Chemistry Reference
In-Depth Information
11.2.2 Catalase
Hydrogen peroxide results from the two-electron reduction of O
2
, and is
ubiquitous in biological systems. A key process leading to hydrogen peroxide
production in vivo is by the reaction between superoxide and SOD, as described
above. In food systems, hydrogen peroxide can be produced non-enzymatically
as a result of polyphenol oxidation (Halliwell, 2008; Long et al., 1999). While
hydrogen peroxide is a non-radical species and is not a particularly potent
oxidant, it is easily reduced by metal catalysis or ultraviolet light to highly
reactive
·
OH radicals.
One of nature's answers to this problem is catalase (CAT), a heme-containing
enzyme found in many biological systems that catalyzes the reduction of
hydrogen peroxide to water by the following pathway:
2H
2
O
2
ÿ! 2H
2
O O
2
In plants, hydrogen peroxide can also be removed by ascorbate peroxidase
via the following mechanism:
2 ascorbate H
2
O
2
ÿ! 2 monodehydroascorbate 2H
2
O
11.2.3 Glutathione peroxidase
Most biological tissues also contain the enzyme glutathione peroxidase (GSH-
Px). GSH-Px is capable of deactivating both hydrogen and lipid peroxides.
GSH-Px contains a selenium ion within its active site and reduced glutathione
(GSH) to reduce hydrogen peroxide or lipid hydroperoxide to water:
H
2
O
2
2GSH ÿ! 2H
2
O GSSG
or
LOOH 2GSH ÿ! LOH H
2
O GSSG
where GSSG is oxidized glutathione and LOH is a fatty acid alcohol.
11.3 Metal chelation
11.3.1 Specialized iron-binding proteins
Redox active transition metals such as iron and copper are capable of catalyzing
the decomposition of hydroperoxides and are important prooxidants in food
lipids. There are several important proteins found in food whose sole biological
function is to chelate, store and/or transport metals (e.g., iron). Many of these
proteins play a critical role in vivo by maintaining a near zero steady-state
concentration of free iron that would otherwise catalyze unwanted oxidation
reactions. Some examples of specialized iron-binding proteins include trans-
ferrin, lactoferrin, ovotransferrin (conalbumin), phosvitin, and ferritin. Trans-
ferrin (blood) and lactoferrin (milk) bind two ferric ions each, while
ovotransferrin binds three ferric ions. Haptoglobins and hemopexins are also
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