1'
FPP
PPO
squalene
synthase
2'
FPP
*
electrophilic addition results
tertiary cation
allylic cation
*
H H
*
3'
*
OPP
loss of proton with formation
of cyclopropane ring
H
*
OPP
*
H
presqualene PP
loss of diphosphate results in
primary cation
*
H
W-M
1,3-alkyl shift
H
*
*
*
H
H
*
H
1,3-alkyl shift generates new
cyclopropane ring and more
favorable tertiary cation
bond cleavage produces
alkene and favorable
allylic cation
H
H
*
Figure by MIT OCW.
*
squalene
*
H
(NADPH)
cation quenched
by attack of
anhydride
squalene
O2
NADPH
cyclizations
O
squalene oxide
protonation of epoxide
allows ring opening to
tertiary cation
electrophilic addition
gives tertiary cation +
six-membered ring
electrophilic addition
gives tertiary cation +
six-membered ring
H
H
HO
HO
O
H
HO
H
squalene oxide
electrophilic addition
gives tertiary cation +
five-membered ring
H
H
H
H
H
HO
H
protosteryl cation
H
HO
electrophilic addition
gives tertiary cation
H
H
W-M rearrangement;
ring expansion despite
tertiary --> secondary
cation
HO
H
Figure by MIT OCW
H
O
HO
H
H
H
chair - boat - chair - boat
squalene oxide
H
H
sequence of W-M
1,2-hydride and
1,2-methyl shifts
HO
HO
H
H
H
H
H
H
protosteryl cation
H
H
H
H
H
HO
HO
H
cyclopropane ring
formation and
loss of proton
from C-10 methyl
sequence of W-M
1,2-hydride and
1,2-methyl shifts
H
H
H
H
H
protosteryl cation
H
loss of H-9 creates
double bond
H
H
HO
H
HO
H
H
H
H
H
H
HO
H
cycloartenol
HO
H
lanosterol
Figure by MIT OCW
Aromatase-catalyzed conversion of androstenedione to estrone
O
CH3
O
O2
NADPH
O
O
HR HS
HO
O2
O
+
+H
-H2O
H
O
-H2O
NADPH
O
+
+H
O
HO H
HO
H
O
-HOH
NADPH
O2
+
+H
O
H2O
+
HCOOH
+
HO
A
Three possible mechanisms for the last step in the aromatase-catalyzed oxygenation of androstenedione.
O O Fe+3
+3
Fe O O
O
+3
Fe O
OH
O
+3
+3
Fe O
OH
Fe OH
H
1
A
O
O
O
O
+4
Fe O
H
+4Fe
O
OH
O
2
A
HO
O
C H
OH
Fe
O
O
+4Fe
O
O
O
O
+
HCOOH
O
+4Fe
CH
O
O
H
H
CH
A
3
O
3+
HO
+4Fe
+4Fe
+4Fe
+
O
O
HO
+
3+
Fe
+ HCOOH
+4Fe
OH
Figure by MIT OCW.
PLANT
HO
HO
OH
O
H
HO
HO
H
cycloartenol
loss of C-4α
methyl
H
C-24 alkylation
opening of
cyclopropane
H
HO
H
O
OH
HO
H
24-methylenecycloartanol
cycloeucalenol
H
obtusifoliol
loss of C-14α methyl and
allylic isomerization
(migration of Δ8 to Δ7)
24-ethyl sterols
H
HO
H
loss of C-4
methyl
HO
H
avenasterol
H
H
further side
chain alkylation
HO
H
citrostadienol
(24-ethylidenelophenol)
H
H
loss of C-4 methyl
H
H
HO
episterol
H
gramisterol
(24-methylenelophenol)
H
24-methyl sterols
YEAST
C-24 alkylation
demethylations as C-4
C-14 demethylation
H
HO
H
lanosterol
HO H
eburicol
(24-methylenedihydrolanosterol)
HO
H
H
HO
4,4-dimethylfecosterol
H
fecosterol
H
Figure by MIT OCW.
HO
H
ergosterol
side chain and
ring B
modifications
A
cyclization is initiated by
protonation of a double bond
resulting in tertiary cation
+
H
squalene
B
rings A and B are formed as 5
membered rings via Markovnikov
addition; they then expand to 6
membered rings via W-M
rearrangements
H2O
H2O
OH
H
H
H
H
H
OH
H
H
H
Figure by MIT OCW
H
H
H
HO
hopan-22-ol
tetrahymanol
H
H
GGPP
PPO
GGPP
electrophilic addition
resulting in tertiary cation
allylic cation
H H
OPP
loss of proton with formation
of cyclopropane ring
H
loss of diphosphate
results in primary
cation
OPP
H
H
H
W-M 1,3-alkyl shift generates
new cyclopropane ring and
more favorable tertiary cation
prephytoene PP
H
H
H
H
proton loss
generates alkene
H
bond cleavage produces
alkene and favorable
allylic cation
Z
Z-phytoene
sequence of desaturation
reactions; in plants and fungi,
the central double bond is
also isomerized Z --> E
E
lycopene
Figure by MIT OCW.
protonation of double bond
results in tertiary cation;
then electrophilic addition
H+
O2
NADPH
a
HO
ε-ring
H
c
H
b
b
H
a
c
opening of
epoxide ring
γ-ring
O2
NADPH
O2
NADPH
O
O
HO
HO
β-ring
pinacol-like rearrangement
generates ketone
H+
OH
HO
O2
NADPH
OH
H
O
H+
HO
(a)
HO
OH
allene generated from
opening of epoxide
ring and loss of
proton
Figure by MIT OCW.
(b)
β-carotene
central cleavage
generates two
molecules of retinal
excentric cleavage (b)
central cleavage (a)
oxidative chain
shortening
O
retinal
excentric cleavage
can generate one
molecule of retinal
O
Abietadiene Synthase
bifunctional diterpene cyclase
from fir
cannot separate functional domains
cloned
(a.a. sequence is)
don't know structure
"homology modeling'
ep.-aristolochene
synthase enzyme as a model
(sequiterpene synthase)
Figure removed due to copyright reasons.
Gibberrellin
phytohormone
bifunctional enzyme in fungi
two enzymes in plant
2 individual enzymes
Higher plants
ionization
protonation
OPP CPS
OPP
FCPS/KS
KS
FCPS/KS
Phaeosphaeria sp. L487
Figure by MIT OCW.
yeast
1 bifunctional enzyme
Figure by MIT OCW.
Index of figures removed due to copyright reasons
Jennewein, S., and R. Croteau. Figure 2 in “Taxol: biosynthesis, molecular genetics, and biotechnological
applications.” Applied Microbiol Biotech 57 (2001): 13-19.
Jennewein, Stefan et al. “Random sequencing of an induced Taxus cell cDNA library for identification of clones
involved in Taxol biosynthesis.” PNAS 101 (2004): 9149-9154.
Brown, Geoffrey D. Scheme 8 in “The biosynthesis of steroids and triterpenoids.” Natural Product Reports 15
(1998): 653-696.
Abe, Ikuro, Michel Rohmer, and Glenn D. Prestwich. Scheme VI in “Enzymatic cyclization of squalene and
oxidosqualene to sterols and triterpenes.” Chemical Reviews 93 (1993): 2189 – 2206.
Rilling, H.C., and Chayet, L.T. “The 19-reaction conversion of lanosterol to cholesterol.” Danielsson, H., and
Sjovall, J., eds. Sterols and Bile Acids. New York, NY: Elsevier, 1985. ISBN: 0444806709.
Hoshino, Tsutomu, and Tsutomu Sato. Figure 3 in “Squalene–hopene cyclase: catalytic mechanism and substrate
recognition.” Chemical Communications (2002): 291 – 301.
Peters, Reuben J. Scheme 1, Figure 1, and Table 1 in “Bifunctional Abietadiene Synthase: Mutual Structural
Dependence of the Active Sites for Protonation-Initiated and Ionization-Initiated Cyclizations.” Biochemistry 42
(2003): 2700-2707.
Williams, David C. et al. Figures 1, 3 and 6 in “Intramolecular proton transfer in the cyclization of geranylgeranyl
diphosphate to the taxadiene precursor of taxol catalyzed by recombinant taxadiene synthase.” Chemistry &
Biology 7 (2000): 969 - 977.