BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 5. STRUCTURE, PROPERTIES, AND FUNCTIONS OF PROTEINS

5.3. Formation of the Three-Dimensional Protein Structure in the Cell

5.3.2. Structure and Functional Role of Chaperones in Protein Folding

During the synthesis of polypeptide chains, their transport across membranes, and the assembly of Oligomeric Proteins, unstable intermediate Conformations prone to aggregation emerge. Newly synthesized Polypeptides contain numerous hydrophobic residues that are normally concealed within the interior of the molecule in the three-dimensional Structure. Therefore, while the native conformation is being formed, the reactive amino acid residues of certain proteins must be isolated from identical groups of other proteins.

In all known organisms, Cell/4.html">From Prokaryotes to higher eukaryotes, proteins capable of binding to proteins in an unstable, aggregation-prone state have been discovered. They are able to stabilize their conformation, thereby facilitating protein folding. These are chaperones.

According to their molecular weight, all chaperones can be divided into six main groups:

✵ high-molecular-weight chaperones - with a molecular weight ranging from 100 to 110 kDa;

✵ Hsp90 - with a molecular weight ranging from 83 to 90 kDa;

✵ Hsp70 - with a molecular weight ranging from 66 to 78 kDa;

✵ Hsp60;

✵ Hsp40;

✵ low-molecular-weight chaperones - with a molecular weight ranging from 15 to 30 kDa.

Among chaperones, a distinction is made between constitutive proteins (whose high basal synthesis does not depend on cellular stress factors) and inducible proteins, whose synthesis is weak under normal conditions but increases dramatically under cellular stress. Inducible proteins belong to heat Shock proteins, which are synthesized rapidly in virtually all Cells exposed to any kind of stress. The term heat shock proteins originated because they were first discovered in cells exposed to high temperatures.

During Protein Synthesis, the N-terminal region of the polypeptide is synthesized prior to the C-terminal region. The formation of a protein conformation requires its complete Amino Acid Sequence. Therefore, while the protein is being synthesized on the ribosome, Hsp70 protects its reactive residues (particularly hydrophobic ones).

Hsp70 is a highly conserved Class of proteins present in all cellular compartments: the Cytoplasm, Nucleus, Endoplasmic reticulum, and Mitochondria. Near the carboxyl terminus of the single polypeptide chain of these chaperones lies a groove-like region formed by amino acid residues. It is capable of interacting with segments of protein molecules and unfolded polypeptide chains that are 7-9 amino acid residues long and enriched in hydrophobic residues. In a nascent amino acid chain, such regions occur approximately every 16 Amino Acids.

The folding of many high-molecular-weight proteins with complex conformations (e.g., a domain structure) takes place in a specialized cavity formed by Hsp60, which Functions as an oligomeric complex consisting of 14 subunits (Fig. 5.23).

Hsp60 forms two rings, each consisting of seven interconnected subunits. An Hsp60 subunit comprises three domains: apical, intermediate, and equatorial. The apical domain features a series of hydrophobic residues directed into the cavity formed by the subunits. The equatorial domain contains an ATP-binding site and exhibits ATPase activity, meaning it is capable of hydrolyzing ATP to ADP and H₃PO₄.

The chaperone complex exhibits a high affinity for proteins whose surfaces display elements characteristic of unfolded molecules (primarily regions enriched in hydrophobic residues). Upon entering the cavity of the chaperone complex, the protein binds to the hydrophobic residues of the apical regions of Hsp60. Within the specific environment of this cavity, isolated from other cellular molecules, various possible protein conformations are explored until the single most energetically favorable conformation is found. The release of the protein with its newly formed native conformation is accompanied by ATP Hydrolysis in the equatorial domain.

If the protein has not acquired its native conformation, it re-binds to the chaperone complex. Such chaperone-dependent protein folding requires a significant expenditure of energy.

Fig. 5.23. STRUCTURE OF THE chaperone complex consisting of 14 Hsp60 protein molecules:

A - side view; B - top view

Thus, PROTEIN SYNTHESIS AND folding occur with the participation of various chaperone groups that prevent undesired Protein Interactions with other cellular molecules and accompany them until the final formation of their native structure (Fig. 5.24).

Chaperones involved in protecting cellular proteins from denaturing factors, as mentioned above, belong to heat shock proteins (HSPs).

Under METABOLISM/18.html">The Influence of various stressors (such as high Temperature, Hypoxia, infection, UV radiation, pH fluctuations, changes in medium osmolarity, toxic chemicals, heavy metals, etc.), cellular synthesis of HSPs is significantly upregulated. Exhibiting a high affinity for hydrophobic regions of partially denatured proteins, they help prevent complete Protein Denaturation and restore native protein conformation.

Fig. 5.24. Involvement of chaperones in protein folding:

A — participation of Hsp70 chaperones in preventing inappropriate hydrophobic interactions between Regions of the nascent polypeptide chain;

B — Formation of the native protein conformation within the chaperone complex

It has been established that short-term stress exposures stimulate HSP production and enhance the body's resistance to prolonged stress. For instance, brief myocardial ischemia induced by moderate exercise training significantly increases myocardial tolerance to long-term ischemia caused by angina or coronary thrombosis. Currently, exploring pharmacological and molecular-biological agents capable of activating cellular HSP synthesis is considered a promising direction in medical research.