Capsaicin;
8-methyl-N-vanillyl-6-nonenamide) is an active component of chili peppers,
which are plants belonging to the genus Capsicum. It is an irritant for
mammals, including humans, and produces a sensation of burning in any tissue
with which it comes into contact. Capsaicin and several related compounds are
called capsaicinoids and are produced as secondary metabolites by chili
peppers, probably as deterrents against certain mammals and fungi. Pure
capsaicin is a non-volatile, hydrophobic, colorless, highly pungent, crystalline
to waxy compound.
Biosynthetic pathway
Plants exclusively of the Capsicum genus
produce capsaicinoids, which are alkaloids. Capsaicin is believed to be
synthesized in the interlocular septum of chili peppers and depends on the gene
AT3, which resides at the pun1 locus, and which encodes a putative
acyltransferase.
Biosynthesis of the capsaicinoids occurs
in the glands of the pepper fruit where capsaicin synthase condenses
vanillylamine from the phenylpropanoid pathway with an acyl-CoA moiety produced
by the branched-chain fatty acid pathway.
Capsaicin is the most abundant
capsaicinoid found in the Capsicum genus, but at least ten other capsaicinoid
variants exist.[31] Phenylalanine supplies the precursor to the phenylpropanoid
pathway while leucine or valine provide the precursor for the branched-chain
fatty acid pathway.[27][28] To produce capsaicin, 8-methyl-6-nonenoyl-CoA is
produced by the branched-chain fatty acid pathway and condensed with
vanillamine. Other capsaicinoids are produced by the condensation of
vanillamine with various acyl-CoA products from the branched-chain fatty acid
pathway, which is capable of producing a variety of acyl-CoA moieties of
different chain length and degrees of unsaturation. All condensation reactions
between the products of the phenylpropanoid and branched-chain fatty acid
pathway are mediated by capsaicin synthase to produce the final capsacinoid
product.
Natural function
Capsaicin is present in large quantities
in the placental tissue (which holds the seeds), the internal membranes and, to
a lesser extent, the other fleshy parts of the fruits of plants in the genus
Capsicum. The seeds themselves do not produce any capsaicin, although the
highest concentration of capsaicin can be found in the white pith of the inner
wall, where the seeds are attached.
The seeds of Capsicum plants are dispersed
predominantly by birds: in birds, the TRPV1 channel does not respond to
capsaicin or related chemicals (avian vs. mammalian TRPV1 show functional
diversity and selective sensitivity). This is advantageous to the plant, as
chili pepper seeds consumed by birds pass through the digestive tract and can
germinate later, whereas mammals have molar teeth which destroy such seeds and
prevent them from germinating. Thus, natural selection may have led to
increasing capsaicin production because it makes the plant less likely to be eaten
by animals that do not help it disperse. There is also evidence that capsaicin
may have evolved as an anti-fungal agent: the fungal pathogen Fusarium, which
is known to infect wild chilies and thereby reduce seed viability, is deterred
by capsaicin, which thus limits this form of predispersal seed mortality.
In 2006, it was discovered that the venom of a certain tarantula species
activates the same pathway of pain as is activated by capsaicin; this was the
first demonstrated case of such a shared pathway in both plant and animal
anti-mammal defense.
