"Perplexed in Peoria"  wrote in message 
news:cppsj2$2qad$1{at}darwin.ediacara.org...
>
> "IRR"  wrote in message 
> news:cpok29$2ekk$1{at}darwin.ediacara.org...
> [ ... Which came first - THF or purines? ...]
>> Thanks for the reply.  A more generic overview is here:
>> http://web.indstate.edu/thcme/mwking/nucleotide-metabolism.html
>> (the section on purine biosynthesis), or a mechanistic view here:
>> http://www.chem.qmul.ac.uk/iubmb/enzyme/reaction/misc/purine1.html
>> N10-formyl-THF is apparently the source of the formyl group (giving THF 
>> as
>> product) in "ring building" in both this early step
pathway and again in 
>> a
>> later antepenultimate step towards IMP biosynthesis.  But my 
>> understanding
>> of THF biosynthesis is that the purine GTP is explicitly used in pterin
>> construction, which are essential THF precursors.  I'm embarrassed that I
>> can't figure this out -- there ought to be a simple solution to this
>> apparent paradox?
>
> Biochemistry is filled with "loops" like this.  There is no
"paradox".
> Only a chicken-egg puzzle in the context of OOL.  The generic solution
> to this kind of puzzle is to assume that the current pathways are not
> the primordial pathways.  In this case, it is pretty clear that guanine
> was being produced by some pathway not involving THF long before the
> modern pathway was "invented".  All we need is some donor of a formyl
> group - THF is a convenient source of formyl groups today, but some
> other source may have been available in early life.  Or, a completely
> different pathway for purine production was used.  Some people even
> claim that purines were produced in quantity by non-biological processes.

Thanks so much for the response.  So I now understand how this circumvents 
my (pseudo) paradox, but this of course requires that these biosynthetic 
precursors have been sustained parent-to-progeny since that primordial 
pathway.  Is that an acceptable formulation from a biologist's perspective? 
It seems a bit fragile to maintain those sorts of contingencies, i.e. rather 
than maintaining some pathway to go from CO2 directly to THF without 
requiring THF-derived intermediates, but I suppose if you're pinning a 
pathway to some essential intermediate, using a DNA base is probably a safe 
bet for its ubiquity and for the prebiotic routes of synthesis you allude 
to.  Is that thinking correct?

I'd be interested to know of other examples of such loops, sounds like a 
potentially interesting lot from an OOL/evolution perspective.
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