Biochemistry, Essential Amino Acids

Excerpt

Proteins are made up of 20 amino acids. Each amino acid has an α-carboxyl group, a primary α-amino group, and a side chain called the R group (see Image. Amino Acid Generic Structure). Unlike other amino acids, proline has a secondary amino group. The side chain varies from one amino acid to the other. Nutritionally, amino acids are divided into 3 groups—essential, nonessential, and semi-essential. Semi-essential amino acids are synthesized by the body but are designated essential during periods of stress.

Nine amino acids, including histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine, are classified as essential amino acids because they cannot be synthesized by human or other mammalian cells. Therefore, these amino acids must be supplied from an exogenous diet. Non-essential amino acids not primarily derived from the diet are synthesized by the body. Semi-essential amino acids are growth-promoting amino acids. They are essential in growing children, pregnant women, and lactating women. Amino acids are further classified into 3 groups depending on the structure of the R-group—neutral, acidic, and basic. In addition to serving as the building block of proteins and peptides, amino acids play crucial roles in various important functions.

Some amino acids are converted to carbohydrates and are called glucogenic amino acids. Certain amino acids give rise to specialized products. For example, tyrosine can be converted to hormones, such as thyroid hormones, epinephrine, norepinephrine, and melanin. Methionine, in its active form known as S-adenosylmethionine, plays a critical role in cellular processes by transferring the methyl group to various substances through a process called transmethylation. Cystine and methionine are the primary sources of sulfur.

Besides 20 amino acids that participate in protein synthesis, recently, 2 more new amino acids have been described—selenocysteine and pyrrolysine. Selenocysteine occurs at the active site of several enzymes, including thioredoxin reductase and glutathione peroxidase. Pyrrolysine is not present in humans but is used in the biosynthesis of proteins in some methanogenic species, such as archaea and bacteria.