[ccp4bb]: meta phosphate ion?!! (fwd)

["S. Parthasarathy" <spartha@sanketh.mbu.iisc.ernet.in>]

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	Dear all,

	This is not a CCP4 related question.  However, I hope some
	could help me out.

	We have refined a TIM-2PG (2-phosphoglycerate) structure at
	1.1 ang.  The asymmetric unit contains a dimer.  The ligand
	2PG properly defined in one of the subunits.  However, the
	electron density for 2PG in the other subunits was broken at
	O-P bond.  Hence, this was built as 2-oxoglycerate (2OG) and
	PO3H.  Breaking of 2PG into these species is not expected of
	TIM.

	my question is concerned to PO3H.  PO3H (HPO3 or PO3-) is a
	metaphosphoric acid or metaphophate  ion.  The electron density
	quite clearly confirm this. My problem is that it is known that
	metaphosphates does not exist as single species due to their
	stability properties. Therefore, we are having a situation where a
	single metaphoshpate ion has been trapped in an enzyme's acitve
	site.
	
	(a) is this possible for a metaphosphate ?

	(b) can any one suggests references where such unexpected,
	unstable species which known to be multimeric have been trapped in
	the enzyme active site as a single molecular species ?    

	(c) There is also a remote chance that the density could also
	be a Acetate ion (used during crystallization).  What are the best
	way and means of distinguishing between a metaphosphate ion and
	an Acetate ion at 1.1 ang. ?

	Thanking you all for ur forthcoming responses.

	S.Parthasarathy.	
	Molecular Biophysics Unit
	Indian Institute of Science
	Bangalore 560 012 India.