Intro to Amino Acids: Videos & Practice Problems

Amino acids are essential molecules characterized by the presence of an amino group, a carboxyl functional group, and a unique R side chain. The central component of an amino acid is the alpha carbon, which serves as the backbone to which these functional groups are attached. Specifically, the alpha carbon is bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom (–H), and the variable R group, which can be a hydrogen, a carbon chain, or a more complex structure.

At physiological pH, which is approximately 7.4, amino acids exist predominantly in a form known as the zwitterion. In this state, the amino group accepts a proton (H⁺), resulting in a positively charged ammonium ion (–NH₃⁺), while the carboxyl group donates a proton, transforming into a negatively charged carboxylate anion (–COO^–). This dual charge creates a zwitterionic structure, where the molecule carries both positive and negative charges, contributing to its unique properties in biological systems.

There are 20 standard alpha amino acids, each distinguished by its specific R group. These R groups vary in size, shape, and chemical properties, allowing for a diverse range of functions and structures in proteins. The combination of these 20 amino acids forms the building blocks of proteins, which are vital for numerous biological processes in living organisms. Understanding the structure and behavior of amino acids, particularly the role of the alpha carbon and the R group, is crucial for exploring the complexity of protein synthesis and function.

In the study of alpha amino acids, it is essential to identify and label their structural components accurately. Each alpha amino acid consists of a carboxyl group (–COOH), an amino group (–NH₂), and an alpha carbon (C_α) that connects these two functional groups. The alpha carbon is also bonded to a hydrogen atom and a unique side chain known as the R group, which varies among the 20 standard amino acids.

For example, in the first structure, labeled as A, the carboxyl group and amino group are directly attached to the alpha carbon, making it a traditional alpha amino acid. The presence of a distinct R group further confirms its classification.

In the second structure, labeled as B, the alpha carbon does not have the amino group directly attached, indicating a deviation from the standard structure of alpha amino acids. This structural difference means that it does not fit into the category of the 20 common amino acids.

In the third structure, labeled as C, the carboxyl group and amino group are again connected to the alpha carbon, which is also bonded to a hydrogen and a unique R group. This confirms that C is also a traditional alpha amino acid.

In summary, structures A and C represent standard alpha amino acids, while structure B deviates from this classification due to the absence of a direct amino group attachment to the alpha carbon.