Chemistry Reference
In-Depth Information
Figure 9.4
Caffeine biosynthesis. XMT, xanthosine N-methyltransferase (also
called 7-methylxanthosine synthase); XN, methylxanthosine nucleotidase; MXMT,
7-methylxanthine-N-methyltransferase (also called theobromine synthase); DXMT,
dimethylxanthine-N-methyltransferase (also called caffeine synthase).
impact of caffeine on human health has been studied extensively. The biosyn-
thetic pathway of caffeine has been elucidated on the genetic level. Caffeine
biosynthesis has been studied most widely in the plant species
Coffea
(coffee)
and
Camellia
(tea) (168, 169).
Xanthosine, which is derived from purine metabolites, is the first committed
intermediate in caffeine biosynthesis (Fig. 9.4). Xanthosine can be formed from
de novo
purine biosynthesis, S-adenosylmethione (SAM) cofactor, the adenylate
pool, and the guanylate pool (169).
De novo
purine biosynthesis and the adenosine
from SAM are believed to be the most important sources of xanthosine (168, 170).
The biosynthesis of caffeine begins with the methylation of xanthosine to
yield N-methylxanthosine by the enzyme xanthosine N-methyltransferase (XMT)
(also called 7-methylxanthosine synthase) (171-173). N-methylxanthosine is
converted to N-methylxanthine by methylxanthine nucleosidase, an enzyme that
has not been cloned yet (174). N-methylxanthine is converted to theobromine
by 7-methylxanthine-N-methyltransferase (MXMT) (also called theobromine
synthase), a second N-methyltransferase (171, 175). Theobromine is converted
to caffeine by a final N-methyltransferase, dimethylxanthine-N-methyltransferase
(DXMT) (also called caffeine synthase) (171).
Coffee and tea plants seem to contain a variety of N-methyltransferase
enzymes that have varying substrate specificity (168, 169). For example,
a caffeine synthase enzyme isolated from tea leaves catalyzes both the
N-methylation of N-methylxanthine and theobromine (176). The substrate
specificity of the methyltransferases can be changed by site-directed mutagenesis
(177), and the crystal structure of two of the N-methyltransferases has been
reported (178).
9.5.2 Metabolic Engineering of Caffeine Biosynthesis
Caffeine may act as a natural insecticide in plants. When the three N-
methyltransferase genes were overexpressed in tobacco, the resulting increase
in caffeine production improved the tolerance of the plants to certain pests
(179). Conversely, coffee beans with low caffeine levels would be valuable
commercially, given the demand for decaffeinated coffee. Because of the
discovery of these N-methyltransferase genes, genetically engineered coffee
