[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Superoxides, etc.

> Date: Mon, 10 Jul 2000 14:53:25 -0600 (MDT)
> From: George Booth <booth at lvld_agilent.com>
> Subject: Re: algae, H2O2, SOD, et al.
> I won't even *try* to translate this into something common folk like me can 
> understand, other than to note that SOD stands for "superoxide dismutase". 

well, maybe i can help.  i studied SOD in yeast briefly, early in my
graduate career.

> Superoxide is O2 with a full negative charge and an extra electron so it is
> deisgnated as O2- with a dot above the -.
> There are other oxygen free radicals such as hydroxyl radical (.OH).  

So the basic scheme is this (Take from Stryer, _Biochemistry_):

O2 --> add e- to form O2*- (superoxide anion) --> add H+ to form HO2*-
(hydroperoxyl radical)  --> react w/another superoxide anion to form H2O2
(hydrogen peroxide)

Superoxide anion is formed when you have only a partial reduction of O2 -
usually you want it to take four electrons, to form two molecules of H2O,
but sometimes only a partial reduction might occur.  So to be safe for the
cell, the catalyst can't release these partially reduced products.  

But sometimes, it will still happen - which is why higher organisms have
things like superoxide dismutase.  Now SOD takes two superoxides, adds two
hydrogens, and turns them into H2O2 and O2 - but the H2O2 is still
damaging to the cells, so that must be converted to something safe.
Catalase takes two H2O2 molecules and converts them to 2 H2O + O2.  

A quote from a recent paper on yeast SOD (LB Corson et al., JBC, 1999)
explains why/how them there radicals can be damaging to the cells:

"Aerobic organisms are chronically exposed to potentially harmful reactive
oxygen species generated as by-products of cellular metabolism...
Superoxide radicals can oxidize iron-sulfur cluster proteins liberating
iron. Thus, Sod1p-deficient yeast (sod1 yeast) may have increased
pools of reactive iron as has been shown for SOD1-deficient
Escherichia coli. "Free" iron can react with hydrogen peroxide (H2O2),
and possibly other reactive oxygen species, to generate toxic hydroxyl
radicals (OH) by Fenton chemistry. Hydroxyl radicals have the
potential to damage proteins, nucleic acids, and membranes."

This extra unpaired electron found in radicals cause it to be a highly
unstable and damaging molecule - b/c it wants to have paired electrons, it
may "steal" them from other molecules it runs into - and that can change
the structure or properties of other molecules, doing damage to DNA, etc.

And, if memory serves (and it may well not), radicals are not consumed in
the reaction, since the stealing of electrons from something else can
cause them to turn into radicals themselves...  or something.  I think to
stop the reaction, two radicals have to run into each other - but the
likelihood of that is too low to be practical...  or something like that.  
If someone cares, I'll look up the details when I get home.  It's been
eight years since my undergraduate chem class.  Plus i've had a 101 F
fever for the last two days.

> Since the rotting of the straw prevents algae from developing, but doesn't
> affect the growth of fish or higher plants, I figured that algae must lack
> the protective mechanisms against oxyradicals such as catalase and SOD found
> in higher organisms.  Since I don't know plant physiology, I do not know
> what enzymes algae may have for defending against oxyradicals.

This makes good logical sense to me.  I'm not a plant person, either - but
I do know that so-called "lower" organisms can use a host of other enzymes
or compounds to combat radicals.  my understanding is that these
substitute solutions are not as efficient as SOD/catalase, but it's not
like they have NO defenses whatsoever.  You can't make it in an aerobic
environment without coming up with some kind of combative strategy.

And my other, purely unrelated question - how is it, if we are all
receiving messages as a digest (according to the welcoming message from
the APD majordomo: "This list is only available as a digest."), that I am
seeing questions and answers to those questions all in the same digest?
Some of you have extrasensory perception, perhaps?  :D