26.1 Structures of Amino Acids

Amino acids are the building blocks of proteins, each with a unique structure and properties. They consist of an amino group, a carboxyl group, and a side chain attached to a central carbon atom. These structural features determine their classification and behavior in biological systems.

At physiological pH, amino acids exist as zwitterions, with both positive and negative charges. This property, along with their classification as essential or nonessential, plays a crucial role in their function within the body. Understanding amino acid structure and properties is key to grasping protein behavior and function.

Amino Acid Structure and Classification

Structural features of amino acids

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• General structure consists of an amino group ($-NH_2$) and a carboxyl group ($-COOH$) attached to the $\alpha$-carbon along with a hydrogen atom and a side chain (R group) specific to each amino acid (Glycine, Alanine)
• Side chain properties determine the classification of amino acids into nonpolar aliphatic (Glycine, Alanine, Valine), aromatic (Phenylalanine, Tyrosine), polar uncharged (Serine, Threonine), positively charged (Lysine, Arginine), and negatively charged (Aspartic acid, Glutamic acid)
• Presence of four different groups attached to the $\alpha$-carbon makes all amino acids except glycine chiral with naturally occurring amino acids being L-stereoisomers (L-Alanine, L-Serine)
• Contain both acidic (carboxyl group) and basic (amino group) functional groups allowing them to act as acids or bases depending on the pH of the solution (Lysine, Glutamic acid)
• Stereochemistry of amino acids plays a crucial role in their biological function and interactions with other molecules

Amino acids as zwitterions

• Exist predominantly as zwitterions at physiological pH (around 7.4) with a proton transferred from the carboxyl group to the amino group resulting in a molecule with both positive and negative charges and a net charge of zero (Glycine, Lysine)
• Exhibit amphiprotic behavior by acting as both acids (donating a proton from the carboxyl group) and bases (accepting a proton at the amino group) in solution (Aspartic acid, Arginine)
• Isoelectric point (pI) represents the pH at which an amino acid has a net charge of zero due to equal numbers of positive and negative charges (Leucine, Histidine)

Essential vs nonessential amino acids

• Essential amino acids cannot be synthesized by the human body and must be obtained through diet including Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine with deficiency leading to health issues such as impaired growth and development
• Nonessential amino acids can be synthesized by the human body from other compounds and include Alanine, Arginine, Asparagine, Aspartic acid, Cysteine, Glutamic acid, Glutamine, Glycine, Proline, Serine, and Tyrosine but still play important roles in various biological functions
• Serve as building blocks of proteins essential for cellular functions, involved in the synthesis of neurotransmitters (Serotonin, Dopamine), hormones (Insulin, Glucagon), and other biologically active compounds, and play a role in energy metabolism and nitrogen balance (Urea cycle, Glucose-Alanine cycle)

Amino Acid Interactions and Properties

• Amino acids can form peptide bonds to create longer chains called polypeptides, which are the building blocks of proteins
• Cysteine residues can form disulfide bonds, which contribute to protein stability and tertiary structure
• Hydrogen bonding between amino acid side chains plays a crucial role in protein folding and maintaining protein structure
• The hydrophobicity of amino acid side chains influences protein folding and interactions with the surrounding environment