Figure 2.
Fig. 2. Homobelactosin C specifically binds to the
chymotryptic-like active site by formation of an ester. (a)
Chemical structures of omuralide and bisbenzyl-protected
homobelactosin C in their native and bound conformation. The
lead structure segments that in particular are involved in
inhibitor binding are depicted in blue; Thr-1 of subunit 5 is in
red. (b) Stereorepresentation of the chymotryptic-like active
site of the yeast 20S proteasome (colored in gray) in complex
with bisbenzyl-protected homobelactosin C (colored in green).
Covalent linkage of the inhibitor with 5-Thr1O^ is drawn
in magenta. The electron density map (colored in blue) is
contoured from 1 around Thr-1 (colored
in black) with 2F[o] – F[c] coefficients after twofold
averaging. Temperature factor refinement indicates full
occupancies of the inhibitor-binding site. The homobelactosin C
derivative has been omitted for phasing. (c) Surface
representation of the chymotryptic-like active site in complex
with omuralide (depicted in brown, Left) and homobelactosin C
(depicted in green, Right), covalently bound to Thr-1 (depicted
in white). Note the overall similarity in the binding mode of
both inhibitors but the different orientations of the generated
C4-hydroxy group upon -lactone ring opening
(indicated by a black arrow). Surface colors indicate positive
and negative electrostatic potential contoured from 15 kT/e
(intense blue) to –15 kT/e (intense red). (d)
Stereorepresentation of the superposition of lactacystin and
bisbenzyl-protected homobelactosin C, including Thr-1 with
respect to subunit 5. Omuralide is shown in
brown, bisbenzyl-protected homobelactosin C is drawn in green,
and the active site Thr-1 is in black. The superposition
indicates that both inhibitors occupy the S1 specificity pocket
in a unique way. The bisbenzyl-protected homobelactosin C is
prolonged to the primed site. Nonprimed and primed sites are
indicated by a black arrow.
5 is in
red. (b) Stereorepresentation of the chymotryptic-like active
site of the yeast 20S proteasome (colored in gray) in complex
with bisbenzyl-protected homobelactosin C (colored in green).
Covalent linkage of the inhibitor with 
is drawn
in magenta. The electron density map (colored in blue) is
contoured from 1 
around Thr-1 (colored
in black) with 2F[o] – F[c] coefficients after twofold
averaging. Temperature factor refinement indicates full
occupancies of the inhibitor-binding site. The homobelactosin C
derivative has been omitted for phasing. (c) Surface
representation of the chymotryptic-like active site in complex
with omuralide (depicted in brown, Left) and homobelactosin C
(depicted in green, Right), covalently bound to Thr-1 (depicted
in white). Note the overall similarity in the binding mode of
both inhibitors but the different orientations of the generated
C4-hydroxy group upon