Proteins and Amino Acids Overview

Proteins

• Proteins are large organic molecules made up of amino acids linked by peptide bonds.
• They contain carbon, hydrogen, oxygen, nitrogen, sulfur, and sometimes phosphorus.
• Proteins are crucial for various functions in living organisms, including structural components, enzymatic activity, antibody production, and hormone regulation.

Amino Acids

• Amino acids are the building blocks of proteins, and they are obtained by hydrolyzing proteins.
• Most naturally occurring amino acids are L-amino acids, with the amino group on the left side.
• D-amino acids are found in bacterial cell walls.

Importance of Amino Acids

• They form the basis of protein structure.
• They are precursors of important biological compounds:
  • Hormones: Thyroid hormones, adrenal cortical hormones, melatonin
  • Vitamins: Niacin
  • Purines and pyrimidines
  • Porphyrins

Classification of Amino Acids

• Amino acids are classified according to:
  • Chemical classification: based on the number of amino and carboxyl groups.
    • Neutral amino acids: One amino group and one carboxyl group.
    • Acidic amino acids: Two carboxyl groups and one amino group.
    • Basic amino acids: Two amino groups and one carboxyl group.
  • Biological classification: based on whether they are essential or non-essential.
    • Essential amino acids: Not synthesized by the body, must be obtained through diet.
    • Non-essential amino acids: Synthesized by the body.
    • Semi-essential amino acids: Can be synthesized by the body, but in insufficient quantities.
  • Metabolic classification: based on their metabolic fate in the body.
    • Glucogenic amino acids: Can be converted into glucose or carbohydrate intermediates.
    • Ketogenic amino acids: Can be converted into ketone bodies.
    • Mixed amino acids: Can be converted into both glucose and ketone bodies.

Chemical Classification of Neutral Amino Acids

• Aliphatic amino acids:
  • Glycine (α-amino acetic acid)
  • Alanine (α-amino propionic acid)
  • Valine (α-amino iso-valeric acid)
  • Leucine (α-amino isocaproic acid)
  • Isoleucine (α-amino β-methyl valeric acid)
• Hydroxy amino acids:
  • Serine (α-amino β-hydroxyl propionic acid)
  • Threonine (α-amino β-hydroxyl butyric acid)
• Sulfur-containing amino acids:
  • Cysteine (α-amino β-thiol propionic acid)
  • Cystine (formed by oxidation of two cysteine molecules)
  • Methionine (α-amino β-thiol butyric acid)
• Aromatic amino acids:
  • Phenylalanine (α-amino β-phenyl propionic acid)
  • Tyrosine (α-amino β-parahydroxy phenyl propionic acid)
  • Tryptophan (α-amino β-indol propionic acid)
• Heterocyclic amino acids:
  • Histidine (α-amino β-imidazole propionic acid)
  • Proline (α-pyrrolidine carboxylic acid)
  • Hydroxyproline

Chemical Classification of Acidic Amino Acids

• Glutamic acid: (α-amino glutaric acid)
• Aspartic acid: (α-amino succinic acid)

Chemical Classification of Basic Amino Acids

• Arginine: (α-amino σ-guanido valeric acid)
• Lysine: (α-ε-diamino caproic acid)
• Hydroxylysine: (α-ε-diamino σ-hydroxy caproic acid)
• Ornithine: (α-σ-diamino valeric acid) - Not involved in protein synthesis, present as an intermediate in urea synthesis.
• Citrulline: Produced during urea synthesis.

Properties of Amino Acids

Physical Properties

• Solubility:
  • Soluble in polar solvents like water and ethanol due to the presence of charged groups, except for cysteine and tyrosine.
  • Soluble in strong acids and bases.
  • Insoluble in alcohols except proline and hydroxyproline.
• Optical activity:
  • All amino acids, except glycine, are optically active due to the presence of asymmetric carbon atoms.
• Optical isomer:
  • Naturally abundant amino acids are L-amino acids.
  • D and L forms are mirror images of each other.
• Amphoterism and Zwitter ion:
  • Amino acids are amphoteric, meaning they can act as both acids and bases.
  • Zwitter ion or Dipolar ion: A form of the amino acid carrying an equal number of positive and negative charges.
• Isoelectric point:
  • The pH at which the amino acid is isoelectric (carrying equal numbers of positive and negative charges).
  • At the isoelectric point, the amino acid does not migrate toward the anode or cathode and will be precipitated.

Chemical Properties

• Reactions due to the carboxyl group:
  • Esterification: Formation of esters with alcohols.
  • Amide formation: Formation of amides with amines.
  • Decarboxylation: Removal of carbon dioxide.
• Reactions due to the amino group:
  • Acylation: Reaction with acyl chlorides or anhydrides.
  • Alkylation: Reaction with alkyl halides.
  • Deamination: Removal of the amino group.
• Reactions due to the side chain:
  • Reactions specific to the side chain functional group.

Important Notes:

• The text highlights the importance of essential amino acids for growth and health, emphasizing their role in diet.
• It includes details about metabolic classification and the fate of amino acids within the body, categorizing them as glucogenic, ketogenic, or mixed depending on their products.
• The text also provides a detailed chemical and physical properties of amino acids, including their solubility, optical activity, amphoterism, and special properties associated with the presence of specific side chains.

This quiz covers the fundamentals of proteins and amino acids, including their structures, functions, and importance in biological processes. Explore the classification of amino acids and their roles in the formation of essential compounds like hormones and vitamins.