Quimica de Proteinas

Protein Chemistry

• Proteins are polymers of amino acids, with more than 50 linked by peptide bonds.
• Amino acids are organic acids with an amino group (NH₂). About 300 exist in nature, but only 20 polymerize in protein structures within mammals.
• All amino acids in mammals are L-amino acids and are alpha-amino acids. D-amino acids are found in bacterial cell walls.
• The R group differentiates the 20 amino acids, influencing their chemical behaviour.
• Amino acids categorize into neutral, acidic, and basic groups based on their chemical structure, each with distinct features. Neutral amino acids have one amino and one carboxyl group. Acidic have two carboxyl and one amino group. Basic have two amino and one carboxyl group.
• Neutral amino acids subcategory includes aliphatic (e.g., Glycine, Alanine, Valine, Leucine, Isoleucine), hydroxy (e.g., Serine, Threonine), and aromatic (e.g., Phenylalanine, Tyrosine).
• Sulfur-containing amino acids include Cysteine and Methionine.
• Heterocyclic amino acids are complex ring-containing structures, such as Histidine and Tryptophan.
• Acidic amino acids include Glutamic acid and Aspartic acid.
• Basic amino acids include Lysine, Ornitine, Hydroxylysine, and Arginine.
• Essential amino acids are not synthesized in the body; they must be ingested. Animal and plant proteins are essential sources. Essential ones include Valine, Isoleucine, Leucine, Lysine, Threonine, Tryptophan, Phenylalanine, Methionine, and Arginine. Histidine is sometimes considered essential.

Classification of Amino Acids

• Amino acids can be categorized based on chemical, biological, or metabolic properties.

Protein Classification

• Biological classification

  • High biological value proteins: Animal and certain plant proteins with all essential amino acids in balanced amounts, are easily digestible. (Examples: milk, eggs, liver, fish, meat, lentils, broad beans)
  • Low biological value proteins: Plant proteins, lacking one or more essential amino acids, or having very low amounts, often difficult to digest.

• Axial Ratio Classification

  • Fibrous proteins: High length-to-width ratio; stable; found structurally in hair, skins, and muscular tissue. (Examples: keratin, myosin)
  • Globular proteins: Low length-to-width ratio; less stable than fibrous proteins; found in solutions and involve transport, metabolic regulation, etc.. (Examples: albumin, insulin)

• Composition Classification

  • Simple proteins: Hydrolysis yields only amino acids (Examples: Albumin, Globulins)
  • Conjugated proteins: Proteins combined with a non-protein group (prosthetic group). (Examples: Phosphoproteins [casein], Lipoproteins [plasma lipoproteins], Glycoproteins, Metalloproteins [ferritin, transferrin, ceruloplasmin], Nucleoproteins [formed from nucleic acid], Chromoproteins [hemoglobin])
  • Derived proteins: Denatured or degraded forms of proteins. (Examples: Gelatin, peptones, polypeptides)

Protein Structure

• Primary structure: Linear sequence of amino acids held by peptide bonds.
• Secondary structure: Local folding of the polypeptide chain held by hydrogen bonds into alpha-helices or beta-pleated sheets.
• Tertiary structure: Three-dimensional structure of the entire polypeptide chain, stabilized by disulfide bonds and secondary bonds. These bonds form a complex network.
• Quaternary structure: The final arrangement of multiple polypeptide chains (subunits) in a functional protein complex, often stabilized by secondary bonds.

Protein Properties

• Denaturation: Loss of secondary, tertiary, and quaternary structure (sometimes partially reversible), often by physical stressors (heat, agitation) or chemical agents (acids, bases, organic solvents).
• Bonds: Peptide bonds (strong, resist denaturation), disulfide bonds (secondary, strong but denatured prone), hydrogen bonds, hydrophobic interactions, electrostatic interactions.

Additional Notes

• Glutathione, a tripeptide (glutamic, cysteine, glycine), has crucial biological functions, including activating enzymes and serving as an antioxidant.