"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.

A parable regarding paradoxes:  I have recently been researching electric
power generation.  There are several methods of generating power, but I
have learned that all of them require steel.  So I looked into how steel
is produced - it turns out that steel is produced in electric arc furnaces!
Paradox???
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