Re: How is H2O2 produced in cells and why is it a problem?

Hello,
I apologize for the slow response.
This is largely taken from Styer's Biocemistry text, 3rd Ed., Page 422

Catalase does not directly and deliberately affect any particular enzyme 
pathway. Its purpose is is to remove toxic peroxide that is derived mainly 
as a byprduct of the production of ATP in the mitochondria from the 
complete oxidation of glucose. Peroxide is an indirect product of the 
occasional incomplete reduction of oxygen to water. The stepwise transfer 
of elections to oxygen is tightly controlled by a series of protein 
complexes in the inner mitochondrial membrane. However, at a low rate, an
electron is transferred from a metal ion inside one of the catalysts to
an oxygen molecule to produce superoxide. Superoxide is very reactive and
can spontaneously damage a nearby protein. It can also be spontaneously
or enzymatically converted to hydrogen peroxide, a less reactive but still
damgerous molecule to have in the cell (see Stryer, noted above). 
Catalases are used to scavenge for and destoy this peroxide.

It is also deliberately produced in a few cases, most notably by 
neutrophils and macrophages to destroy bacteria. However, catalase seems
to be around mostly to scavenge for peroxide that is a byproduct of the 
oxidative phosphorylation process.

Superoxide and peroxide are dangerous because they are very reactive and
can oxidize a large number of substances including side chains of proteins
and lipid derivatives that are a part of the membrane, thus damaging them.
DNA bases are also susceptible to oxidation by peroxide.

Regards,
Jeff