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Reaction mechanisms of thiamin diphosphate enzymes: new insights into the role of a conserved glutamate residue
Author(s) -
Shaanan Boaz,
Chipman David M.
Publication year - 2009
Publication title -
the febs journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.981
H-Index - 204
eISSN - 1742-4658
pISSN - 1742-464X
DOI - 10.1111/j.1742-4658.2009.06965.x
Subject(s) - chemistry , enzyme , residue (chemistry) , cofactor , stereochemistry , glutamate receptor , biochemistry , active site , carboxylate , mutagenesis , mutation , receptor , gene
Subsequent to the demonstration in the late 1950s of the catalytic power of the C2 anion/ylid of thiamin diphosphate, further convincing evidence was provided demonstrating that the 4′‐aminopyrimidine group plays a vital role in activation of this cofactor. Structural evidence from several crystal structures of thiamin diphosphate‐dependent enzymes emphasized the presence of a glutamate residue in hydrogen‐bonding distance from N1′ as a conserved element in these enzymes. The important role of this conserved glutamate in promoting C2‐H ionization and activation of thiamin diphosphate was emphasized by site‐directed mutagenesis studies. This role was further elaborated by spectroscopic studies of the 4′‐aminopyrimidine–iminopyrimidine tautomerization. The low polarity of the environment of the protein‐bound thiazolium is an additional factor in the stabilization of the C2 anion/ylid. The recently determined crystal structure and mutagenesis studies of glyoxylate carboligase, in which the position of the conserved glutamate is occupied by valine, now show that, for the multi‐step reaction catalyzed by this enzyme, the advantages of accelerating the ionization of C2‐H by re‐introducing a carboxylate are outweighed by the apparent overstabilization of intermediates.