Chaperones

Folding problem and protein aggregation

At all stages of a protein's life — from synthesis on the ribosome and transport across membranes to oligomer assembly — intermediate, extremely unstable conformations inevitably arise. In such states, hydrophobic residues, which are hidden deep inside the globule in a normally folded protein, end up on the surface.

Since the cell maintains a high concentration of various molecules, these exposed hydrophobic regions tend to immediately bond with each other. This leads to aggregation (clumping) of poorly soluble proteins. Spontaneous renaturation under such conditions is very difficult, so nature created special protective proteins — chaperones — to solve this problem.

Their basic tasks:

• Ensuring optimal folding (formation of the native structure).
• Stabilizing the native conformation during protein function.
• Maintaining structure during cellular homeostasis disruption.

Classification of molecular chaperones

Chaperones are usually divided according to two main criteria: subunit mass and type of synthesis depending on the presence of stress factors.

By subunit molecular mass:

1. High-molecular-weight (60–110 kDa). These are entire families of related proteins. The best-studied representatives are the Hsp60, Hsp70, and Hsp90 classes. For example, the Hsp70 class includes proteins ranging in mass from 66 to 78 kDa.
2. Low-molecular-weight (15–40 kDa).

By stress response:

• Constitutive. They have a high and constant (basal) level of synthesis, which does not depend on whether the cell is experiencing stress or not.
• Inducible (heat shock proteins, HSPs). Under normal conditions, they are synthesized in small amounts, but their production increases sharply during stress. They got their name because they were first discovered during temperature exposure that threatened proteins with denaturation.

Mechanism of action of Hsp70 chaperones

The Hsp70 class is highly conserved. These molecules are present in all cell compartments: the nucleus, cytoplasm, mitochondria, and endoplasmic reticulum.

Structurally, Hsp70 proteins consist of a single polypeptide chain with a peptide-binding groove at the carboxyl terminal. They recognize short segments 7 to 9 amino acids long, rich in hydrophobic residues. In typical globular proteins, such sites occur approximately every 16 amino acids.

Role of Hsp70 in protein synthesis (Stage A): During translation on the ribosome, the N-terminus of the protein appears earlier than the C-terminus, but proper folding requires the entire chain. Hsp70s bind to the polypeptide co-translationally (directly during synthesis). They cover dangerous hydrophobic sites with their groove, preventing premature aggregation before synthesis is fully completed.

Structure and functions of Hsp60 chaperones

Hsp60 chaperones come into play after chain synthesis is complete (Stage B). Their main task is to ensure the folding of high-molecular-weight proteins with complex conformations (e.g., those with a domain structure).

Unlike Hsp70s, Hsp60s function as a huge oligomeric complex consisting of 14 subunits. They form a cylindrical cavity from two stacked rings (7 subunits in each).

Each subunit has three domains:

1. Apical. Facing the inside of the cavity, enriched with hydrophobic residues for initial binding of the unfolded protein.
2. Intermediate.
3. Equatorial (at the base). This is where ATP hydrolysis occurs.

The protein with hydrophobic markers enters the isolated cavity of this complex. A specific environment is created inside, where a continuous search for possible spatial structures takes place until the single most energetically favorable form is found. This process requires significant energy expenditure in the form of ATP.

Additional functions of chaperones

Protection against denaturation and folding of new chains are not the only tasks of these proteins. Chaperones are also necessary for the cell to:

• Properly assemble complex oligomeric proteins from multiple subunits.
• Transport proteins across cellular and organellar membranes.
• Regulate the functional activity of protein complexes.
• Recognize irreversibly denatured proteins and subsequently target them for degradation in lysosomes.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of co-translational binding of Hsp70 to the ribosomal complex
• Folding energetics: detailed role of the Hsp60 equatorial domain in ATP hydrolysis
• Involvement of chaperones in the assembly of complex oligomeric proteins
• Mechanisms of denatured protein transport into lysosomes via chaperones
• Differences between constitutive chaperones and heat shock proteins at the level of genetic regulation

Other topics in the section "Structure, Properties, and Functions of Proteins"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.