Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Enzymatic Redox Chemistry 137
Cyclohexanone mono-oxygenase is the enzyme used by Pseudomonas for
the biodegradation of aliphatic hydrocarbons containing cyclohexane rings.
The lactone product is subsequently hydrolysed and broken down into small
fragments which can be utilised for growth.
6.5 CASE STUDY: Glutathione and trypanothione reductases
Glutathione is a tripeptide l-Glu-g-l-Cys-Gly which is found at concentrations
of about 1 mm in all mammalian cells. Its function is to protect the cell against
‘oxidative stress’, or the presence of activated oxygen species which would
otherwise have harmful eVects upon the cell. In the course of its reaction with
these activated oxygen species, the thiol side chain of glutathione (GSH) is
converted into an oxidised disulphide (GSSG). Reduced glutathione is regenerated
in the cell by the enzyme glutathione reductase (see Figure 6.22), which
uses NADPH as reducing equivalent and contains one equivalent of tightly
bound FAD.
Glutathione reductase is a dimer of identical 50-kDa protein sub-units.
Each sub-unit contains FAD- and NADPH-binding domains which are composed
of a babab secondary structural motif common to many nucleotidebinding
proteins. The active site of the enzyme lies at the interface of the two
protein sub-units. On one face of the Xavin cofactor is the NADPH cofactor
responsible for generation of reduced FADH 2 , as shown in Figure 6.23a. On
the other face of the Xavin cofactor are two active site cysteine residues (Cys-46
and Cys-41) which form an active site disulphide at the start of the catalytic
cycle.
H 3 N +
H 3 N
+
CO 2
−
CO 2
−
O
O
H
N
N
H
S
S
O
O
oxidised glutathione
N
H
H
N
CO 2
−
CO 2
−
NADPH
glutathione
reductase
FAD
NADP +
2
+
H 3 N
CO 2
−
H
N
O
HS
O
N
H
reduced glutathione
CO 2
−
Figure 6.22 Glutathione reductase.