The 20 Standard Amino Acids: The Foundational Building Blocks
Amino acids are organic compounds that serve as the building blocks of proteins, which perform a vast array of critical functions throughout the body. For decades, biochemistry textbooks have highlighted the 20 standard, or proteinogenic, amino acids encoded by the universal genetic code. These 20 amino acids are grouped into three categories based on the body's ability to synthesize them:
- Essential Amino Acids: There are nine essential amino acids that the human body cannot produce on its own. These must be obtained through your diet and include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
- Non-Essential Amino Acids: These are the amino acids the body can synthesize, so they are not essential to consume in food. Examples include alanine, asparagine, aspartic acid, and serine.
- Conditionally Essential Amino Acids: This category includes amino acids that are typically non-essential but become essential under specific circumstances, such as illness, stress, or rapid growth. Cysteine and tyrosine are examples, as they can become essential when their precursor amino acids (methionine and phenylalanine, respectively) are limited.
Selenocysteine: The 21st Genetically Encoded Amino Acid
The concept of only 20 proteinogenic amino acids was challenged with the discovery of selenocysteine (Sec). This amino acid, containing selenium in place of sulfur, is now recognized as the 21st genetically encoded amino acid. Its incorporation into proteins is a highly specific process that involves a special tRNA and a unique mRNA sequence known as a SECIS element, which directs the ribosome to incorporate Sec instead of terminating protein synthesis at what would normally be a stop codon.
The Discovery and Function of Selenocysteine
In the mid-1980s, researchers observed that the gene for the selenoprotein glutathione peroxidase contained an in-frame stop codon (UGA) that directed the incorporation of selenium in the form of selenocysteine. This was a groundbreaking discovery that expanded the known rules of the genetic code. Selenocysteine is an analog of cysteine but with increased reactivity, making it crucial for the function of selenoproteins involved in important biological processes, particularly those related to redox reactions and oxidative stress protection. Humans have at least 25 known selenoproteins, underscoring the physiological importance of this unique amino acid.
The Verdict: Are the 20 or 21 Amino Acids Essential?
The answer to whether 20 or 21 amino acids are essential is nuanced and depends on the context. From a nutritional perspective, there are only nine amino acids considered truly essential for healthy human adults, as they must be obtained from the diet. The concept of 20 or 21 refers to the total number of amino acids the genetic code uses to build proteins, not how many are required in the diet.
The debate over the number of amino acids is rooted in biochemistry, not dietary requirements. While selenocysteine is a genetically encoded amino acid in humans, it is produced from the modification of serine after it is attached to a specific tRNA. This means that we don't need to consume selenocysteine directly; rather, we need to consume enough selenium, an essential trace element, for its synthesis. In this way, selenocysteine is not a dietarily essential amino acid in the same way as histidine or tryptophan.
The Curious Case of Pyrrolysine
To add further complexity, some microorganisms have also been found to incorporate a 22nd genetically encoded amino acid, pyrrolysine (Pyl), which is encoded by the UAG stop codon. However, this is not relevant to human health, as the necessary machinery for its production is not present in humans.
Comparison: Standard Amino Acids vs. Selenocysteine
| Feature | Standard Amino Acids (e.g., Leucine) | Selenocysteine (the 21st Amino Acid) |
|---|---|---|
| Genetic Encoding | Encoded by specific codons (e.g., UUA, UUG for Leucine) found universally in the genetic code. | Encoded by the UGA stop codon, but only in the presence of a special SECIS element on the mRNA. |
| Dietary Requirement | The nine essential amino acids must be consumed in the diet because the body cannot synthesize them. | Not dietarily essential; requires dietary intake of the trace element selenium for its synthesis. |
| Incorporation Method | Directly incorporated into the growing polypeptide chain by a ribosome in a standard process. | Incorporated via a specialized mechanism where a seryl-tRNA is modified to selenocysteyl-tRNA. |
| Function in Proteins | Serves a wide variety of structural, enzymatic, and regulatory roles based on its chemical properties. | Specializes in redox reactions due to its unique chemical properties involving selenium. |
Conclusion: Clarity on the Count
In summary, the distinction between 20 and 21 amino acids revolves around a deep-seated biological mechanism, not dietary needs. The nine essential amino acids are a critical nutritional concern for all humans, but the recognition of selenocysteine as the 21st genetically encoded amino acid is a detail important to molecular biologists and those studying selenium metabolism. For the average person, focusing on obtaining adequate amounts of the nine essential amino acids and the mineral selenium through a balanced diet is what truly matters for health.
