Amino Acids

Introduction to Amino Acids

Amino acids are the fundamental building blocks of proteins, which are essential macromolecules in all living organisms. Each amino acid contains a central carbon atom (the α-carbon) bonded to an amino group, a carboxylic acid group, a hydrogen atom, and a unique side chain (R group) that determines its properties.

• α-Carbon: The central carbon atom to which all functional groups are attached.

• Amino Group (-NH2): Acts as a base and can accept a proton.

• Carboxyl Group (-COOH): Acts as an acid and can donate a proton.

• Side Chain (R group): Unique to each amino acid; determines the amino acid's identity and properties.

At neutral pH (~7), the amino group is typically protonated (-NH3+) and the carboxyl group is deprotonated (-COO-), resulting in a zwitterion (a molecule with both positive and negative charges but overall neutral).

Structure of α-Amino Acids

Chemical Structure and Zwitterions

The general structure of an α-amino acid can be represented as:

• General formula:

• At physiological pH, the structure is:

• Zwitterion: A molecule with both a positive and a negative charge, but overall electrically neutral.

Example: Glycine is the simplest amino acid, with R = H.

Amino Acid Stereochemistry

Chirality and Stereoisomers

Most α-amino acids (except glycine) have four different groups attached to the α-carbon, making it a chiral center (asymmetric carbon). This gives rise to two possible stereoisomers, known as enantiomers:

• L-amino acids: The form found in proteins in nature.

• D-amino acids: The mirror image of L-amino acids; rarely found in proteins.

• Enantiomers: Non-superimposable mirror images of each other.

Example: L-alanine and D-alanine are enantiomers.

Classification of Naturally Occurring Amino Acids

The 20 Common Amino Acids

Proteins are composed of 20 standard amino acids, each with a unique side chain. These amino acids can be classified based on the properties of their side chains:

• Nonpolar, Aliphatic: Glycine, Alanine, Valine, Leucine, Isoleucine, Proline, Methionine

• Aromatic: Phenylalanine, Tyrosine, Tryptophan

• Polar, Uncharged: Serine, Threonine, Cysteine, Asparagine, Glutamine

• Positively Charged (Basic): Lysine, Arginine, Histidine

• Negatively Charged (Acidic): Aspartate, Glutamate

Each amino acid is represented by a three-letter and a one-letter code (e.g., Alanine: Ala, A).

Table: Classification of the 20 Common Amino Acids

Additional info: The classification helps predict the behavior of amino acids in proteins, such as their location (interior or surface) and their role in protein function.