Amino Acid Groups: Videos & Practice Problems

Congratulations on completing Unit 1! As we transition into a new unit focused on proteins, it's essential to understand the foundational role of amino acids in biochemistry. There are 20 standard amino acids, also known as alpha amino acids, which serve as the building blocks of proteins. Each amino acid is characterized by its unique R group, which determines its chemical properties.

Amino acids can be categorized based on the characteristics of their R groups, particularly at physiological pH, which is approximately 7. At this pH, the common backbone of every amino acid exists as a dipolar ion, meaning it contains two different charged species. The structure of an amino acid includes a central carbon atom, known as the alpha carbon, which is bonded to a hydrogen atom, an amino group, and a carboxyl group. The amino group carries a positive charge, while the carboxyl group contributes a negative charge, reinforcing the dipolar nature of amino acids at physiological pH.

Understanding these fundamental concepts about amino acids is crucial as we delve deeper into protein structure, techniques, and the role of enzymes and their regulation in subsequent lessons. Each amino acid's unique R group not only defines its properties but also influences how proteins are formed and function in biological systems.

Amino acids can be categorized based on the chemical properties of their R groups, and there are multiple valid systems for grouping them. This flexibility means that different textbooks or professors may use slightly different classifications, but the core concepts remain consistent. Understanding these groupings is essential for mastering amino acid properties and functions.

There are two primary methods for grouping amino acids: functional grouping and structural grouping. Functional grouping focuses on the biological roles and behaviors of the R groups, such as whether they are basic or acidic, polar or non-polar. For instance, basic amino acids have R groups that can accept hydrogen ions, while acidic amino acids can donate hydrogen ions. Polar amino acids have R groups that interact well with water, whereas non-polar amino acids do not.

On the other hand, structural grouping emphasizes the arrangement and presence of atoms within the R groups. This includes classifications like aromatic and aliphatic R groups. Aromatic amino acids contain ring structures, while aliphatic amino acids consist of straight-chain hydrocarbons.

Due to the nature of these groupings, some amino acids may fit into multiple categories. However, in the system used here, each amino acid is assigned to only one group for clarity. The four major groupings identified are non-polar, aromatic, polar, and charged. Non-polar and polar classifications fall under functional groupings, while aromatic is categorized as a structural grouping.

Additionally, each amino acid is represented by a three-letter and a one-letter abbreviation, which aids in memorization and identification. Understanding these groupings and their characteristics will be crucial as you progress in your studies of amino acids and their functions in biological systems.