Structure
of Estrogen Related Receptor Gamma
in complex with Bisphenol
A
Ian Richardson and Myles Alderman
Contents:
I.
Introduction
Estrogen-related
Receptor-γ
(ERRγ) is a nuclear
receptor
protein
that has
demonstrated
regulatory
control
over
genes in
the breast and bone. Despite
sharing structural homology with
the
Estrogen Receptor family
of proteins, ERRs do not bind estrogen and
thus allow for transcription regulation by small lipophilic molecules
which can act as agonists
or inverse agonists.
Bisphenol A (BPA) is an agonist for ERRγ and is falsly
recognized as estrogen by the protein. Since BPA mimics the
hormone, it is very tightly bound by ERRγ and thereby
ensures the continuous activation of the protein. This
control over ERRγ has led
to
studies suggesting that
BPA increases the risk of breast cancer and may
be associated with a variety of health problems.
Tamoxifen
(4-hyrdoxytamoxifen) is an antagonist to the ERR family of proteins.
Tamoxifen is useful because it blocks estrogen from the
cancer cells via competitive antagonism, and since some breast cancers
require estrogen to grow, it thereby inhibits cancer growth.
It is commonly perscribed as hormone therapy for recovering breast
cancer patients.
II.
General Structure
ERRγ
is
composed of
four chemically identical polypeptide subunits
,
each with 456 amino
acids, and these combine to form two homodimers. Helicies
9-10 appear to be important in dimerization.
Each
subunit
has its own ligand binding domain (LBD)
,
and binding of a ligand
to the
LBD of one unit does not affect the binding affinity of any of the
other LBDs in the protein. With a volume of 280 cubic
Angstroms,
the
LBD of ERRγ is extremely
small - the LBD volume of classic Estrogen Receptors is much larger at
370 cubic Angstroms.
Due
to
the small size of
ERRγ's LBD, and the
fact
that it is largely composed of hydrophobic
residues
ERRγ's LBD, and the
fact that it is largely composed of hydrophobic residues
,
the LBD
provides a favorable environment for small, hydrophobic molecules.
Thus the phenyl rings and methyl groups of BPA
and Tamoxifen
are strongly bound to the LBD of ERRγ.
The highly
conserved portions of the DNA binding domain
(DBD) and the ligand
binding
domain
(LBD) may
explain why the ERR family of proteins are constituitively active.
The core DBD
is composed of two zinc
fingers,
specialized folds in the protein which are stabilized by two
zinc atoms embedded in the protein
. This core DBD is
composed of an alpha helix
that rests in the major
groove of the DNA. Sequence specific
interactions with DNA occur via H-bonds with Lys-128, Lys-124, and
Glu-121.
.
Since the ERR family of
proteins binds to DNA as a monomer, it is neccesary to have additional
contact with the DNA to ensure sequence specific binding. The
result is a unique feature in the ERR family: the C-terminal extension
(CTE)
.
The CTE provides an AT-hook motif that allows sequence
specific binding with the minor groove of the DNA, adjacent to the DBD.
Additional H-bonds are formed with the CTE and the DNA with
Arg-179, Gly-180, Arg-182
.
Once both the DBD and the CTE have been bound to DNA, a
hydrophobic interaction occurs between Tyr-185 of the CTE and Leu-169
and Val-117 of the DBD
. It is thought that this
interaction stabilizes and combines the interactions of the DBD and the
CTE.
III.
Bisphenol A Binding
ERRγ
has
relatively
high affinity
for BPA.
BPA - the highest of any phenol/polyphenol tested.
BPA
is a small ligand and fits tightly
in the LBD of ERRγ
.
The strong
interactions
between ERRγ and
BPA arise from a set of Pi stacking and hydrogen
bonds.
Pi
stacking occurs between the ligand and residues Tyr-326 and Phe-435
.
There are additional
hydrophobic interactions between the phenol rings of BPA and the
hydrophobic carbon side chains of Leu-342 and Leu-345
.
Hydrogen bonding occurs
between BPA and Glu-275, Arg-316, Tyr-326, and Asn-346
.
The
hydrogen bond with Tyr-326 is enabled by a hydrogen bond between
Tyr-326 and Asn-346 that pulls the Tyr-326 residue close enough to BPA
to enable an interaction between them
.
IV.
Tamoxifen Binding
Tamoxifen
is an effective breast cancer
drug which works via competitive
antagonism against
estrogen
in Estrogen Receptor Alpha (ERα),
but it binds much tighter to ERRγ. The variation in
binding affinity is due to structural differences between
ERRγ
and ERα. The most significant difference in the
binding pocket is the
Phe-435 residue in ERRγ
which corrosponds to
Leu-525 in ERα.
This
difference in amino acids is the primary reason that estrogen cannot
bind to ERRγ, the steric strain from the PHE prevents it.
The
binding
interactions between Tamoxifen and ERRγ are
comprised
primarily of three
hydrogen bonds.
ERRγ's
Asp-273,
Glu-275, and Arg-316
form hydrogen
bonds with the ligand.
The
binding interactions between
Tamoxifen and ERRγ are
stabilized
by the prominent hyrophobic residues seen earlier in the LBD.
VI.
References
Abad,
Marta C., et al. "Structural determination of estrogen-related
receptor γ in the presence of phenol derivative compounds."
The Journal of steroid biochemistry and molecular biology 108.1 (2008):
44-54.
Brisken,
Cathrin. "Endocrine Disruptors and Breast Cancer." CHIMIA
International
Journal for Chemistry 62.5
(2008):
406-09.
Print.
Gearhart,
Micah D., Signe M.A. Holmbeck, Ronald M. Evans, H.Jane Dyson, and Peter
E. Wright. "Monomeric Complex of Human Orphan Estrogen Related
Receptor-2 with DNA: A Pseudo-dimer Interface Mediates Extended
Half-site Recognition." Journal of Molecular Biology 327.4 (2003):
819-32. Print.
Wang, Liping, et al. "X-ray crystal
structures of the estrogen-related
receptor-γ ligand binding domain in three functional states
reveal the molecular basis of small molecule regulation." Journal of
Biological Chemistry 281.49 (2006): 37773-37781.