Amino acids are the foundational molecules that link together to form proteins, which are responsible for countless tasks within a biological system. The specific function of a protein is intimately tied to its unique three-dimensional structure. The chemical properties of the individual amino acids in a protein chain dictate how it folds into its final, active shape. These properties provide a basis for classifying amino acids into different groups.
Defining Nonpolar Amino Acids
Every amino acid shares a common backbone structure but is distinguished by a unique side chain, also known as an R-group. This side chain determines the amino acid’s specific chemical properties. Polarity refers to the distribution of electrical charge within a molecule; polar molecules have an uneven charge distribution, while nonpolar molecules have a more even distribution. Nonpolar amino acids possess side chains that lack significant electrical charge.
These side chains are primarily composed of carbon and hydrogen atoms, forming hydrocarbon groups. Because carbon and hydrogen share electrons relatively evenly, these side chains do not readily form hydrogen bonds with water. This characteristic makes them hydrophobic, or “water-fearing,” causing them to avoid contact with water in an aqueous environment like a cell.
There are nine common nonpolar amino acids:
- Glycine
- Alanine
- Valine
- Leucine
- Isoleucine
- Methionine
- Phenylalanine
- Tryptophan
- Proline
The side chains of Alanine, Valine, Leucine, and Isoleucine are aliphatic, consisting of simple hydrocarbon chains that increase in size. Phenylalanine and Tryptophan have bulky aromatic rings, while Methionine contains a sulfur atom within its hydrocarbon chain. Proline is unique because its side chain loops back to connect with the amino acid’s backbone, creating a rigid ring structure.
The Hydrophobic Effect and Protein Folding
The tendency of nonpolar amino acids to avoid water is a primary driver of protein folding, a process known as the hydrophobic effect. Within the watery environment of the cell, a newly synthesized, linear chain of amino acids must fold into a stable, three-dimensional structure to become functional.
To minimize their exposure to water, the hydrophobic side chains of nonpolar amino acids cluster together. This inward aggregation forces the protein chain to fold, burying the nonpolar residues away from the aqueous solvent. This process is a directed, energetically favorable collapse.
This clustering results in the formation of a tightly packed hydrophobic core in the protein’s interior, composed almost exclusively of the nonpolar amino acids. The stability of this core, maintained by weak van der Waals interactions between the packed side chains, is a significant factor in maintaining the protein’s folded architecture.
Function in Biological Structures
The hydrophobic nature of nonpolar amino acids is fundamental to the function of many biological structures, particularly those that interact with nonpolar environments. A primary example is their role in transmembrane proteins, which are embedded within cell membranes. Cell membranes are composed of a lipid bilayer, a structure whose interior is made of nonpolar fatty acid tails.
For a protein to reside within or pass through this membrane, it must have segments that can comfortably exist in this oily environment. Transmembrane proteins possess stretches of nonpolar amino acids, often arranged in an alpha-helical structure, that span the membrane. These hydrophobic regions anchor the protein securely within the lipid bilayer, allowing it to perform functions like transporting molecules or receiving signals.
Beyond membrane anchoring, nonpolar amino acids facilitate interactions between different proteins. The surfaces of proteins can have hydrophobic patches where nonpolar residues are exposed. Two proteins with complementary hydrophobic patches can bind to each other, an interaction important for the assembly of larger, multi-protein complexes.
Dietary Significance and Synthesis
The human body can produce some amino acids, while others must be acquired from food. Amino acids the body can synthesize are termed non-essential, and those that must be obtained from the diet are essential. This classification is based on synthetic capability, not importance, as all 20 are required for health.
Among the nine nonpolar amino acids, six are essential: Valine, Leucine, Isoleucine, Phenylalanine, Tryptophan, and Methionine. The remaining three—Alanine, Glycine, and Proline—are non-essential, as the body can synthesize them from other molecules.
To meet the dietary requirement for essential nonpolar amino acids, a variety of foods can be consumed. Complete protein sources, which contain all nine essential amino acids, include animal products like meat, eggs, and dairy, as well as plant-based options like soy and quinoa. Other plant sources like legumes, nuts, and grains can be combined to provide all essential amino acids.