Proteins play vital roles in numerous bodily functions. They serve as essential building blocks for constructing and repairing the body, being indispensable for human development and the repair and renewal of damaged cells. Additionally, proteins can be broken down to provide energy for vital bodily activities.
[Amphiphilic Nature]
Proteins are macromolecules formed by α-amino acids linked via peptide bonds. The presence of amino and carboxyl groups within the protein molecule makes proteins, like amino acids, amphiphilic substances.
【Capable of Hydrolysis Reactions】
Proteins undergo hydrolysis reactions under the influence of acids, bases, or enzymes. This process breaks down the protein into peptides and ultimately yields various α-amino acids. During hydrolysis, the reaction must locate the "break points" within the structural bonds, resulting in the partial or complete cleavage of peptide bonds.
【Hydrophilic Colloidal Properties】
Certain proteins dissolve in water (e.g., egg white dissolves in water) to form solutions.
Protein molecules reach colloidal particle sizes (10⁻⁹ to 10⁻⁷ m), endowing them with colloidal properties.
【Precipitation】
Causes: Adding high concentrations of neutral salts, organic solvents, heavy metals, alkaloids or acids, or thermal denaturation.
Small amounts of salts (e.g., ammonium sulfate, sodium sulfate) can promote protein solubility.Adding concentrated inorganic salt solutions to protein aqueous solutions reduces protein solubility, causing precipitation—a process called salting out.
Proteins precipitated by salting out remain soluble in water without altering their original properties, making salting out a reversible process. This property enables protein separation and purification using stepwise salting-out methods.
Denaturation Under the influence of heat, acids, alkalis, heavy metal salts, ultraviolet light, etc., proteins undergo structural changes and coagulate. This coagulation is irreversible, preventing restoration to the original protein state. This transformation is termed denaturation. After denaturation, ultraviolet absorption, chemical activity, and viscosity increase, while solubility decreases. Denatured proteins become more susceptible to hydrolysis.
Once denatured, proteins lose their original solubility and thus their physiological functions. Therefore, protein denaturation and coagulation constitute an irreversible process.
【Causes of Protein Denaturation】
Physical factors include: heating, pressurization, stirring, shaking, UV irradiation, X-rays, ultrasonic waves, etc.
Chemical factors include: strong acids, strong bases, heavy metal salts, trichloroacetic acid, ethanol, acetone, etc.
【Color Reactions】
Proteins can undergo color reactions with numerous reagents.
For example, adding concentrated nitric acid to an egg white solution turns the solution yellow.This occurs due to the color reaction between proteins (containing benzene ring structures) and concentrated nitric acid. The biuret reagent can also be used for testing, forming a purple complex when encountering proteins.
【Odor Reaction】
When proteins decompose upon burning, they produce a distinctive odor resembling burnt feathers.
This property can be utilized to identify proteins.