Proteins: Structure, Functions, and Classification

Posted on Nov 3, 2024 in Biology

Proteins

Amino Acids

The hydrolysis of protein molecules releases amino acids, whose union forms polypeptide chains. Amino acids have an amino group (-NH2) and a carboxyl group (-COOH).

Properties of Amino Acids:

  • Amphoteric character: They behave as an acid or base depending on the pH. If the carboxyl group has an acidic character, it releases H+; if the amino group has a basic character, it accepts H+.
  • Stereoisomerism: Since the alpha carbon is asymmetric, it forms stereoisomers. Two forms are considered: D (if the NH3 group is on the right) and L (if it’s on the left).
  • Other properties: The presence of polar groups (amino and carboxyl) allows the formation of hydrogen bonds between amino acids. This results in higher boiling and melting points and increased solubility in water.

Classification of Amino Acids

  • Neutral: These amino acids have no extra amino or carboxyl groups. They can be nonpolar (hydrophobic) or polar (hydrophilic).
  • Acidic: These have an extra carboxyl group.
  • Basic: These have an extra amino group.

Peptide Bond

The carboxyl group of one amino acid interacts with the amino group of another, releasing water. The resulting molecule is called a dipeptide. The union of many amino acids forms a polypeptide.

Structures of Proteins

Polypeptide chains undergo folding to form different protein structures.

Primary Structure

This refers to the linear sequence of amino acids in the polypeptide chain.

Secondary Structure

  • Alpha-helix: A clockwise spiral folding. Each turn contains 3.6 amino acids. The folding is stabilized by hydrogen bonds between the NH and OC groups.
  • Beta-sheet: A zigzag fold (pleated sheet). Collagen has this structure.

Tertiary Structure

This refers to the three-dimensional arrangement of the polypeptide chain in space. Four types of bonds are involved:

  • Disulfide bonds: Covalent bonds between SH groups.
  • Electrostatic forces: Ionic bonds between COO- and NH3+ groups.
  • Hydrogen bonds: Nonionic bonds between polar groups.
  • Van der Waals forces: Weak bonds between nonpolar amino acids.

Tertiary structures can be globular or fibrous.

Quaternary Structure

This structure consists of several polypeptide chains interacting with each other.

Properties of Proteins

  • Solubility: Globular proteins are soluble, while fibrous proteins are not.
  • Alteration of spatial structure: If any bond in the secondary, tertiary, or quaternary structure is broken, it can affect the protein’s functionality. These bonds can be broken by changes in temperature, pressure, salt concentration, or pH.
  • Specificity: Each species has unique proteins that are not found in other species.

Functions of Proteins

  • Structural: Form cell structures.
  • Amino acid storage.
  • Physiological functions.
  • Genetic regulation.
  • Catalysis.
  • Immunity.

Classification of Proteins

Holoproteins or Simple Proteins

  • Globular: Albumin (reserve), globulins, histones (associated with DNA).
  • Fibrous: Keratin (part of structures), collagen, myosin (muscle contraction), elastin (elasticity).

Heteroproteins

  • Phosphoproteins.
  • Glycoproteins.
  • Antibodies.
  • Lipoproteins (transport).
  • Chromoproteins: Porphyrin compounds (hemoglobin, myoglobin, and cytochromes).