94
H. Zhu et al.
HN
O
O
O
HN
O
O
O
O
O
Fl
HN
O
O
O
O
HN
O
O
O
O
O
O
Fl
HN
O
O
H
O
O
O
HN
O
O
O
O
O
HN
O
O
O
O O
HN
O
O
O
O
O
HN
O
O
O
O
O
Fl-4a-OO
Fl-4a-OO
B:
B-H
Fl-4a-OH
Fl-4a-OH
B-H
B-H
HN
O
OAc
OH
OH
O
PyiG (CYP)
PyiD (CYP)
PyiH (OXR)
PyiB (OAT)
HN
O
OAc
OH
O
O
CYP3 (From
P. oryzae Guy 11)
A
B
CcsB
C1
C2
C3
164 (pyrichalasin H)
C4
131 (cytochalasin Z16)
Scheme 4 Enzymes involved in late-stage modifications. (A) The proposed mechanism of CcsB
catalyzing the formation of carbonate. (B) Left: tailoring enzymes on the pyi gene cluster. The
arrows point to the modifying sites of each enzyme. Right: CYP3, a cytochrome P450 from another
gene cluster, catalyzes the epoxidation to give an unnatural product C4 through combinatorial
biosynthesis