Amino Acids and Their Functions

Amino Acids

• Proteins are made from a common set of 20 amino acids, each with unique side chains.
• Amino acids serve as the building blocks of protein, akin to an alphabet in written language.
• Key functions of proteins include catalysis, transport, structural support, immune response, and regulation.

Structural Features of Amino Acids

• Each amino acid contains a chiral α carbon with four substituents: a carboxyl group, an amino group, a hydrogen atom, and a unique R group (side chain).
• Glycine is unique as it has a second hydrogen atom instead of an R group.

Stereoisomers

• Amino acids exist as L (levorotatory) stereoisomers, with two enantiomers being optically active.

Classification of Amino Acids

• Amino acids are classified into five main groups based on R groups:
  • Nonpolar, aliphatic (7)
  • Aromatic (3)
  • Polar, uncharged (5)
  • Positively charged (3)
  • Negatively charged (2)

Properties of R Groups

• Nonpolar, aliphatic R groups enhance protein stability due to the hydrophobic effect.
• Aromatic R groups absorb UV light at 270–280 nm; this property can be used to measure concentration.
• Polar, uncharged R groups can form hydrogen bonds; cysteine can form disulfide bonds which are reversible.

Disulfide Bonds

• Disulfide bonds can be intramolecular or intermolecular, playing key roles in protein structure.

Uncommon Amino Acids

• Some uncommon amino acids have notable functions, being modified during or after protein synthesis (e.g., 4-hydroxyproline in collagen, pyrrolysine).

Amino Acid Abbreviations

• Standard amino acids have three-letter abbreviations (e.g., Gly for Glycine) or one-letter symbols.

Acid-Base Properties of Amino Acids

• Amino acids can act as acids or bases due to the presence of ionizable groups.
• At neutral pH, amino acids exist predominantly in zwitterionic form, where they carry both positive and negative charges.

Titration and pKa Values

• The pKa values indicate the strength of the acidic or basic nature of the amino acids' functional groups.
• pK1 (carboxyl group) ranges between 1.8-2.4; pK2 (amino group) ranges between 8.8-11.0.
• The isoelectric point (pI) is where the net charge is zero, influencing solubility and migration in an electric field.

Buffering Capacity of Amino Acids

• Amino acids can serve as buffers, maintaining pH around their pKa values.

Peptides and Proteins

• Peptides are chains of amino acids connected by peptide bonds, which are covalent linkages formed via condensation and broken through hydrolysis.
• Peptide types classified by number of amino acids include:
  • Dipeptides (2)
  • Tripeptides (3)
  • Oligopeptides (few)
  • Polypeptides (many, < 10 kDa)
  • Proteins (thousands, > 10 kDa)

Peptide Naming

• Peptides are named starting from the N-terminal to C-terminal, using full names or abbreviations (three-letter or one-letter).

Ionization Behavior of Peptides

• Peptides have one free α-amino group and one free α-carboxyl group, influencing their ionization.

Biologically Active Peptides

• Naturally occurring peptides vary in size and composition, serving various biological functions, with molecular weights ranging widely.

Amino Acid Composition in Proteins

• Composition varies significantly across different proteins, impacting their structure and function.

Estimating Amino Acid Residues

• The number of amino acid residues can be estimated by dividing molecular weight by 110, accounting for the water lost during peptide bond formation.

Conjugated Proteins

• Conjugated proteins contain additional chemical groups beyond amino acids, which can include lipids, carbohydrates, phosphate groups, and specific metals, enhancing their functionality.

Explore the fascinating world of amino acids, the building blocks of proteins. This quiz covers their structural features, classification, and key functions in biological systems. Test your understanding of the unique properties and roles of various amino acids.