Chemistry Reference
In-Depth Information
5.2 CHEMISTRY
Much chemistry research in plant terpenoids has been to elucidate the structure,
define the biosynthetic pathways, characterize the enzymes involved, and develop
systems for the large-scale production of medicinally or industrially important
terpenoids (4). Progress on the identification and the study of plant terpenoids is
reviewed regularly in the journal
Natural Product Reports
, and the biosynthesis
of terpenoids, including plant terpenoids, has been reviewed comprehensively (5).
Plant terpenoids can be volatile or nonvolatile, lipophilic or hydrophilic, cyclic or
acyclic, chiral or achiral, and they often have double-bond stereochemistry. The
chemical diversity of terpenoid structures originates largely from the terpenoid
synthase enzymes that stabilize different carbocation intermediates, allow rear-
rangements or water termination, and direct stereochemistry; the diversity also
originates from the many different terpenoid-modifying enzymes.
Two major complementary approaches to studying plant terpenoids have been
established. One approach involves the isolation and the structural identifica-
tion of terpenoid chemicals of interest from plant tissues based on traditional
natural products research followed by targeted search for the relevant enzymes
and genes that control biosynthesis. The second approach explores the emerging
plant genome sequences to discover complete sets of genes that encode terpenoid
biosynthetic enzymes. The combination of these two approaches is the most pow-
erful approach to a comprehensive understanding of plant terpenoid chemistry
and its biosynthetic origins.
The diversity of plant terpenoids reflects the complexity and the diversity of the
pathways that biosynthesize them. The recent sequencing of the genomes of four
different plant species and large collections of expressed sequence tags (ESTs)
from many other plants may indicate the diversity of pathways and chemicals we
might expect in any one species. For example, the genes that encode putatively
active terpenoid synthases (TPS) comprise at least 32 in the Arabidopsis (
Ara-
bidopsis thaliana
) genome (6), at least 15 in the rice (
Oryza sativa
) genome (7),
at least 47 in the poplar (
Populus trichocarpa
) genome (8), and at least 89 in the
genome of a highly inbred grapevine (
Vitis vinifera
) Pinot Noir variety (9). The
large gene family of TPS, which is important to generating structural diversity of
terpenoid chemicals in plants, apparently results from repeated gene duplication
and subsequent neofunctionalization or subfunctionalization (10, 11). Most TPS
produce more than one product from a single substrate, and these products are
often modified by the action of additional enzymes such as cytochromes P450
and reductases. Thus, the number of distinct terpenoids found in any one plant
species is predicted to be manifold higher than the number of TPS genes present
in that species. Genomics approaches, which can identify the candidate genes for
terpenoid production, together with functional characterization of heterologously
expressed enzymes and the identification of the resulting plant terpenoids, can
enhance the discovery of the biochemical pathways substantially
in planta
as
has been demonstrated in recent years with research in Arabidopsis (12), rice
(13), and grapevine (14). Ideally, the functional genomics approach is combined
