Nonstandard amino acids

Nonstandard amino acids refer to those amino acids that have been chemically modified after they have been incorporated into a protein (called a “posttranslational modification”) and those amino acids that occur in living organisms but are not found in proteins. Among these modified amino acids is γ-carboxyglutamic acid, a calcium-binding amino acid residue found in the blood-clotting protein prothrombin (as well as in other proteins that bind calcium as part of their biological function). The most abundant protein by mass in vertebrates is collagen. Significant proportions of the amino acids in collagen are modified forms of proline and lysine: 4-hydroxyproline and 5-hydroxylysine.

Arguably, the most important posttranslational modification of amino acids in eukaryotic organisms (including humans) is the reversible addition of a phosphate molecule to the hydroxyl portion of the R groups of serine, threonine, and tyrosine. This event is known as phosphorylation and is used to regulate the activity of proteins in their minute-to-minute functioning in the cell. Serine is the most commonly phosphorylated residue in proteins, threonine is second, and tyrosine is third.

Proteins with carbohydrates (sugars) covalently attached to them are called glycoproteins. Glycoproteins are widely distributed in nature and provide the spectrum of functions already discussed for unmodified proteins. The sugar groups in glycoproteins are attached to amino acids through either oxygen (O-linked sugars) or nitrogen atoms (N-linked sugars) in the amino acid residues. The O-linked sugars are attached to proteins through the oxygen atoms in serine, threonine, hydroxylysine, or hydroxylproline residues. The N-linked sugars are attached to proteins through the nitrogen atom in asparagine.

Finally, there is the case of selenocysteine. Although it is part of only a few known proteins, there is a sound scientific reason to consider this the 21st amino acid because it is in fact introduced during protein biosynthesis rather than created by a posttranslational modification. Selenocysteine is actually derived from the amino acid serine (in a very complicated fashion), and it contains selenium instead of the sulfur of cysteine.

Analysis of amino acid mixtures

The modern biochemist has a wide array of methods available for the separation and analysis of amino acids and proteins. These methods exploit the chemical differences of amino acids and in particular their ionization and solubility behaviour.

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metabolism: Amino acids

A typical determination of the amino acid composition of proteins involves three basic steps:

1. Hydrolysis of the protein to its constituent amino acids.
2. Separation of the amino acids in the mixture.
3. Quantification of the individual amino acids.

Hydrolysis is accomplished by treatment of a purified protein with a concentrated acid solution (6N HCl) at a very high temperature (usually 110 °C [230 °F]) for up to 70 hours. These conditions cleave the peptide bond between each and every amino acid residue.

The hydrolyzed protein sample is then separated into its constituent amino acids. Methods important for amino acid separations include ion exchange chromatography, gas chromatography, high-performance liquid chromatography, and most recently, capillary zone electrophoresis.

The sensitivity of the analysis of separated amino acids has been greatly improved by the use of fluorescent molecules that are attached to the amino acids, followed by their subsequent detection using fluorescence spectroscopy. For example, amino acids may be chemically “tagged” with a small fluorescent molecule (such as o-phthalaldehyde). These approaches routinely allow as little as a picomole (10⁻¹² mole) of an amino acid to be detected. Most recently, this range of sensitivity has been extended to the attomole (10⁻¹⁸ mole) range.

Some common uses

The industrial production of amino acids is an important worldwide business. The first report of the commercial production of an amino acid was in 1908. It was then that the flavouring agent monosodium glutamate (MSG) was prepared from a type of large seaweed. This led to the commercial production of MSG, which is now produced using a bacterial fermentation process with starch and molasses as carbon sources. Glycine, cysteine, and d,l-alanine are also used as food additives, and mixtures of amino acids serve as flavour enhancers in the food industry. The amino acid balance of soy or corn protein for animal feed is significantly enhanced upon the addition of the nutritionally limiting amino acids methionine and lysine.

Amino acids are used therapeutically for nutritional and pharmaceutical purposes. For example, patients are often infused with amino acids to supply these nutrients before and after surgical procedures. Treatments with single amino acids are part of the medical approach to control certain disease states. Examples include l-dihydroxyphenylalanine (l-dopa) for Parkinson disease; glutamine and histidine to treat peptic ulcers; and arginine, citrulline, and ornithine to treat liver diseases.

Certain derivations of amino acids, especially of glutamate, are used as surfactants in mild soaps and shampoos. d-Phenylglycine and d-hydroxyphenylglycine are intermediates used for the chemical synthesis of β-lactam antibiotics (e.g., synthetic versions of penicillin). Aspartame is a sweetener prepared from the individual component amino acids aspartic acid and phenylalanine.