全 文 ：植物环肽及其可能的生物合成途径?
杜良成1 * * , 谭宁华2
??
, 许文彦1 ,2 , 娄丽丽1
(1 内布拉斯加州立大学林肯分校化学系 , 林肯 内布拉斯加 68588; 2 中国科学院昆明植物研究所
植物化学与西部植物资源持续利用国家重点实验室 , 云南 昆明 650204 )
摘要 : 植物环肽是一个庞大的小分子天然产物家族 , 通常由 4～10 个氨基酸残基组合而成。该类化合物广
泛存在于全球多种植物的根、茎、枝、叶及种子中 , 中草药中也时有发现。由于对其生物合成途径及机理
研究较少 , 环肽分子的利用价值尚未得到有效的开发。和常见的非环状基因编码的多肽或蛋白质相比 , 环
肽结构更为复杂。本文将对植物环肽的生物合成途径及其机理做初步探讨。
关键词 : 植物环肽 ; 生物合成 ; 内生菌 ; 非核糖体环肽
中图分类号 : Q 946 文献标识码 : A 文章编号 : 0253 - 2700 (2009) 04 - 374 - 09
Plant Cyclopeptides and Possible Biosynthetic Mechanisms
DU Liang-Cheng1 * * , TAN Ning-Hua2 * * , XU Wen-Yan1 , 2 , LOU Li-Li1
(1 Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588 , USA ; 2 State Key
Laboratory of Phytochemistry and Plant Resources in West China, Kunming Instituteof Botany,
Chinese Academy of Sciences, Kunming 650204 , China)
Abstract : Plant cyclopeptides area largegroupof small molecular weight natural products, typicallywith 4 - 10 amino ac-
ids, isolated from the leaves, stem barks, roots, and seeds of a wide variety of plant species throughout the world . The
peptides arepresent inmanyChinesemedicinal plants, andtheir potentials havenot beenwell exploitedbecauseof the lack
of knowledge in their biosynthetic originandmechanism . The cyclopeptides often havecomplex chemical structures distinct
from common polypeptides ( proteins) that are non-cyclic and gene-coded . This reviewdiscussed the potential origin of the
cyclopeptides and their possible biosynthetic mechanisms .
Key words: Plant Cyclopeptide; Biosynthesis; Endophyte; Nonribosomal peptide
Plant cyclopeptides represent a new, largely un-
tapped source for human medicines . Many of the natu-
ral products have fascinating structures and potent bio-
logical activities, such as sedative sanjoinine-A ( Han
et al. , 1989 ) , immunosuppressive cycloleonurinin
(Morita et al. , 1997) , antitumor RA-VII ( Itokawa et
al. , 1983; Hitotsuyanagi et al. , 2004 ) and uterotonic
kalataB1 ( Saether et al. , 1995 ) . These are a large
group of natural products and classified into eight types
based on their structures and distributions in plants
(Tan and Zhou, 2006) . However, none of the cyclo-
peptides, except the Type-VIII ( cyclotides) that are
gene-coded products, have been studied for their bio-
synthetic mechanisms . An understanding of the biosyn-
thetic mechanismfor the bioactive compounds is an es-
sential step toward utilization of the natural products as
human medicines . This is because thebiosynthesis is a
feasible approach for utilization of these products .
云 南 植 物 研 究 2009 , 31 (4) : 374～382
Acta Botanica Yunnanica DOI : 10 .3724?SP. J . 1143 .2009.09093
?
?? ?Author for correspondences; E-mail : ldu@ unlserve. unl . edu; nhtan@ mail . kib. ac. cn
Received date: 2009 - 05 - 11 , Accepted date: 2009 - 05 - 15
作者简介 : 杜良成 (1965 - ) 男 , 副教授 , 主要从事生物合成研究。 ?
Foun ?dation items: Theresearch in theDu lab has been supported by NSF (MCB-0614916) , NIH (AI073510) , Nebraska Research Initiatives and NSFC
(No . 30428023) . The research in the Tan lab has been supported by NSFC (30725048 , China-AustraliaSpecial Fund for S & T Cooperation)
Many of the cyclopeptides have complex chemical struc-
tures that precludethe chemical total synthesis approach
from commercial application . Their bioavailability is
generally low that makes it unsustainable for large scale
extraction of the products directly from plants . The
knowledge of the biosynthetic mechanismwill help de-
sign new and rational approaches to increase and im-
prove the yield of the compounds in the native hosts or
to produce the compounds in user-friendly heterologous
hosts throughgenetic engineering . Thebiosynthetic ap-
proachmay also lead to a lower toxicity of some of the
compounds, such as the potent antitumor RA-VII ,
which was in phase II clinical trial but droppedoff due
to the toxicity .
Another need of studying the biosynthetic mecha-
nism for plant cyclopeptides comes from the fact that
many of the cyclopeptides are isolated from traditional
Chinese herb medicines . There are more than 5 , 000
plants and plant products in Chinese herb pharmaco-
poeia, and many of the plants have been used to treat
various diseases for thousands of years without knowing
the exact naturesof theactive factors (Bensky, 1993) .
The understanding of their biosynthesis would provide
new molecular insights into the understanding of the
pharmacological andmedical theories of traditional Chi-
nese medicines . This in turn will help develop safer
and more defined use of the medicines to treat human
diseases .
1 Small Molecular Weight Peptides
Small molecular weight peptides can be classified
into two groups according to their biosynthetic mecha-
nisms, the ribosomal peptides ( RPs) and nonribosomal
peptides (NRPs) . RAs are synthesized by translation
of mRNA on the ribosome . These peptides are derived
from gene-encoded precursors ( pre-peptides) that un-
dergo posttranslational modifications, including proteol-
ysis, dehydration, and cyclization . For example, the
antibiotic microcin MccB17 produced by various E . coli
strains is derived fromthe69-aaMccB17 precursor Mc-
bA (encoded by mcbA) (Li et al. , 1996; Milne et al
. , 1999 ) . Three genes, mcbB, mcbC , and mcbD,
encode the three components of the MccB17 synthetase
required for posttranslational modifications of McbA to
produce the 43-residue final product, MccB17 . The
modifications consist of the removal of the 26-residue
leader peptide and the formation of eight heterocycles,
four oxazole rings and four thiazole rings . Theoxazoles
and thiazoles are formed via cyclization-dehydration-ox-
idationof serine residues and cysteine residues, res-
pectively, with adjacent residues . Extensive posttrans-
lational modifications are also found in the biosynthesis
of lantibiotics, such as lacticin 481 that is matured
from a 51-aa prepeptide (LctA ) through removal of the
24-aa leader peptide, dehydration of serines and thre-
onines, and intramolecular cyclic thioether formation
(Xie et al. , 2004; Xie and van der Donk, 2004 ) .
Amazingly, this process is catalyzed by one single en-
zyme, LctM . Because of the gene-encoded nature,
RPs are composedonly fromthe 21 proteinogenic amino
acids .
NRPs are assembled by specific enzymes, inde-
pendent of the ribosome . For example, glutathione, an
important antioxidant in cells, is a linear tripeptide
synthesized by two ATP-dependent enzymes, γ-glu-
tamylcysteine synthetase ( glutamate cysteine ligase)
and glutathionesynthetase . Glutamate cysteine ligase is
a heterodimeric enzyme consisting of a catalytic and
modulatory subunit . The most actively studied NRPs
are a vast array of small peptides isolated from mi-
crobes . They are assembled on enzyme complexes
called nonribosomal peptide synthetases ( NRPS ) ,
which are modular enzymes composed from a series of
functional units ( domains) (Marahiel et al. , 1997 ) .
These peptides are highly diverse in their chemical
structures and biological activities . More than 300 pre-
cursors are known to serve as building blocks for these
NRPs .Theseincludemany nonprotenogenic amino acids
and modified proteinogenic amino acids . Themodifica-
tions include hydroxylation, acylation, N-methylation,
glycosylation, halogenation, and epimerzation (D-ami-
no acids) . The huge precursor pool contributes to the
vast structural diversity of these peptides, including li-
popeptides, depsipeptides, and peptidolactones that
can be linear, cyclic, or cyclic branched . Interesting-
ly, many plant cyclopeptides alsoshare these structural
5734 期 DU Liang-Cheng et al. : Plant Cyclopeptides and Possible Biosynthetic Mechanisms
features, which are hallmarks for the nonribosomally
synthesized peptides . Thepeptides possess an extreme-
ly broad rangeof biological activities andpharmacologi-
cal properties, including antibiotics ( such as penicil-
lin, vancomycin and daptomycin) , anticancer agents
(such as epothilone and bleomycin) , immunosuppres-
sants (such as cyclosporine A) , siderophores (such as
entorobactin and myxochelin A ) , and toxins (such as
microcystins) .
2 Plant Cyclopeptides
Plant cyclopeptides are a largegroup of small mo-
lecular weight natural products, typically containing 4
- 10 amino acid residues . They havebeen isolated from
the leaves, stembarks, roots, and seedsof awideva-
riety of plant species throughout the world . Tan and
Zhou reviewed 455 plant cyclopeptides that were isolat-
ed from 120 species, spreading in 65 different genera
of 26 plant families (Tan and Zhou, 2006) . The pep-
tides exhibit numerous biological activities, including
anticancer, antibacterial , antifungal , anti-HIV , anti-
malarial , and sedative effects . These plant cyclopep-
tides are grouped into eight types ( I to VIII ) based on
their chemical structures and distributions ( Fig. 1 )
(Tan and Zhou, 2006 ) . Type VIII ( cyclotides) is a
distinct group with a chain (usually 30 residues) much
longer than the rest of cyclopeptides . This group is also
thebest understood plant cycloptides in terms of their
biosyntheticmechanism . Similar to microcins and lanti-
biotics from bacteria, cyclotides aregene-products syn-
thesized via the ribosomal mechanism ( Jennings et al. ,
2001) . For example, the prototypic kalata B1 is a 29-
residue cyclopeptide isolated from Oldenlandia affinis .
It is derived from a 124-residue precursor protein that
is encoded by the Oak1 gene . The precursor protein
consists of a 20-residue endoplasmic reticulum signal
sequence, a 65-residue non-conserved pro-region, a
highly conserved 3-residue region known as the N-ter-
minal repeat, the 29-residue cyclotidedomain and a 7-
residue hydrophobic C-terminal tail . The final mature
kalata B1 is formed through a series of posttranslational
proteolysis and disulfide bond formation, which forms
the three characteristic interlocked disulfidebonds, the
“cyclic cystine knot”. The six cysteine residues that
formthe knots are absolutely conserved throughout the
cyclotides .
So far, the Type VIII cyclopeptides ( cyclotides)
are the only group whose biosynthetic mechanism has
been studied . Noneof theother seven types have been
studied for their biosynthetic mechanism ( note that the
term“cyclopeptides”refers to Type I-VII and is the
target of the rest discussion in the article) . Structural-
ly, these cyclopeptides aremoresimilar to typical non-
ribosomal peptides found in microorganisms . For exam-
ple, at least 34 amino acids are found in these cyclo-
peptide alkaloids (Type I ) , many of themare nonpro-
teinogenic amino acids or modified proteinogenic amino
acids, such asβ-hydroxyl , N-methyl , N-aldehyde,
N, N-dimethyl , N-aldehyde- N-methyl , N-oxo- N, N-
dimethyl (N→O) amino acids . An especially notable
and frequently occurring type of amino acids is the so-
called“ringbond amino acids”, which are linked to the
rings of other amino acids or moieties by bonds other
than an amidebond, such as the ether, carbon-carbon,
or carbon-nitrogen bond (Fig. 1) . In addition, the cy-
clopeptides are generally smaller than 10 amino acid
residues . These structural features make them distinct
from cyclotides and more closely related to NRPs .
3 Cyclopeptides Isolated from Microbial
Endophytes of Plants
Before discussing the biosynthetic mechanism for
plant cyclopeptides, it is important to briefly mention a
groupof cyclopetides that are isolated from microorga-
nisms living within plants, the so-called endophytes .
These compounds could be regarded as a link between
the biosynthetically well-known, microbe-originated
NRPs and the plant cyclopeptides, for which the bio-
synthetic mechanism and origin remain unclear . Al-
though the plant cyclopeptides are isolated from plant
materials, there are three possiblebiosynthetic origins,
the plants, potential endophytic microbes, and both .
Several dozens of plants’genomes havebeen sequenced
or are being sequenced . Within the two genomes com-
pleted ( Arabidopsis thaliana and Oryza sativa Japonica
group) , there is nosignof presenceof NRPS-typegenes
673 云 南 植 物 研 究 31 卷
Fig . 1 Chemical structures of plant cyclopeptides . One representative compound from each of the seven groups (Type I to VII ) is shown .
The NRPs-like, unusual structural features are highlighted by colors, and the activities and occurrences of the peptides are also indicated
( Blastp search by Du, unpublished observation ) .
While it is not known if thesetwo plantsproducecyclo-
peptides, it is clear that these plants do not contain
such giant, multi-domain enzyme complexes as NRPS
found inmicrobes . Although it is possible that different
groups of cyclopeptides may have different biosynthetic
mechanisms and origins, the structural features suggest
that at least some of the plant cyclopeptides might be
originated from endophytic microbes using the NRPS
mechanism .
Numerous natural products have been isolated
from endophytes (Strobel , 2002; Ge and Tan, 2009;
Zhang et al. , 2006 ) , including cyclopeptides of plant
endophytes ( Fig. 2 ) . Cryptocandin A, a potent anti-
mycotic, was isolated from Cryptosporiopsis quercina,
which is an endophytic fungus of the medicinal plant
Tripterygium wilfordii ( Strobel et al. , 1999 ) . This
compound contains several unusual hydroxylated amino
acids and a novel amino acid, 3-hydroxy-4-hydroxy-
methyl proline . Pseudomycin A is oneof the antifungal
lipodepsipeptides produced by Pseudomonas syringae
MSU 16H, a plant-associated pseudomonas ( Ballio et
al. , 1994 ) . The compound contains several unusual
amino acids, including chlorothreonine, hydroxyaspar-
tic acid, and diaminobutryic acid . Epichlicin, another
potent antifungal cyclopeptide, was isolated from Epi-
chloe typhina, an endophytic fungus of the timothy
plant, Phleumpretense (Seto et al. , 2007 ) . Thepep-
tide contains an unusualβ-amino acid, 3-amino tetra-
decanoic acid . Inturin A2 , A3 and A6 , with a struc-
ture and activities similar to epichlicin, were isolated
from Acinetobacter baumannii , an unusual endophytic
bacterium of the medicinal plant Cinnamomum cam-
phora (Liu et al. , 2007) . Besides, anumber of struc-
turally not-well-resolved cyclopeptides have also been
isolated from plant endophytes . For example, munum-
7734 期 DU Liang-Cheng et al. : Plant Cyclopeptides and Possible Biosynthetic Mechanisms
Fig . 2 Chemical structures of cyclopetides isolated from endophytic microorganisms of plants . The unusual structural features are high-
lighted by colors . All the compoundsexhibit antifungal activities . Cryptocandin A (9 ) was isolated from Cryptosporiopsisquercina, which is
an endophytic fungus of Tripterygiumwilfordii , a medicinal plant native to Eurasia ( Strobel et al. , 1999) . Pseudomycin A ( 10) was iso-
lated from Pseudomonas syringaeMSU 16H that isassociatedwith plants (Ballio et al. , 1994 ) . Epichlicin (11) was isolated from Epichloe
typhina, which is an endophytic fungus of plant Phleumpretense ( Seto et al., 2007 ) . Intruin A2 (12 ) was isolated from Acinetobacter
baumannii LCH001 , which is an endophytic bacterium of plant Cinnamomumcamphora (Liu et al. , 2007 ) .
bicins, broad spectrumpeptide antibiotics, were isola-
ted from Streptomyces sp . strain NRRL 30562, which
is a novel endophyte of snakevine, Kennedia nig-
riscans, a bush medicine in Australia (Castillo et al. ,
2002 ) . Coronamycins, another complex of peptides
with antifungal activities, were isolated fromaverticil-
late Streptomyces sp ., which is an endophyte from an
epiphytic vine, Monstera sp (Ezra et al. , 2004) . These
prior examples suggest that endophytes are alikely source
for many of the cyclopeptides isolated from plants .
Inmarinenatural products, it has been recognized
that some compoundsoriginally isolated frommarine in-
vertebrates are actually synthesized by microbes associ-
ated with the invertebrates ( Schmidt, 2005 , 2008 ) .
Someplant terpenoids and polyketides that were origi-
nally isolated fromtheplants turned out to beof micro-
bial origins . For example, maytansinoids, the 19-
membered macrocyclic lactams, were initially isolated
from the tropic plant Mallotus nudiflorus . Recent evi-
dence suggests that the corestructureof maytansinoids,
which is related to ansamycin antibiotics of microbial
origin, is synthesized by endophytic microbes (Yu et
al. , 2002) . Recently, it was shown that anovel fungal
endophyte isolated fromthe inner bark of the medicinal
plant Camptotheca acuminata was able to produce
camptothecin and analogs through fermentation (Kusari
873 云 南 植 物 研 究 31 卷
et al. , 2009 ) . Camptothecin is a potent antineoplastic
agent, originally isolated fromtheplant . Interestingly,
theyield of the compounds sharply decreased over se-
ven successivesubculturegenerations of the endophyte,
suggesting that the production of these metabolites
probably requires unknown factorsof the host plant ori-
gin . Very little is known about the interactions between
endophytes and their hosts . The understanding the in-
teractions is clearly important in order to use endo-
phytes as alternate sources of plant secondary metabo-
lite production .
4 Heterophyllin B and RA-VII—Two Prototypical
Plant Cyclopeptides
For thepurposeof illustrating the possible biosyn-
thetic mechanism, we choose to focus on two prototypi-
cal plant cyclopeptides, heterophyllin B ( 6) and RA-
VII ( 8 ) , in the following sections . Heterophyllins
(Type VI ) are isolated from the Chinese medicinal
plant, Tai-Zi-Shen ( Pseudostellaria heterophylla) ( Tan
et al. , 1993; Morita et al. , 1994) , which is used to
treat palpitation, sweating, fatigue, cough, and lossof
appetite (Fig. 1) . At least 12 cyclopeptides, most with
inhibitory activities against tyrosinase or melanogene-
sis, havebeen isolated fromthis plant . HeterophyllinB
(HB) is the best known one and has served as amodel
cyclopeptide in many studies (Tan and Zhou, 2006 ) .
Besides, HB is theonly plant cyclopeptidefor which an
in vitro enzymatic assayand a tissueculturewith theHB
biosynthetic activity have been established ( J ia et al. ,
2006) . Structurally, HB contains three sequential pro-
line residues, which is an unusual feature in NRPs . An
understandingof HB biosyntheticmechanismwill set the
foundation to exploit this group of cyclopeptides in Tai -
Zi-Shen as well as other type cyclopeptides .
The RAs are a group of bicyclic hexapeptides
(Type VII ) isolated from several Rubia species in
1980s . Among them, R. yunnanensis is a traditional
Chinese medicinal plant (Xiao-Hong-Shen) ( Fig. 1 ) ,
which is used to treat anemia, injury, rheumatism,
gastritis, lipoma, and menoxenia . At least 18 cyclo-
peptides ( RA-I through RA-XVIII ) have been identi-
fied . Most RAs have the same amino acid sequence
and differ only in the numbers of modification groups
(methyl and hydroxyl ) . The bioavailability of RAs is
extremely low, about 0 .001 to 0 .00001% of dry pla-
nts . Themost recent member of this family is RA-XVI-
II , which was isolated from the dried roots of R. cor-
difolia ( Lee et al. , 2008 ) . From 55 kg dried roots,
4 . 8 mg RA-XVIII was obtained through a series of ex-
traction and separation . TheRAs areprobably themost
interesting plant cyclopeptides in terms of their biologi-
cal activities and structural features . They all exhibit
very potent anticancer activities ( IC50 at nM level) and
have distinctive structural features . The most interes-
ting structural feature is the unusually strained 14-
membered cycloisodityrosine that was proposed to be
the pharmacophore for its activities ( Boger et al. ,
1991; Boger and Yohannes, 1993; Boger and Zhou,
1995) . The anticancer activities wereobserved with P-
388 leukemia, ascites tumors, L1210 , B-16 melanoma
and solid tumors, colon 38 , Lewis lung carcinoma,
and Ehrlich carcinoma ( Itokawa, 1984) . The effective
dose ranges ( five days of i . p . administration) were
0 .01 - 4 .0 mg?kg . Among them, RA-V ( deoxybouvar-
din, 7) was especially potent on MM2 mammary carci-
noma in mice ( two of six mice given 5 mg?kg and one
of seven micegiven 10 mg?kg recovered) . RA-VII (8)
was in phase II clinical trials in Japan in the 1980s
( Itokawa, 1984; Inoue, 1986 ) . It exhibited almost
the same chemosensitivity compared tothat of fivestan-
dard anticancer drugs ( adriamycin, mitomycin C, cis-
platin, vinblastine and 5-FU) . However, RA-VII also
caused nausea and vomiting, fever, stomachache, mild
hypotension and slight abnormality of electric-cardio-
gram . The modeof action is believed through interact-
ing with eukaryotic 80S ribosomes to inhibit protein
synthesis ( SirDeshpande and Toogood, 1995 ) . Re-
cently, it was shown to cause conformational changesof
F-actin and stabilization of actin filaments to induced
G2 arrest ( Fujiwara et al. , 2004 ) .
5 Biosynthetic Genes and Possible Nonribo-
somal Biosynthetic Mechanism for Plant Cy-
clopeptides
None of theplant cyclopeptides have been studied
9734 期 DU Liang-Cheng et al. : Plant Cyclopeptides and Possible Biosynthetic Mechanisms
for their biosynthetic genes and enzymes . However, bio-
synthetic genes for peptides?polyketides have been
cloned from endophytes of insects and marine animals
(Schmidt, 2008; Piel , 2006 ) . Pederin was the first
natural product from an endophyte whose biosynthetic
gene cluster has been sequenced ( Piel , 2002 ) . It has
to be emphesized that the cyclopeptides exemplified in
Fig. 2 were isolated from plant endophytes that are
“culturable”. Their identification followed the common
routeof identifying“ interesting plants”, isolating and
identifying microbes fromthe plants, culturing the mi-
crobes and isolating bioactive natural products . Thus,
this approach can only identify those products made by
culturable endophytes and would miss the“uncultu-
rable”endophytes . Furthermore, even within the cul-
turable endophytes, the approach could also miss those
products that are not synthesized under theculture con-
ditions . Thus, the uncertainty always exists between
what an endophyte produces in cultures and what it
may produce in nature . Oneway to circumvent the di-
sadvantages is to use the metagenomic approach (Wang
et al. , 2008) , which has been successfully used to ex-
ploit biosynthetic genes from marine sponges, insects,
and human microbiota ( microbiome) . This approach
has recently been exploited in plant endophytes by en-
riching the microbiota from plants ( Wang et al. ,
2008) .
Below, we will use RA-VII as an example to pro-
pose a possible NRPS-catalyzed biosynthetic pathway
(Fig. 3 ) . This proposal is based on the general para-
digm established for microbial cyclopeptides and the
structural features of RA-VII . The biosynthesis follows
the“clockwise”cycleof the cyclopeptide . L-Tyrosine-
1 is proposed to be the starter . This is mainly for the
convenience of discussion, because the biosynthesis
could start from other amino acids and is only be sure
by experimental determination . The putative RA-VII
synthetase could consist of the starter module ( activat-
ing, thiolating and N-methyl transferring to L-Tyr-1)
and fiveelongationmodules ( incorporating D-Ala-2 , L-
Ala-3 , L-Tyr-4 , L-Ala-5 , and L-Tyr-6 ) . Among
them, module-2 would contain an epimerase domain to
convert L-Ala to D-Ala, and module-1 , 4 and 6 would
contain a methyltransferase domain to add a methyl
group to the amide nitrogen . The physical distribution
of themodules could not be predicated, as they could
beonmultiple proteins or on asingleprotein . Thebio-
synthesis would be terminated by the nucleophilic at-
tack of L-Tyr-1 nitrogen on the carbonyl of L-Tyr-6 to
produce a cyclohexapeptide precursor . This could be
catalyzed by a thioesterase domain located at the endof
the synthetases . The cyclized precursor would be fur-
ther processed to become a mature cyclopeptide . This
includes the O-couplingof the two phenyl rings of Tyr-
1 and Tyr-6 . This is probably the most interesting step
in the biosynthesisof RA-VII , as it leads to the forma-
tion of the unusually strained 14-membered cycloisodi-
tyrosine unit, which is important for its anticancer ac-
tivity . This typeof phenolic O-coupling is known from
lignins and could be readily rationalized through oxida-
tive radical couplingmechanism . The reaction could be
catalyzed by a peroxidase or a cytochrome P450-type
enzyme . In addition, the cyclized precursor would
require two O-methylations at the phenolic oxygen of
Tyr-1 and Tyr-4 . This type of methylation is usually
catalyzed by a separate methyltransferase, rather than
the MT domain of the NRPS .
6 Final Remarks
Finding alternative drug sources is obviously an
important goal in light of the constant occurrence of
multi-drug resistant pathogens and tumor cells . This is
especially significant for multi-drug resistant pathogens
and cancers . Unlikeother types of plant natural pro-
ducts, cyclopeptides represent an untapped source for
new drugs and drug leads that couldbe used in thebattle
against the multi-drug resistant pathogens and cancers .
To realize thisgoal , it is crucial to haveanunderstand-
ing of the biosynthetic origin and the biosynthetic
mechanism . This knowledge is essential for developing
rational approaches toward the utilization of the natural
resources through metabolic engineering . In the re-
view, we intend to propose an endophytic origin and a
nonribosomal peptide biosynthetic mechanism . Howev-
er, the real answer to thequestionswill ultimately have
to come from the experiments .
083 云 南 植 物 研 究 31 卷
Fig . 3 A proposed biosynthetic pathway for RA-VII . Abbreviations: A , adenylation domain; C , condensation domain;
E, epimerasedomain; MT , methyltransferase domain; T, thiolation domain ( peptidyl carrier protein) .
Acknowledgments: This work was supported by a grant to Du
andTan fromState Key Laboratory of Phytochemistry and Plant
Resources in West China, Kunming Institute of Botany, Chinese
Academy of Sciences, China .
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