Protein Structure Levels and Denaturation Explained

Protein Structure Levels and Denaturation

Tertiary Structure

Tertiary Structure: Disposition that takes the structure in the secondary space, resulting in the final three-dimensional arrangement. The tertiary structure dictates the protein's function, so any changes in this layout can lead to a loss of biological activity. This structure arises from the union of specific areas of the chain. These unions are formed through links between the side chains (R groups) of the amino acids (AAs). It is maintained by hydrogen bridges, disulfide bonds, and Van der Waals forces.

Quaternary Structure

Quaternary Structure: This structure is present in proteins made up of a number of chains or subunits. The Quaternary Structure is the arrangement of protein subunits when they come together. The union between them is made by the same types of bonds holding the tertiary structure, established in this case between the side chains of AAs of different subunits. It is maintained by noncovalent bonds.

Protein Denaturation

Protein Denaturation: When proteins lose their secondary, tertiary, or quaternary structure, they can be denatured. This means they lose their conformation reversibly (all bonds except covalent bonds break) and adopt a primary structure in which they cannot perform their role and become inactive. This process can be induced by changing the pH of the medium, the protein concentration of a solution, or the temperature of the medium. Restoration involves recovering the native protein structure and its functionality.

Functions of Proteins

Functions of Proteins:

• Structural: They are the main material constituting almost all structures, e.g., the plasma membrane.
• Enzyme: Chemical reactions are catalyzed by enzymes.
• Transport and Storage: Many small molecules and ions are transported by specific proteins, e.g., hemoglobin and hemocyanin transport oxygen.
• Hormone: Several hormones are proteins, e.g., insulin.
• Contractile: They are part of contractile systems that produce coordinated movements, e.g., muscle contraction through actin and myosin.
• Defense and Protection: Antibodies.
• Reserves: Some proteins serve to reserve AAs, e.g., casein in milk.

Amino Acids (AAs)

Amino Acids (AAs): These are organic acids ($\text{ác}$) that have an amino group ($\text{-NH}_2$) at C-2, which is the carbon adjacent to the carboxyl group ($\text{-COOH}$). On this C-2 carbon, there is another hydrogen atom ($\text{H}$) and a distinctive group, the radical-R. They are crystallizable, non-hydrolyzable substances of varied taste. They are classified into:

• AAs with nonpolar R groups (hydrophobic).
• AAs with polar R groups, but uncharged.
• AAs with polar R groups and negatively charged (acidic). They dissociate in solution to give anions.
• AAs with polar R groups and positively charged (basic). They dissociate in solution to give cations.