Biology Reference
In-Depth Information
Figure 17:
The elucidated structure of MnSOD of
Lyngbya valderian
a BDU20041 is a dimer with 6 helices and 3 sheets. Depicted
above is a monomer showing helices (red) and strands (blue). Structures are visualized using WebLab ViewerLite 4.2 software
[http://in.msi.com/]. Picture with the kind permission of D. Prabaharan, National Facility for Marine Cyanobacteria (Sponsored
by Department of Biotechnology, Govt. of India), Department of Marine Biotechnology, School of Marine Sciences, Bharathidasan
University, Tiruchirappalli, Tamil Nadu, India. [Priya
et al.
(2010)
Saline Systems
6:
6; doi:10.1186/1746-1448-6-6].
Color image of this figure appears in the color plate section at the end of the topic.
Asp-Pro-Ala) that provides a unique identity to Ni-DODs. These essentially possess four α-helices
held by a short connecting α-helix (Priya
et al
., 2007).
The overall disproportionation reaction (Reaction 1) is accomplished by the SODs in two steps
which are of the fi rst order with respect to O
2
•-
anion. This is explained by the 'ping-pong' mode
of enzyme action. The two steps are sequential and the enzyme gets reduced (Reaction 2a) in the
fi rst step and this reduced enzyme participates in the second step by binding another O
2
•-
anion
with the generation of H
2
O
2
and the release of the oxidized enzyme. These two steps can be
represented as:
Ez
Ox
+ O
2
•-
+ H
+
E
red
(H
+
) + O
2
(Reaction 2a)
Ez
red
(H
+
) + O
2
•-
+ H
+
Ez
OX
+ H
2
O
2
(Reaction 2b)
where Ez
Ox
and Ez
red
represent the oxidized and reduced states of the SODs. Accordingly, Fe
3
and
Fe
2+
, Cu
2+
Zn
2+
and Cu
+
Zn
2+
, Mn
3+
and Mn
2+
, Ni
3+
and Ni
2+
represent the oxidized and reduced states
respectively of the metal ions. The net positive electrostatic potential near the active site of the SODs
and the negative charge of O
2
•-
radical facilitates substrate binding. The ping-pong mechanism of
action exhibited by SODs is explained on the basis that the reduced enzyme is still be able to bind
the O
2
•-
in the second step due to preservation of the electrostatic attraction of the positive charge
upon metal ion reduction by the coupled uptake of a proton. It means that O
2
•-
binding is favoured
to two different versions of the same active site. The uptake of one proton in the fi rst half of the
reaction seems to provide suffi cient driving force for substrate reduction (and binding) and the
excess energy available is thereby utilized in the second step (Miller, 2004).
