Amino acids are the foundational molecules that combine to form proteins. When the body digests proteins, these amino acids are what remain, ready to be used for numerous processes. Think of them as the letters of an alphabet; just as letters combine to form words, amino acids link together to create a vast array of proteins. Each protein has a specific function determined by its unique sequence of amino acids.
The Chemical Structure of Amino Acids
Every amino acid shares a common structural foundation. This structure consists of a central carbon atom (the alpha-carbon) bonded to four components: an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain called the R-group. The amino group is basic and the carboxyl group is acidic, which contributes to the molecule’s chemical properties.
The identity of each of the 20 common amino acids is determined by its unique R-group. This side chain varies in its chemical composition, size, and electrical charge. For instance, the R-group can make an amino acid nonpolar and hydrophobic, like valine, or polar and hydrophilic, like serine. This variability in the R-group is what dictates the final three-dimensional shape and chemical behavior of a protein.
Essential vs. Non-Essential Amino Acids
Amino acids are categorized based on whether the human body can synthesize them. The primary classifications are essential and non-essential, a distinction directly related to dietary needs, human nutrition, and metabolic capabilities.
Essential amino acids cannot be produced by the body and must be obtained from food. Without a consistent dietary supply, the body cannot build or repair proteins effectively. The nine essential amino acids are:
- Histidine
- Isoleucine
- Leucine
- Lysine
- Methionine
- Phenylalanine
- Threonine
- Tryptophan
- Valine
Conversely, non-essential amino acids can be synthesized by the body and include alanine, asparagine, aspartic acid, and glutamic acid. A third category, conditionally essential amino acids, also exists. These are non-essential but become necessary from the diet during illness or stress when the body cannot produce them in sufficient amounts. Examples include arginine, cysteine, and glutamine.
The Biological Function of Amino Acids
The primary function of amino acids is to serve as the building blocks of proteins. They are linked together in long chains, called polypeptides, through peptide bonds. A peptide bond forms when the carboxyl group of one amino acid connects to the amino group of another. The specific sequence of amino acids is determined by genetic information and dictates how the polypeptide folds into a functional protein.
Some proteins act as enzymes, which speed up biochemical reactions like digestion. Others provide structural support; for example, collagen is a fibrous protein that gives strength to skin and bones. Transport proteins, such as hemoglobin, carry oxygen in the blood.
Beyond protein synthesis, amino acids serve as precursors for other biological molecules. For instance, tryptophan is used to produce the neurotransmitter serotonin, which regulates mood and sleep. Phenylalanine is a precursor for hormones like epinephrine (adrenaline). Amino acids can also be used as a source of energy when necessary.
Dietary Sources and Imbalances
Dietary proteins are described as either “complete” or “incomplete” based on their amino acid composition. Complete proteins contain all nine essential amino acids in adequate amounts. Animal-based foods such as meat, fish, dairy, and eggs are common sources of complete proteins.
Most plant-based foods are incomplete proteins because they are low in one or more of the essential amino acids. It is possible to obtain a full profile from plant sources by combining different foods. A classic example is pairing rice and beans, as together they provide all the essential amino acids.
A diet deficient in one or more essential amino acids limits the body’s ability to synthesize necessary proteins. This is why consuming a varied diet with a mix of protein sources is important. It is not necessary to eat all essential amino acids in one meal, but rather to ensure a sufficient intake of each over the course of a day.