interactions.
pH shifts can protonate/deprotonate residues, altering charge and stability.
Denaturation, Refolding, and Chaperones
Denaturation disrupts noncovalent interactions (hydrogen bonds, ionic interactions, hydrophobic packing) but leaves the primary sequence intact.
Some proteins can refold if the environment allows proper interactions.
Others require chaperone proteins to assist folding; folding is hindered if chaperones are absent.
Examples:
A classic example is egg white albumin: heating causes unfolding and new interactions (coagulation), preventing return to native state. This illustrates how denaturation changes structure without changing the amino acid sequence.
In contrast, some proteins (e.g., hemoglobin) can self-assemble under certain conditions, but misfolding and aggregation can occur without proper folding aids.
Protein Examples and Visual Cues
GFP (green fluorescent protein) helps visualize 3D structure.
Proteins often display an N-terminus and a C-terminus.
In multimeric proteins, subunits may be distinct (heterodimers) but still assemble into a quaternary structure.
A local change (e.g., a valine in a pocket) can alter interactions with neighboring subunits or ligands, affecting overall assembly.
RNA vs Protein in Secondary Structure
Secondary structure arises from the amino acids and their side chains, via backbone hydrogen bonding.
It is not RNA-based.
RNA precedes protein synthesis, but once a protein is formed, its secondary structure is determined by its amino acid sequence and side-chain chemistry.
Some complexes do involve RNA and protein together, but the protein
’s secondary structure is defined by its own backbone and R-group interactions.