Amino Acid Properties and Protein Structure

Posted on Jan 30, 2025 in Human Biology

Amphoteric Character of Amino Acids

Item 5 – Properties of Amino Acids: The amphoteric character of amino acids allows for pH regulation because they behave as acids or bases, as appropriate to the organism.

The Peptide Bond

Characteristics of the peptide bond:

  • The peptide bond is a covalent bond, shorter than most other C-H bonds.
  • It has some double bond character, which prevents it from rotating freely.
  • The four atoms of the peptide group and the two carbon atoms are located on the same plane, maintaining fixed distances and angles.
  • The only bonds that can rotate are those formed by Cα and N-C.

Elementary Protein Structure

The elementary structure is the most important because it determines the other protein structures with higher levels of organization.

Secondary Structure

  • Stability of the alpha-helix depends on the presence or absence of proline or hydroxyproline residues.
  • Beta or folded sheet conformation: all peptide bonds are involved in this conformation, conferring great stability to the structure. Polypeptide chains can bind in two different ways: parallel or antiparallel.
  • Collagen: The individual chains are wound clockwise.

Tertiary Structure

The characteristics of a protein and its biological functions depend on its tertiary structure. Domains are joined together by a flexible protein portion that serves as a hinge and are very stable.

Quaternary Structure

Formed by two or more polypeptide chains, known as subunits, protomers, or monomers. They may be the same or different. They are held together by hydrogen bonds, van der Waals forces, and even disulfide bonds.

Protein Properties

  • Solubility: Fibrous proteins are insoluble in water, while globular proteins are water-soluble.
  • Denaturation: Caused by changes in pH or temperature, or by treatment with denaturants such as urea. Denatured proteins lose their biological activities.
  • Specificity: This is the most characteristic property of proteins. Specific functions also occupy certain positions that constitute their linear sequence. Specificity of species means that proteins are unique to each species.
  • Buffer Capacity: Proteins behave like amphoteric compounds, similar to the amino acids that form them.

Holoproteins

Those which are composed exclusively of amino acids. Examples include collagen fibers, myosin, keratins, fibrin, and elastin. Globular proteins are more complex and include actin, albumin, globulins, and histones.

Heteroproteins

Their composition consists of one protein part and another non-protein portion called the prosthetic group.

  • Nucleoproteins: Proteins whose prosthetic group is a nucleic acid, maintaining the structure of DNA.
  • Glycoproteins: The prosthetic group is a carbohydrate covalently bound to the polypeptide chain.
  • Phosphoproteins: The prosthetic group is phosphoric acid. Examples include casein in milk and vitellin in egg yolk.
  • Lipoproteins: The prosthetic group is a lipid. They are responsible for transporting insoluble lipids.
    • LDL (Low-Density Lipoproteins): Carry cholesterol and phospholipids from the liver to the tissues to form cell membranes. They can form deposits on the walls of the arteries.
    • HDL (High-Density Lipoproteins): Their action is contrary to LDL. They carry cholesterol to the liver, removing it from the arterial walls, thus decreasing the deposits.