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The complex formed between the enzyme ribonuclease T1 (EC 3.1.27.3) and its
specific inhibitor 2'-guanylic acid (2'-GMP) has been refined to R = 0.180 using
x-ray diffraction data to 1.9-A resolution. The protein molecule displays a
compact fold; a 4.5 turn alpha-helix packed over an antiparallel beta-pleated
sheet shields most of the hydrophobic interior of the protein against the
solvent. The extended pleated sheet structure of ribonuclease T1 is composed of
three long and four short strands building up a two-stranded minor beta-sheet
near the amino terminus and a five-stranded major sheet in the interior of the
protein molecule. In the complex with ribonuclease T1, the inhibitor 2'-guanylic
acid adopts the syn-conformation and C2'-endo sugar pucker. Binding of the
nucleotide is mainly achieved through amino acid residues 38-46 of the protein.
The catalytically active amino acid residues of ribonuclease T1 (His40, Glu58,
Arg77, and His92) are located within the major beta-sheet which, as evident from
the analysis of atomic temperature factors, provides an environment of minimal
local mobility. The geometry of the active site is consistent with a mechanism
for phosphodiester hydrolysis where, in the transesterification step, His40
and/or Glu58 act as a general base toward the ribose 2'-hydroxyl group and
His92, as a general acid, donates a proton to the leaving 5'-hydroxyl group.
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