Selenoenzyme
Glutathione Peroxidase
Sam Pletz '13 and
Jimmy Chapman '13
Contents:
I.
Introduction
Glutathione
peroxidase belongs to the
selenoprotein family and is observed in mammals, birds, and fish. This
enzyme along with other members of the selenoprotein family have an
integral role in the defense mechanisms of these organisms.
Specifically,
the Selenoenzyme
Glutathione Peroxidase from
bovine plasma plays a role in catalyzing the reduction of
hydroperoxides. This enzyme also protects biomembranes and cell
structures from oxidative damage. Glutathione
Peroxidase accomplishes this
through the reduction of lipid hydroperoxides to their corresponding
alcohols and through the reduction of free hydrogen peroxide to water
using glutathione as the reducing substrate. The enzyme reduces lipid
hydroperoxidases and hydrogen peroxide through the following proposed
mechanism:
ROOH
+ 2GSH → ROH + H2O + GSSG
The
crystal structure of Seleno-Glutathione
Peroxidase from human plasma has
been characterized and differs only slightly from that of Glutathione
Peroxidase from bovine plasma
(Rin et al. 1997). However, the physilogical role of Glutathione
Peroxidase in humans is still
unclear because of the low levels of reduced glutathione and the low
reactivity of the enzyme (Rin et al. 1997).
II.
General Structure
Glutathione
peroxidase is a tetramer consisting of 4 identical single polypeptide
chains each with 178 amino acid residues
.Each
monomer contains two
parallel and two anti-parallel pleated β-sheets
surrounded
by
four α-helices
.
α helices 1,
2,
and 4,
exist on one side of the β
sheet complex whereas
α4
exists on the other side
.
The catalyitically
active selenocysteine is
located near the first turn of α1
.
The second
region of the monomer consists of α3 and two anti-parallel
pleated β-sheets,
and several β turns. Hydrogen
bonding between β1 and β3 stabilizes and completes
the four stranded β complex.
III.
Subunit Contacts
A
unique feature of the
protein is the tetrahedral quaternary
structure
that is a planar arrangement of four identical monomers
.
The contact regions of
each subunit to another consists of 16 amino acid residues; both polar
and non-polar amino acid residues can be found between them.
Four
hydrogen bonds can also be formed across the local axis by residues Glu-77,
Arg-86
and the symmetry equivalent pair, Glu-277
and Arg-286
.
(Epp et al. 1983). Therefore, each monomer has 20 identical
interactions with each other.
Polar
side chains and other backbone
atoms contribute to more subunit contacts between each monomer. The
interactions between each monomer decrease the accessible surface area
and is the main source of free energy keeping the monomers together
(Epp et al. 1983). Because each monomer is chemically identical, one
would predict roughly identical values of the individual binding
constants between the GSH peroxidase monomers (Epp et al. 1983).
IV.
Active Centers
GSH
peroxidase has high reaction rates. The reason for this is that the
catalytically active selenocysteine residues
on the molecular
surface
are exposed which allows for easy access for substrates
.
Aromatic
residues in
the
immediated vicinity
allow for hydrogen bonding with the selenocysteine
.
This helps
to stabilize the active-site.
The
selenocysteine lies
in the area where the carboxy ends of two
parallel Beta-sheets
and the N-terminal end of one Alpha-helix
1
meet
.
This is the area where
substrate binding occurs.The
alpha1 helix combined
with two adjacent parallel Beta strands form a Beta-Alpha-Beta
substructure.
Thus secondary structure
formation is favorable
due to the carboxy ends
of the Beta strands near the binding region.
The macrodipole created by alignment of the alpha helix parallel to the
helix axis stabilizes the active-site selenolate.
When
treated with
cyanide, the charge distribution at the active site is shifted. The
seleno-amino acid has a negative charge taken away.
This results in a 0.4-nm shift of the Met-101
side chain
located
on the
opposite surface of the monomer compared to the selenium
site
.
The shift is consistent with the formation of hydrogen bonds of the
selenoloate to 148-Trp
N
and to 70-Gln N.
This results in stabilization
of the active-site
.
V.
Substrate Binding
Highly
oxidized Gluthatione peroxidase binds the donor substrate
glutathione
with
high affinity (Ren
et al. 1983). In contrast, reduced
GSH peroxidase binds glutathione with low affinity. In an oxidized
environment, GSH peroxidase binds four glutathione molecules per
subunit by a selenosulfide linkage. The specific binding interaction
between a subunit of the enzyme and glutathione can be characterized by
several important amino acid interactions. Amino acids Arg-40
and Arg-167
form a salt bridge with the glutathione
(Ren
et al. 1997).
Residue Gln-130
forms a hydrogen bond with the N-terminus of the ligand
(Ren
et al. 1997).
VI.
References
Epp Otto, Ladenstein Rudolf, and Wendel
Albrecht.
1983.
The Refined Structure of the Selenoenzyme Glutathione Peroxidase at
0.2-nm Resolution. Biochemistry
133:51-69.
Ladenstein Rudolf, Epp Otto, Bartels Klaus,
Jones Alwyn, Huber Robert and Wendel Albrecht.
1979.
Structure Analysis and Molecular Model of the Selenoenzyme Glutathione
Peroxidase at 2.8 Angstrom Resolution. Journal
of Molecular Biology
134:199-218.
Ren
Bin, Huang Wenhu, Akesson Bjorn, Ladenstein
Rudolf.
1997.
The Crystal Structure of Seleno-Glutathione Peroxidase from Human
Plasma at 2.9 A Resolution. Journal
of Molecular Biology
268:869-885.
Sharadamma
C. K., Purushotham B., Radhakrishna M.P., Abhilekha
M.P., and Vagdevi M.H.
2010.
Role of Selenium in Pets Health and Nutrition: A Review. Asian
Journal
of Animal Sciences
5:64-70.