Molecular Structure of Proteins and DNA

Protein Structure and Function

Proteins are composed of amino acid monomers. Through a condensation reaction, they liberate one H₂O molecule to form a peptide bond, which is a functional amide group. They also contain a carboxylic acid functional group.

Levels of Protein Organization

Primary and Secondary Structures

Primary Structure: This is the amino acid sequence that shows the component amino acids and the order they are in within the protein.

Secondary Structure: This reports the disposition of amino acids (from the primary structure) in space, specifically the folding it adopts through hydrogen bonds (PteH):

• Alpha Helices: A dextrorotatory helical structure formed by hydrogen bonds between the NH group of an amino acid "n" and the C=O group of amino acid "n+4". This stabilizes the structure.
• Beta-folds (Beta-sheets): This provides positioning for two chains where the NH group of one amino acid forms a hydrogen bond with the C=O group of an amino acid on the opposite chain.

Tertiary and Quaternary Structures

Tertiary Structure: This reports on the secondary structure when it is folded upon itself. It leads to two main configurations:

• Fibrous Configuration: Structural functions, such as collagen and elastin.
• Globular Configuration: Transport functions, such as membrane proteins.

Links and Bonds: These include strong intermolecular forces (covalent, ionic, and molecular) and weak atomic bonds:

• Disulfide Bonds: Formed between sulfur (S) atoms existing between radicals.
• Hydrogen Bonds (PteH): Formed between hydrogen and oxygen atoms in the radicals.
• Van der Waals Forces: Stable forces of attraction and repulsion.
• Electrostatic Forces: Occur when positive radical groups are attracted to negative groups.

Quaternary Structure: This reports on the union of several chains with a weak structure, denominated as protomers, which results in the formation of a protein complex.

Protein Denaturation and Biological Roles

Denaturation: This involves the total or partial loss of structures higher than the primary level. It is caused by changes in pH and temperature (Tº) that break weak bonds. It does not affect peptide bonds, but as the spatial arrangement is lost, the protein loses its biological functions.

Biological Functions:

• Catalysts: Enzymes affect the velocity (V) of reactions in living organisms (nearly all enzymes are proteins, except ribozymes).
• Transport: Hemoglobin (transports O₂ from cells and CO₂ to lung alveoli).
• Immunological: Antibodies.
• Hormonal: Insulin and Glucagon.

Nucleic Acids and Nucleotide Formation

Nucleic Acids are composed of nucleotide monomers joined by condensation with phosphodiester bonds.

Components:

• Phosphoric Acid (H₃PO₄): Represented as O=P(OH)₃.
• Monosaccharide: A pentose sugar.
• Nitrogenous Bases: Purines and pyrimidines.

Formation: The NH group of the base is joined to the C₁ of the pentose via an N-glycosidic bond. The phosphate group binds to the C₅ of the pentose, releasing H₂O and forming a phosphodiester bond.

Polymerization: Occurs by condensation between the C₅ of one nucleotide and the C₃ of the next nucleotide.