Structural Biochemistry/Enzyme Catalytic Mechanism/Glutathione Reductase

General Information
Oxidation-reduction enzyme

Uses coenzymes NADP+/NADPH and FAD/FADH2

In humans, this is a dimer of 478-amino acid subunits linked by disulfide bond

Two-stage reaction

1st stage : E + NADPH + H+ EH2 + NADP+

Oxidized E binds NADPH, immediately reduces FAD, producing NADP+.

E contains redox-active disulfide bond (Cys58-Cys63), which has accepted an electron pair in EH2 to form a dithiol (one S- is in a charge transfer complex with FADH-)

2nd stage : EH2 + GSSG E + 2GSH

GSSG binds to EH2

Cys58 nucleophile attacks one S of GSSG yielding mixed disulfide, promoted by His467' acting as general base

One GSH is kicked-off by protonation by His467' (general acid)

Cys63 nucleophile attacks Cys58 to form redox-active disulfide, kicking off second GSH

Reference
http://books.google.com/books?id=fSORbNITJ0QC&pg=PA6&lpg=PA6&dq=acid+base+enzyme+catalysis&source=bl&ots=SA3h6Afvdc&sig=0VpMXdejjvFQ7HdfgjSTt3602lQ&hl=en&ei=j48YS8qECouotgPajcyHBw&sa=X&oi=book_result&ct=result&resnum=8&ved=0CCsQ6AEwBw#v=onepage&q=acid%20base%20enzyme%20catalysis&f=false