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From: chiaravi@copper.ucs.indiana.edu (Lucius Chiaraviglio)
Newsgroups: sci.chem,sci.bio
Subject: Re: Thermodynamics of reduction of phosphate to phosphine
Summary: Found some info:  reducing phosphate to phosphine isn't going to work
Keywords: marsh gas, wierd organisms
Message-ID: <61152@iuvax.cs.indiana.edu>
Date: 30 Sep 90 04:23:44 GMT
References: <60883@iuvax.cs.indiana.edu> <4067@kitty.UUCP>
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Organization: Department of Biology at Indiana University, Bloomington
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In article <4067@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
>In article <60883@iuvax.cs.indiana.edu>, chiaravi@copper.ucs.indiana.edu
>(Lucius Chiaraviglio) writes:
>> 	Something that has intrigued me (and some microbiologists) for a while
>> is the process, so far as I know almost completely uncharacterized, of
>> phosphine production in marshes.  [. . .]
>
>	It is my belief that an explanation of the mechanism behind the
>biological production of phosphine requires consideration of the various
>processes involved in putrefaction.  It is also my feeling that there
>exists more than one putrefactive mechanism for the production of phosphine.
>
>	Consider that the majority of phosphoglycerides are phosphatidates.
>Further consider that E. coli and other bacteria produce phosphatidylserine
>from L-serine and (I believe) cytidine diphosphoacylglycerol.

	So far, all phosphates. . .

>	Putrefaction chiefly involves deamination and/or decarboxylation.
>The decarboxylation products of phosphatidylserine are carbon dioxide and
>phosphatidylethanolamine.  While free serine apparently will not undergo
>decarboxylation (or deamination, for that matter), the phosphatidyl form
>will.

	That sounds bizarre -- I have read that some methylotrophic bacteria
are able to transaminate serine just fine, to get hydroxypyruvate (meanwhile
converting another alpha-keto-acid into an amino acid), which is then
phosphorylated to 3-phosphoglycerate (which then goes into the glycolytic
pathway -- more explanation in another message, if people are interested).  On
the other hand, phosphorylating the serine in the first place would certainly
work; maybe the bacteria actually do it that way (or maybe both ways), and
somebody determining the serine-involving methylotrophic pathway messed up.
(Sorry, I won't be able to check the references until the University of
Chicago (where I actually am now) online catalog comes back on line :-( ).
Any particular reason why decarboxylation or deamination of serine shouldn't
work?

>	It is therefore my speculation that as a putrefactive process,
>phosphine results from the successive decarboxylation of phosphatidylserine,
>through phosphatidylethanolamine, eventually into phosphine (and perhaps,
>diphosphine).

	Out of curiosity, how would this work?  Phosphatidylethanolamine could
be oxidatively deaminated to O2-phosphatidylhydroxyacetaldehyde, which could
then be oxidized to O2-phosphatidylglycolate (with the energy conserved as a
high-energy phosphate by way of the O1-phospho-O2-phosphatidylglycolate
intermediate which might be expected to be formed by bacterial oxidation of an
aldehyde).  After that, I suppose you could do something really wierd like
decarboxylate this compound (I don't know if that would work) to a methyl
phosphatide, but I don't know what the profit would be in doing that, and the
phosphate still hasn't been reduced.  What next (or where would you branch off
from the pathway I described)?  (Hey, isn't thinking up biochemical pathways
and their reaction mechanisms fun?)

>	I know of no reference to support this speculation, but I suspect
>a modest amount of literature research will readily prove or disprove its
>feasibility.

	Well, last night I stumbled on some information (that is, I managed to
find what actually appears to be a decent chemistry text) which shows that no
reduction of phosphate to phosphine is going to be energetically profitable,
and in fact it would be rather difficult for terrestrial biochemistry to do at
all (and if it did it, it would have to be for something other than an energy-
generation pathway).  See below, and despair.  :-)

>	While most biological phosphorous is in the form of phosphate
>esters and diesters, there are some exceptions.  Consider as an example,
>2-aminoethylphosphonic acid, which is found in some protozoa.

	Anyone know what this does for those protozoa, or how they make it?

>	I don't believe that any free hydrogen under any biological
>conditions is going to result in, or otherwise aid, the formation of
>phosphine.

	For the reasons coming up, you're right.

>	Why don't you do a little investigation along the lines I suggested?
>If you publish a paper as a result, you can include a note of thanks.  :-)

	I stumbled on this information last night, but decided to wait on
posting until others had had a chance to squirm in it.  :-)  Basically, no
matter what the pH in aqueous solution, the reduction of phosphate to
phosphine is strongly endergonic.  Under standard conditions (pH = 0), the
reaction H[3]PO[4] + 8H(+) + 8e(-) --> PH[3] + 4H[2]O consumes 45.1 kcal/mole;
if you modify the conditions so that pH is now physiological (~7), the
reaction will consume much more (because hydrogen ion concentration is
reduced, and it is eighth-order in the equilibrium constant).  Not having
handy at the moment a calculator with a natural logarithm (or any logarithm)
function on it, and having eight minutes to catch my last bus, I will leave it
as an exercise for the reader to determine how much more energy the reaction
would consume at pH = 7.  :-)

|   Lucius Chiaraviglio    |    Internet:  chiaravi@copper.ucs.indiana.edu
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