CRYSTAL-STRUCTURE OF INORGANIC PYROPHOSPHATASE FROM THERMUS-THERMOPHILUS

The 3-dimensional structure of inorganic pyrophosphatase from Thermus thermophilus (T-PPase) has been determined by X-ray diffraction at 2.0 Angstrom resolution and refined to R = 15.3%. The structure consists of an antiparallel closed beta-sheet and 2 alpha-helices and resembles that of the yeast enzyme in spite of the large difference in size (17 4 and 286 residues, respectively), little sequence similarity beyond t he active center (about 20%), and different oligomeric organization (h exameric and dimeric, respectively). The similarity of the polypeptide folding in the 2 PPases provides a very strong argument in favor of a n evolutionary relationship between the yeast and bacterial enzymes. T he same Greek-key topology of the 5-stranded beta-barrel was found in the OB-fold proteins, the bacteriophage gene-5 DNA-binding protein, to xic-shock syndrome toxin-1, and the major cold-shock protein of Bacill us subtilis. Moreover, all known nucleotide-binding sites in these pro teins are located on the same side of the P-barrel as the active cente r in T-PPase. Analysis of the active center of T-PPase revealed 17 res idues of potential functional importance, 16 of which are strictly con served in all sequences of soluble PPases. Their possible role in the catalytic mechanism is discussed on the basis of the present crystal s tructure and with respect to site-directed mutagenesis studies on the Escherichia coli enzyme. The observed oligomeric organization of T-PPa se allows us to suggest a possible mechanism for the allosteric regula tion of hexameric PPases.