POPLAR OXIDOREDUCTASES INVOLVED IN THE OXIDATIVE STRESS RESPONSE: A CRYSTALLOGRAPHIC SNAPSHOT TOWARDS THE UNDERSTANDING OF THE CATALYTIC MECHANISMS
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Date
2008-05
Authors
KOH, CHA SAN
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Abstract
The production of reactive oxygen species (ROS) is an unavoidable consequence of
living in an aerobic environment and once produced, it can be removed by several different
enzyme systems. Three oxidoreductases (glutathione peroxidase, Gpx; thioredoxin, Trx and
glutaredoxin, Grx) from Populus trichocarpa x deltoides (poplar tree) were characterized
using biochemistry and X-ray crystallography approaches. Gpxs are a group of enzymes that
regulate the levels ofROS in cells, and protect them against oxidative damage. In this study, I
have determined the crystal structures of the reduced and oxidized form of poplar Gpx5
(PtGpx5). Comparison of both redox structures indicates that a drastic conformational change
is necessary to bring the two distant cysteine residues together to form an intramolecular
disulfide bond. Trxs are a family of ubiquitous enzymes which regulate various protein
partners through the thiol-disulfide(s) reduction. The aim of this study is thus to precisely
describe the catalytic mechanism of a new isoform of Trx that has been characterized in
poplar, PtTrxh4, since it has been demonstrated recently to be reduced by Grx but not by the
typical NADPH:thioredoxin reductase reducing system. PtTrxh4 contains three cysteines; one
localized in an N-terminal extension (Cys4) and two (Cys58 and Cys61) in the usual Trx
active site (WC1GPC2). Analyses of two PtTrxh4 crystal structures solved in this study, wildtype
and C61S mutant, allow us to propose a four-step disulfide cascade catalytic mechanism
in accordance with enzymatic studies. Grxs are highly conserved redox-proteins that utilize
electrons from GSH particularly to catalyze thiol-disulfide exchange reactions. Here, I present
the structure of glutathionylated PtGrxS12, the first structure of plant Grx of subclass 1 with
an atypical 28 WCSYS32 active site. This protein possesses an additional cysteine (Cys87) in
which the role of this extra active-site cysteine remains obscure. Details of the GSH binding
site are summarized in this study. Protein structures solved here shed lights to our
understanding of the redox mechanism in plants and to the enzyme-substrate interactions
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Keywords
STRESS RESPONSE , UNDERSTANDING