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The molecular surface of IDE is represented by
light yellow. The N- and C-terminal domains of IDE are colored
green and red, respectively. The beta-amyloid (blue) is entrapped
inside the degradation chamber of the IDE molecule.
Yuequan Shen, Univ. of Chicago
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"The structure of insulin-degrading enzyme
tells us a lot about how it works, which is somewhat unorthodox," said
Wei-Jen Tang, Ph.D., associate professor in the Ben May Institute for
Cancer Research at the University of Chicago and director of the study.
"Understanding how it works gives us clues about how to design drugs
either to inhibit or activate it."
"By introducing small, targeted mutations, we have
already been able to increase the enzyme's activity by as much as
40-fold," he said. "That gives us a blueprint for the next step,
trying to devise a drug that would produce a similar effect."
Ever since I. Arthur Mirsky discovered IDE in 1949,
physicians have sought ways to manipulate it. Mirsky thought that by
inhibiting the enzyme he could help diabetics by making their insulin
remain active longer. More recently, as scientists realized that IDE
was also involved in clearance of amyloid-beta, they have begun
searching for ways to supercharge the enzyme to see if it could
prevent the build-up of the amyloid plaques that are a hallmark of
Alzheimer's disease.
Despite more than half a century of intensive
research, however, insulin-degrading enzyme has remained "an
especially elusive pharmacological target," biochemist Malcolm
Leissring of the Scripps Research Institute and neurobiologist Dennis
Selkoe of Harvard Medical School wrote in a commentary that
accompanies the Nature article.
Using the Advanced Photon Source at Argonne
National Laboratory, Tang and colleagues were able to solve the
structures of this enzyme in complex with insulin and with amyoid-beta,
as well as amylin and glucagon. These "high-resolution crystal
structures open the door to the rational design of pharmacological
modulators of this important protease," wrote Leissring and Selkoe.
The enzyme, Tang's team reports, resembles the
video-game character "Pac-Man," with two bowl-shaped halves joined by
a hinge at one end and held closed, most of the time, by a latch of
hydrogen bonds on the other end. When the bowls come together, like a
shut mouth, they enclose a chamber, shaped like a triangular prism,
with a base that measures 35 x 34 x 30 angstroms and a height of 36
angstroms, large enough to contain relatively small peptides, such as
insulin or amyloid-beta, which have fewer than 50 amino acids.
Although it can cleave larger molecules, the
proteins IDE degrades most readily fit neatly within this chamber.
Negative electrical charges on their outer surfaces help to align them
with the positive charges on one inner surface of the chamber. Once
they are in place, the enzyme slices them multiple times into tiny
pieces, which are then discarded.
Although the enzyme's structure is similar to
Pac-Man, its behavior differs. Pac-Man keeps his mouth wide open to
gobble up anything in his path. With IDE the mouth is usually closed.
The hydrogen-bond latch that holds the jaws together protects its
active, or catalytic site.
But in a series of experiments, Tang and colleagues
were able to make small mutations of IDE that altered only the latch,
disrupting the alignment of contacts that normally keep the enzyme
closed. Three of these altered versions of wide-open IDE proved to be
30 to 40 times more active than the normal version of the enzyme.
"This suggests that the rate-limiting step may be
the speed at which the enzyme can reopen and then clamp down on a new
morsel rather than the time it takes to chew something up," said Tang.
"This makes us think that if we can slightly alter its shape, we can
substantially boost its activity."
The researchers are now searching for small
molecules that can duplicate the effects of those mutations, shifting
the balance toward the open rather than the closed state. "Such
compounds," the authors note, "might facilitate the clearance of
amyloid-beta and other pathologically relevant IDE substrates."
"By revealing IDE's active site in unprecedented
detail," notes the commentary, "the new structures provided by Tang
and coworkers hold great promise for finally realizing Mirsky's dream." |
