The Alkaloids Volume 76

341 Pages·2016·30.67 MB·English

by  Knölker

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Dedicated to the memory of Ekkehard Winterfeldt (1932–2014)
j
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CONTRIBUTORS
GregoryL.Adams
MerckResearchLaboratories,WestPoint,PA,USA
JamesM.Cook
UniversityofWisconsin-Milwaukee,ChemistryDepartment,Milwaukee,WI,USA
PetraKer(cid:1)cmar
SandozBiopharmaceuticals,Menge(cid:1)s,Slovenia
MarikoKitajima
GraduateSchoolofPharmaceuticalSciences,ChibaUniversity,Chiba,Japan
OjasA.Namjoshi
RTIInternational,CenterforDrugDiscovery,ResearchTrianglePark,NC,USA
AmosB.Smith,III
DepartmentofChemistry,LaboratoryforResearchontheStructureofMatter,University
ofPennsylvania,Philadelphia,PA,USA;MonellChemicalSensesCenter,Philadelphia,
PA,USA
JoachimSto€ckigt
YunnanProvincialKeyLaboratoryofEntomologicalBiopharmaceuticalR&D,Dali
University,Dali,Yunnan,P.R.China;CollegeofPharmaceuticalSciences,Zhejiang
University,Hangzhou,Zhejiang,P.R.China
HiromitsuTakayama
GraduateSchoolofPharmaceuticalSciences,ChibaUniversity,Chiba,Japan
FangruiWu
YunnanProvincialKeyLaboratoryofEntomologicalBiopharmaceuticalR&D,Dali
University,Dali,Yunnan,P.R.China;DepartmentofPharmacology,BaylorCollege
ofMedicine,Houston,TX,USA
ChengguiZhang
YunnanProvincialKeyLaboratoryofEntomologicalBiopharmaceuticalR&D,
DaliUniversity,Dali,Yunnan,P.R.China
j
ix
PREFACE
In thematic volumes of The Alkaloids seminal developments in areas of
exceptionalinterestaresummarizedandhighlighted.Thepreviousthematic
volumeofthisseries,Volume71,focusedonhalogenatedalkaloids.InVol-
ume76,allfourchaptersaredevotedtoindolealkaloids,oneofthesupreme
fields in alkaloid chemistry.
In the first chapter, Fangrui Wu, Petra Ker(cid:1)cmar, Chenggui Zhang, and
Joachim Sto€ckigt describe the enzyme-catalyzed biosynthetic pathway to
sarpagan–ajmalan-type monoterpenoid indolealkaloids.Thebiosynthesisof
alkaloidsinRauvolfiaserpentinawascoveredpreviouslyinthisseriesbyJoachim
Sto€ckigtinChapter2ofVolume47(publishedin1995).ToniM.Kutchan
brieflydiscussedthebiosynthesisoftheajmalan-andsarpagan-typealkaloids
in his treatise on the “Molecular Genetics of Plant Alkaloid Biosynthesis”
(Chapter 7, Volume 50, 1998). Short compilations of the biosynthesis of
thesarpagineandajmalinegroupsofindolealkaloidsweresubsequentlypro-
vided by Mauri Lounasmaa et al. in Chapter 2 of Volume 52 (1999) and
Chapter1ofVolume55(2001).Inthepresentchapter,Sto€ckigtetal.present
the current knowledge of the biosynthetic routes to sarpagan–ajmalan-type
indoles. Application to synthesis leads to biomimetic and chemoenzymatic
approaches which open up the way to generate compound libraries with
novelalkaloidstructures.
OjasA.NamjoshiandJamesM.CooksummarizeinChapter2thepro-
gress on “Sarpagine and Related Alkaloids.” Sarpagine alkaloids were
covered in this series many years ago by J. Edwin Saxton in his review
“TheIndoleAlkaloids”(Chapter10,Volume7,1960)andbyW.I.Taylor
in his two articles on “The Ajmaline–Sarpagine Alkaloids” (Chapter 22,
Volume8,1965andChapter2,Volume11,1968).Themostrecentcover-
agewasprovidedbyMauriLounasmaaandcoworkersintheirreview“The
SarpagineGroupofIndoleAlkaloids”(Chapter2,Volume52,1999).Thus,
an update summarizing the recent developments in the chemistry of this
important family of alkaloids was more than overdue.
InChapter3,GregoryL.AdamsandAmosB.SmithIIIgiveanoverview
on the recent progress in “The Chemistry of the Akuammiline Alkaloids.”
Akuammiline alkaloids have been discussed previously in “The Alkaloids”
by J. Edwin Saxton (Chapter 10, Volume 7, 1960; Chapter 7, Volume 8,
1965; Chapter 11, Volume 10, 1968; Chapter 4, Volume 14, 1973) and
j
xi
xii Preface
more recently by Toh-Seok Kam and Kuan-Hon Lim (Chapter 1, Volume
66,2008).Inthepresentchapter,AdamsandSmithdescriberecentisolations
ofakuammilinealkaloids,theirbiologicalactivity,andnovelachievementsin
total synthesis.
In Chapter 4, Mariko Kitajima and Hiromitsu Takayama present
“MonoterpenoidBisindoleAlkaloids.”Previousfullcoveragesofthebisin-
dole alkaloids from terrestrial plants have been published in this series by
Geoffrey A. Cordell and J. Edwin Saxton in Chapter 1 of Volume 20
(1981) and by Toh-Seok Kam and Yeun-Mun Choo in Chapter 4 of
Volume63(2006).Inthepresentarticle,theauthorsreviewedtheresearch
onmonoterpenoidbisindolealkaloidspublishedfromthemiddleof2006till
themiddleof2015,atimespanwithahighlydynamicdevelopmentinthis
area.
Hans-Joachim Kno€lker
Technische Universit€at Dresden, Dresden, Germany
CHAPTERONE
Sarpagan-Ajmalan-Type Indoles:
Biosynthesis, Structural Biology,
and Chemo-Enzymatic
fi
Signi cance
Fangrui Wu*,x,1, Petra Ker(cid:1)cmar{, Chenggui Zhang*
and Joachim Sto€ckigt*,jj,1
*YunnanProvincialKeyLaboratoryofEntomologicalBiopharmaceuticalR&D,DaliUniversity,Dali,
Yunnan,P.R.China
xDepartmentofPharmacology,BaylorCollegeofMedicine,Houston,TX,USA
{SandozBiopharmaceuticals,Menge(cid:1)s,Slovenia
jjCollegeofPharmaceuticalSciences,ZhejiangUniversity,Hangzhou,Zhejiang,P.R.China
1Correspondingauthors:E-mail:[email protected];[email protected]
Contents
1. Introduction 2
2. RauvolfiaAlkaloidsdSources,TherapeuticValue,andEnzymeDetection 3
2.1 ChoosinganEfficientBiologicalSystem 4
2.2 TheAlkaloidPatternofRauvolfiaCellCultures 4
2.3 ChemicalSynthesesofBiosyntheticIntermediates 7
2.4 DetectionofEnzymeActivitiesandEnzymePurification 9
3. EnzymesoftheAjmalinePathway(AP) 10
3.1 StrictosidineSynthase(STR) 10
3.2 StrictosidineGlucosidase(SG) 18
3.3 SarpaganBridgeEnzyme/GeissoschizineDehydrogenase(GDH) 20
3.4 PolyneuridineAldehydeEsterase(PNAE) 21
3.5 VinorineSynthase(VS) 24
3.6 VinorineHydroxylase(VH)FormingVomilenine 26
3.7 VomilenineReductases(VR,DHVR) 27
3.8 AcetylajmalineEsterase(AAE) 29
3.9 NorajmalineNa-methyltransferase(NAMT) 30
4. SideRoutesoftheAjmalinePathway 31
4.1 VellosimineSideRoutes 31
4.1.1 VellosimineReductase(VER) 31
4.1.2 DeoxysarpagineHydroxylase(DH) 33
4.2 VinorineSideRoute 33
TheAlkaloids,Volume76
ISSN1099-4831 ©2016ElsevierInc. j
http://dx.doi.org/10.1016/bs.alkal.2015.10.001 Allrightsreserved. 1
2 FangruiWuetal.
4.3 VomilenineSideRoutes 34
4.3.1 VomilenineGlucosyltransferse(VGT) 34
4.3.2 RaucaffricineGlucosidase(RG) 35
4.3.3 PerakineReductase(PR) 39
4.4 SideRoutesBeyondPerakineandRaucaffrinoline 42
4.5 TheRouteBeyondAjmaline 43
4.5.1 SideRoute:FromAjmalinetoRaumaclines 43
4.5.2 BiosynthesisofRaumaclines 43
4.5.3 DialdehydeReductase 44
5. MonitoringBioconversionsbyInvivoNMR 45
6. Chemo-EnzymaticApproaches 49
6.1 FromSingletoBunchdFromStrictosidineSynthase(STR)toNovelAlkaloids 49
6.2 Chemo-EnzymaticApplicationofRaucaffricineGlucosidase(RG) 52
7. Summary 54
Acknowledgments 55
References 55
1. INTRODUCTION
Despite its therapeutic uses as an antiarrhythmic drug to balance
arrhythmic heart failures,1 its well-known properties to reduce high blood
pressure, and the very long history of the ancient plant Rauvolfia in South
and South-East Asian traditional medicine,2 very little was known about
the biosynthesis of the monoterpenoid indole alkaloid ajmaline (1). Our
limited knowledge of this alkaloid’s biosynthesis was based on a very few
in vivo feeding experiments that have used labeled, putative biosynthetic
precursorssuchastryptophanfortheindolepartofthealkaloid.3Mostinter-
estingwastheunusualcomplexchemicalstructureof1harboringahexacy-
clic carbon skeleton that contains the chiral Nb atom and nine other
asymmetric carbon centers (Figure 1).4
Thecomplexityoftheajmalinestructuremakesitdifficulttoidentifying
itsbiosyntheticpathway.Thesolutiontothisproblemisanin-deptheluci-
dation of each reaction step and each enzyme involved in the catalysis.4e9
This article reviews the relevant research on the ajmaline’s main biosyn-
thetic pathway and its side routes, the route beyond ajmaline, the applica-
tion of in vivo NMR, the chemo-enzymatic significance of the involved
enzymes and, their reaction mechanisms and the enzyme X-ray crystal
structures.
Sarpagan-Ajmalan-TypeIndoles 3
Figure 1 Chemical structure of ajmaline (1), the typical ajmalan-type alkaloid of
Rauvolfia.(Thechiralcentersaremarkedwithatomnumbers).
2. RAUVOLFIA ALKALOIDSdSOURCES, THERAPEUTIC
VALUE, AND ENZYME DETECTION
The previous pioneering work by Court and coworkers resulted in
the most comprehensive phytochemical analysis of Rauvolfia species. For
instance, during their work on 10 mainland African species, 133 individual
indole alkaloids were identified.10e18 Moreover, recently eight new indole
alkaloids have been isolated from Rauvolfia species of mainland China and
the structures of them were identified by Liu and colleagues.19,20 Seven
novel indole alkaloids from the leaves and twigs of Rauvolfia verticillata
were also identified.13
During the recent decades,attempts were made to detect novel biolog-
ical and pharmacological activities of alkaloids from the genus Rauvolfia.21
The results demonstrated that ajmaline and derivatives interact with ion
channels.22e24 Moreover, Rauvolfia alkaloids from a molecular library
were computationally screened by molecular docking studies to search for
inhibitors of the site of human aldose reductase (AR). Potent inhibition of
AR may lead to effective therapies against diabetes.25 Two peraksine-type
alkaloids were screened out and identified as leads of AR inhibitor from
the library.26
Recently a small collection of alkaloids isolated from the Marquesan
plant Rauvolfia nukuhivensis (Fosberg & Sachet) Lorence & Butaud was
arranged in a postulated biosynthetic scheme highlighting the role of
16-epi-vellosimine in the biosynthetic formation of ajmalan- and sarpagan-
type alkaloids of this particular Rauvolfia species (see also Sections 3.4 and
4.1). Moreover, docking experiments were performed in this study to
underline the biological activity observed on human ion channel structure
(hERG).27Thesefindingsdocumentedagrowingphytochemicalandphar-
macological interest in Rauvolfia alkaloids research.
4 FangruiWuetal.
2.1 Choosing an Efficient Biological System
The biological material can be the key to successfully investigate a biosyn-
thetic pathway at the molecular level. This is especially true for pathways
operating in higher plants due to their slow growth characteristics.
Compared to microbial systems as for instance bacteria, yeasts and fungi,
plant cells have a much bigger size and exhibits a much lower duplication
rate, exhibiting a much slower metabolism of natural secondary products.
ImpressiveexamplesareplantspeciesofthegenusRauvolfiasuchasRauvolfia
serpentina Benth. ex Kurz, the major example for the delineation of the
ajmaline pathway (see Figure 6).
R.serpentinaisatraditionalmedicinalplantwhichhasbeenusedinIndia
forw3000years,sincethe“pre-Vedic”and“Ayurvedic”periods.28Atthat
time it was known as the Rauvolfia root (Radix rauwolfiae). However,
Rauvolfia has also an important, long-standing place in the Traditional
Chinese Medicine (TCM) where it is one of the 50 fundamental herbs.29
Differentiated plant material of Rauvolfia is not very useful for enzymatic
biosynthetic research because of in general low physiological activity of
plants. They need several years (6e8) to accumulate the alkaloids, which
are then isolated for medicinal purposes by the pharmaceutical industry
(suchasreserpine,ajmaline,raubasine,etc.).Moreover,cultivatingtheplants
in green-houses or in laboratories (phytotrones) is difficult and time
consuming. The insufficient amount of physiologically active cell material
wasprobablythemajorreasonthathamperedtheresearchprogressonRau-
volfia alkaloid biosynthesis in the past.
A breakthrough occurred, however, in the research on plant natural
product biosynthesis when tissue and cell suspension cultures were devel-
oped by M.H. Zenk.30,31 Such cultures easily provided continuously fresh
Rauvolfia cell material at kg level in a few weeks in cell culture laboratories
where Erlenmeyer flasks were used for cell growth (Figure 2).
2.2 The Alkaloid Pattern of Rauvolfia Cell Cultures
Alkaloidanalysisofcellsuspensionsshowedthegreatadvantageofsuchplant
cell systems in providing a diverse collection of indole alkaloids (see
Figure 3).8,30,32
In addition, as a second plant source, intergeneric hybrid cell culture
between R. serpentina and Rhazya stricta was established by somatic
Sarpagan-Ajmalan-TypeIndoles 5
Figure2 MassproductionofRauvolfiafreshplantmaterial(cellsuspension).Enzyme
productioninEscherichiacoliisconductedinthesimilarmanner.
hybridization/protoplastfusion.Theircontentofmonoterpenoidindoleal-
kaloidswasingenerallow,butcouldbeenhancedbyinductionwithmeth-
yljasmonate.33 The detected alkaloids are displayed in Figure 4.
Moreover, a third Rauvolfia source for alkaloids was identified, the so
called“hairyroot”cultures.Theseweregeneticallyengineeredroots,which
showedfastergrowthcomparedtonon-engineeredroots.Thephytochem-
ical investigation of “hairy roots”34e37 revealed a bunch of structures
including novel alkaloids as illustrated in Figure 5.37
Conclusively, these three plant systems of Rauvolfia including cell sus-
pension cultures, hybrid cell suspension cultures, and hairy root cultures
became available for the first analysis of their alkaloid pattern, including
isolation and identification of the single alkaloid metabolites in concentra-
tions of about 100mg to w1.6 per liter nutrition medium.38
The alkaloid libraries derived from this phytochemical system were
rare, valuable, and unavailable from commercial sources. They provided
forthefirsttimeputativeenzymesubstratesandproductstypicalofRauvol-
fia, which were later employed for establishing numerous enzyme assays.
These enzyme activity assays are the ultimate prerequisite for enzyme
detection, enrichment, purification, sequencing, heterologous expression
and crystallization, followed by X-ray structure analysis. These alkaloids
were also, together with alkaloid samples provided over the years by col-
leagues worldwide especially from the UK, France, Japan, the USA, and
Germany, the prerequisite for establishing an excellent “alkaloid tool” in
elucidating the ajmaline pathway (AP) by detecting most of the enzymes