Protein Folding, Molecular Chaperones, and Post-Translational Processing

Protein Folding

When a ribosome translates mRNA, it produces a linear polypeptide chain. For this polypeptide to become biologically active, it must fold into its unique three-dimensional (native) conformation

Anfinsen’s Dogma

The primary amino acid sequence contains all the information necessary to dictate the native 3D structure of a protein

Thermodynamic Driving Force

The primary force driving protein folding is the hydrophobic effect. Nonpolar (hydrophobic) amino acid side chains collapse inward away from the aqueous cytosol to form a hydrophobic core, while polar and charged residues remain exposed on the exterior surface.

The Folding Problem

Inside a crowded cell, nascent (newly forming) polypeptide chains risk misfolding or aggregating with neighboring proteins before complete synthesis occurs.

Molecular Chaperones (Heat Shock Proteins)

are specialized proteins that assist in the non-covalent folding, unfolding, and assembly of other proteins without becoming part of the final functional structure. They are called Heat Shock Proteins (Hsp) because their expression increases during cellular stress (heat, oxidative stress) to refold denatured proteins and prevent toxic aggregation

Hsp70 Family

Binds short hydrophobic segments of nascent polypeptides as they emerge from the ribosome, preventing premature aggregation.

ATP-dependent

Works co-translationally (during protein synthesis); requires co-chaperones like Hsp40

Hsp60 Family (Chaperonins / GroEL-GroES in bacteria)

Forms a barrel-shaped cage structure that sequesters misfolded proteins in an isolated hydrophilic cavity, allowing them to fold safely without interference

ATP-dependent

Acts post-translationally on fully synthesized proteins that failed to fold properly.

Hsp90 Family

Assists in the late-stage folding, stabilization, and activation of specific client proteins (e.g., steroid hormone receptors, signal transduction kinases).

ATP-dependent

Target of anti-cancer drugs because cancer cells heavily rely on Hsp90 to stabilize mutant signaling proteins.

Targeting

Signal peptides (short N-terminal hydrophobic sequences) direct newly synthesized proteins to their specific cellular destinations (e.g., endoplasmic reticulum, mitochondria, nucleus) via Signal Recognition Particles (SRP).

Ubiquitination

Covalent attachment of the small protein ubiquitin to lysine residues of misfolded or damaged proteins. This tags the protein for destruction by the 26S Proteasome (the Ubiquitin-Proteasome System)

Residue Modifications

Covalent addition of chemical groups to specific amino acid side chains:

Phosphorylation

Glycosylation

Hydroxylation

Phosphorylation

Addition of phosphate to Serine, Threonine, or Tyrosine by kinases (reversible regulatory switch).

Glycosylation

Addition of carbohydrates in the ER and Golgi (N-linked to Asparagine; O-linked to Serine/Threonine)

Hydroxylation

Addition of -OH groups to Proline and Lysine in collagen (requires Vitamin C).

Residue Trimming / Proteolytic Cleavage

Cleavage of peptide bonds to convert inactive precursor proteins (zymogens or prohormones) into active forms:

• Proinsulin → Insulin + C-peptide

• Pepsinogen → Pepsin

Prion Diseases (Transmissible Spongiform Encephalopathies / CJD)

Ingestion or spontaneous conversion of normal alpha-helical protein (PrP^c) into infectious, insoluble beta-sheet-rich aggregates (PrP^sc).

Alzheimer's Disease

Extracellular deposition of Amyloid-beta plaques and intracellular hyperphosphorylated Tau neurofibrillary tangles

Parkinson's Disease

Intracellular accumulation of misfolded $\alpha$-synuclein forming Lewy bodies.

Cystic Fibrosis

The DeltaF508 deletion in the CFTR gene causes misfolding of the CFTR protein. It is retained and degraded by Endoplasmic Reticulum-Associated Degradation (ERAD) via the proteasome before it can reach the cell surface