

We compared the pKa of H2O bound to a hemepeptide to pKas in 
hemoproteins in J. Inorg. Biochem, 27, 245-254 (1986)

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From: Steve Cabaniss <[email-masked]>

     The hydrolysis constant of a metal can be written as an acid dissociation
constant, comparable to the  Ka for any 'normal' acid.  The pKa's can thus be
looked up in any standard compendium (i.e., Smith and Martell).  I believe the
most authoritative review is "Hydrolysis of Cations" by Baes and Mesmer.
(Of course, this is all simple aqueous solution chemistry, not the active sites
 of proteins.)  Fe(III) thus 'induces' a water pKa of 2-3.

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From: [email-masked] (Yajun Zheng)

 You asked about references concerning pK's of metal-coordinated waters in 
 enzymes.  I have several references. The most famous case is Zn 
 metalloenzymes. The pKa for Zn-Water in human carbonic anhydrase II (HCAII) 
 is 7. In alcohol dehydrogenase (ADH) is about 7.6. In SOD the Cu-Water is 
 about 9. The pKa values usually vary depending on active site environments. 
 In xylose isomerase the value is about 7-9 (in this case, the metals are 
 either Mg2+ or Mn2+). There are also other examples such as DNA related 
 enzymes. 

     For ADH, Kvassman, J.; Pettersson, G. Eur. J. Biochem. 1980, 103, 565.
     also, in Zinc Enzymes; Bertini, I. et al. 1986.

     For HCAII, See the above book or Zheng and Merz, JACS 1992, 114, 10498.

     For SOD, I do not have the exact reference, but you could check Joan 
     Valentie papers.  

     For xylose isomerase and DNA related enzymes, there is no exact numbers 
     and most are just guesses. 

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From: [email-masked] (Ulf Ryde)

For horse liver alcohol dehydrogenase,
G. Pettersson, CRC Crit. Rev. Biochem. 21 (1987)349-389
gives an excellent reviev of pKa values of zinc bound water
(an other ligands) in different complexes.

For carbonic anhydrase, 
Campbell et al., J. Mol. Biol. 90 (1974) 469-489 reports
a pKa between 7 and 8.

For carboxypeptidase the situation is more complicated, but
Auld et al. Biochem. 31 (1992) 3840 gives a pKa of 9.0

Similarily, for thermolysin,
Mock et al., Biochem. J. 302 (1994) 57-68 
provides a recent discussion of pKa values.

Aquist and Warshel, J. Am. Chem. Soc. 112 (1990) 2860-2868
have a more general discussion about pKa values for 
metal bound water molecules in different enzymes.

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