Amino acids have three main functional groups, each with unique properties: an amino group (-NH2), a carboxyl group (-COOH), and a side chain (or R group).
Core Functional Groups of Amino Acids
Every amino acid contains these core groups which attach to a central alpha-carbon atom:
-
Amino Group (-NH2):
- This group is basic in nature.
- It can accept a proton (H+), acting as a base.
- At physiological pH (around 7.4), the amino group is usually protonated and carries a positive charge (-NH3+).
-
Carboxyl Group (-COOH):
- This group is acidic in nature.
- It can donate a proton (H+), acting as an acid.
- At physiological pH, the carboxyl group is usually deprotonated and carries a negative charge (-COO-).
Example:
A basic amino acid, lysine, contains a second amino group in its side chain, making it more basic compared to the other 19 proteinogenic amino acids. An acidic amino acid, such as aspartate, will have a second carboxylic group in its side chain, making it more acidic.
The Role of the R Group (Side Chain)
- The R group is the variable part of an amino acid and it determines its unique chemical properties.
- R groups can vary greatly in terms of:
- Size: From a single hydrogen atom (in glycine) to bulky aromatic rings (in phenylalanine).
- Shape: Linear, branched, cyclic.
- Charge: Can be neutral, positive, or negative at physiological pH.
- Polarity: Can be polar or nonpolar.
- These variations in the R group result in 20 different amino acids with diverse functions within proteins.
Alpha-Amino Acid Structure
- Because the amino and carboxyl groups are attached to the same carbon atom (the alpha carbon), amino acids are called alpha-amino acids.
- This shared structure allows amino acids to form peptide bonds.
Summary
| Functional Group | Formula | Property | Role |
|---|---|---|---|
| Amino Group | -NH2 | Basic | Proton acceptor; forms part of the peptide bond |
| Carboxyl Group | -COOH | Acidic | Proton donor; forms part of the peptide bond |
| R Group | Variable | Varies | Determines the unique properties of each amino acid |
In essence, the amino and carboxyl groups form the backbone of proteins, while the R groups determine the overall structure and function of the protein.
