Molecular Components of Cells: Proteins

Molecular Components of Cells

• The study focuses on inorganic and organic compounds, specifically proteins.

Proteins

• Structures of Proteins:

  • Primary structure: a linear sequence of amino acids linked by covalent bonds. Determined by the nucleotide sequence in DNA. Its alteration affects protein function. All copies of the same protein have identical primary structures.
  • Secondary structure: folding of amino acid portions into α-helices, β-sheets, and irregular structures. Stabilized by hydrogen bonds (except for supercoiling, which has covalent bonds). A specific amino acid sequence always folds the same way.
  • Tertiary structure: folding of helical and non-helical regions into precise, 3-dimensional positions. Hydrophobic amino acids are hidden, while charged amino acids are exposed. Brings distant segments together to form active domains. Stabilized by hydrogen bonds, ionic bonds, hydrophobic interactions, salt bridges, and disulfide bridges.
  • Quaternary structure: assembly of two or more polypeptide subunits (identical or different). Stabilized by weak bonds that are sometimes covalent. Impacts protein activity/deactivation.
  • Protein Denaturation: Unfolding and separation of proteins by physical agents (low pH, high temperature) and chemical agents (detergents, urea). Proteins lose activity, and this denaturation is often irreversible unless mild conditions.

• Classification of Proteins:

  • By composition: Holoproteins (only amino acids) vs. Heteroproteins (amino acids + other molecules, like lipoproteins or hemoproteins).
  • By structure: Fibrous (insoluble in water, e.g., keratin, collagen) vs. Globular (soluble in water, e.g., globulin, albumin).
  • By function: Diverse functions including structural, defense, regulatory, transporter, catalytic, and contractile roles.

Inorganic Compounds

• Inorganic compounds are evenly distributed throughout cell compartments.
• Mineral salts can interact with organic compounds.
• Water is not a trace element.

Amino Acids

• Amino acids are amphiphilic macromolecules, with a pH-dependent electric charge.
• Covalent bonds link amino acids together in polypeptides.
• Solubility of amino acids depends on their side chains.

Polypeptide Chain

• The primary structure is stabilized through hydrogen bonds.
• They fold into 3D structures that can switch between active and inactive conformations. This is not a helix or a sheet along its entire length.