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6. Hydrogenases, oxigenases, oxidoreductases, peroxidases and dismutases

Overview

Hydrogenases (often associated with the cofactors NADH or FADH 2)

-
+
-
+
+
-
-
H 2 ' 2H + 2e more correct: H 2 ' H + H (followed by: H → H + 2e )

May be coupled with the transfer/abstraction of hydrogen to/from a substrate
(hydrogenation/dehydrogenation):
-
+
substrateH 2 ' substrate + 2H + 2e

Oxidoreductases generally catalyse oxidations (electron abstraction) and/or reductions
(electron delivery), such as the iron-sulphur proteins or the cytochromes.

Some oxidoreductases use oxygen for the dehydrogenation of a substrate (oxidases) or water
for the hydrogenation of a substrate (reductases):
SubstratH 2 + ½O 2 ' Substrat + H 2O

2-
Oxigenases transfer/insert, usually starting from oxygen O 2, oxo groups (O ) to/into a
substrate:
substrate + O 2 ' substrateoxide/-hydroxide
+
-
often coupled to: ½O 2 + 2H + 2e → H 2O
2-
The reverse process, i.e. the removal of O from a substrate, is catalysed by deoxygenases.
-
Substrates can be organic in nature (RH → ROH; (CH 3) 2S → (CH 3) 2S=O), or inorganic (NO 3
-
→ NO 2 ).

Peroxidasen employ H 2O 2 for oxygenation:
substrate + H 2O 2 → substrateoxide/-hydroxide + H 2O

Dismutases disproportionate oxygen species with the oxygen in the oxidation states –I
(peroxide) and -1/2 (superoxide):
Catalases: H 2O 2 → H 2O + ½O 2
+
Sub steps: H 2O 2 → O 2 + 2H + 2e - (oxidation)
-
+
H 2O 2 + 2H + 2e → 2H 2O (reduction)
- +
Superoxidedismutases: 2O 2 + 2H → H 2O 2 + O 2
-
Sub steps: O 2 → O 2 + e - (oxidation)
+
-
-
O 2 + e + 2H → H 2O 2 (reduction)

Iron-only hydrogenase HN
H δ+
This enzyme catalyses the charge separation
in the H 2 molecule via polarisation between S S [4Fe,4S]
the NH function of the bridging O C H δ- S
aminedithiolate and one of the iron centres, Fe Fe
and finally the bond cleavage to form a C C N
+
-
hydridic Fe-H and a protic R 2NH 2 O C C C
-
intermediate. Electrons from the H are then
transferred off via a [4Fe,4S] ferredoxins in N O O
direct contact with the hydrogenase.