Canavanine
Encyclopedia
L--(S)-Canavanine is a non-proteinogenic α-amino acid
found in certain leguminous plants. It is structurally related to the proteinogenic
α-amino acid L-arginine
, the sole difference being the replacement of a methylene
group in arginine with an oxa group (i.e., an oxygen
atom) in canavanine. Canavanine is accumulated primarily in the seed
s of the organisms which produce it, where it serves both as a highly deleterious defensive compound against herbivores and a vital source of nitrogen
for the growing embryo (see also L-canaline
). The mechanism of canavanine's toxicity is that organisms that consume it typically mistakenly incorporate it into their own protein
s in place of L-arginine, thereby producing structurally aberrant proteins that may not function properly.
Some specialized herbivores tolerate L-canavanine either because they metabolize it efficiently (cf. L-canaline) or avoid its incorporation into their own nascent proteins. An example of this ability can be found in the tobacco budworm Heliothis
virescens
the larvae of which can tolerate massive amounts of dietary canavanine. These larvae fastidiously avoid incorporation of L-canavanine into their nascent proteins (presumably by virtue of highly discriminatory arginine-tRNA ligase
, the enzyme
responsible for the first step in the incorporation of arginine into proteins). In contrast, larvae of the tobacco hornworm Manduca
sexta can only tolerate tiny amounts (1.0 microgram per kilogram of fresh body weight) of dietary canavanine because their arginine-tRNA
ligase has little, if any, discriminatory capacity. No one has examined experimentally the arginine-tRNA ligase of these organisms. But comparative studies of the incorporation of radiolabeled L-arginine and L-canavanine have shown that in Manduca sexta, the ratio of incorporation is about 3 to 1.
Dioclea
megacarpa seeds contain high levels of canavanine. The beetle Caryedes brasiliensis is able to tolerate this however as it has the most highly discriminatory arginine-tRNA ligase known. In this insect, the level of radiolabeled L-canavanine incorporated into newly synthesized proteins is barely measurable. Moreover, this beetle uses canavanine as a nitrogen source to synthesize its other amino acids to allow it to develop.
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
found in certain leguminous plants. It is structurally related to the proteinogenic
Proteinogenic amino acid
Proteinogenic amino acids are those amino acids that can be found in proteins and require cellular machinery coded for in the genetic code of any organism for their isolated production. There are 22 standard amino acids, but only 21 are found in eukaryotes. Of the 22, 20 are directly encoded by...
α-amino acid L-arginine
Arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
, the sole difference being the replacement of a methylene
Methylene
Methylene is a chemical species in which a carbon atom is bonded to two hydrogen atoms. Three different possibilities present themselves:* the -CH2- substituent group: e.g., dichloromethane ....
group in arginine with an oxa group (i.e., an oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
atom) in canavanine. Canavanine is accumulated primarily in the seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
s of the organisms which produce it, where it serves both as a highly deleterious defensive compound against herbivores and a vital source of nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
for the growing embryo (see also L-canaline
Canaline
L-Canaline ) is a non-protein amino acid. The compound is found in legumes that contain [canavanine], from which it is produced by the action of arginase. The most common-used source for this amino acid is the jack bean, Canavalia ensiformis...
). The mechanism of canavanine's toxicity is that organisms that consume it typically mistakenly incorporate it into their own protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s in place of L-arginine, thereby producing structurally aberrant proteins that may not function properly.
Some specialized herbivores tolerate L-canavanine either because they metabolize it efficiently (cf. L-canaline) or avoid its incorporation into their own nascent proteins. An example of this ability can be found in the tobacco budworm Heliothis
Heliothis
Heliothis is a genus of moths, whose larvae are agricultural pests on crop species such as tobacco, cotton, soybean and pigeon pea. Several of the species formerly placed in this genus are now in the genus Helicoverpa.-Species:...
virescens
Heliothis virescens
The Tobacco Budworm is a moth of the Noctuidae family. It is found in Virginia, Georgia, Florida, Texas, Mexico, Guatemala, Panama, Brazil and the Antilles. The wingspan is 28-35 mm....
the larvae of which can tolerate massive amounts of dietary canavanine. These larvae fastidiously avoid incorporation of L-canavanine into their nascent proteins (presumably by virtue of highly discriminatory arginine-tRNA ligase
Arginine-tRNA ligase
In enzymology, an arginine-tRNA ligase is an enzyme that catalyzes the chemical reactionThe 3 substrates of this enzyme are ATP, L-arginine, and tRNA, whereas its 3 products are AMP, diphosphate, and L-arginyl-tRNA....
, the enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
responsible for the first step in the incorporation of arginine into proteins). In contrast, larvae of the tobacco hornworm Manduca
Manduca
Manduca is a moth genus of the Sphingidae family. The Carolina Sphinx is often used in biological research....
sexta can only tolerate tiny amounts (1.0 microgram per kilogram of fresh body weight) of dietary canavanine because their arginine-tRNA
Transfer RNA
Transfer RNA is an adaptor molecule composed of RNA, typically 73 to 93 nucleotides in length, that is used in biology to bridge the three-letter genetic code in messenger RNA with the twenty-letter code of amino acids in proteins. The role of tRNA as an adaptor is best understood by...
ligase has little, if any, discriminatory capacity. No one has examined experimentally the arginine-tRNA ligase of these organisms. But comparative studies of the incorporation of radiolabeled L-arginine and L-canavanine have shown that in Manduca sexta, the ratio of incorporation is about 3 to 1.
Dioclea
Dioclea
Dioclea is a genus of flowering plants in the pea family, Fabaceae, that is native to the Americas. The seeds of these legumes are buoyant drift seeds, and are dispersed by rivers....
megacarpa seeds contain high levels of canavanine. The beetle Caryedes brasiliensis is able to tolerate this however as it has the most highly discriminatory arginine-tRNA ligase known. In this insect, the level of radiolabeled L-canavanine incorporated into newly synthesized proteins is barely measurable. Moreover, this beetle uses canavanine as a nitrogen source to synthesize its other amino acids to allow it to develop.