Crystal versus solution structures of enzymes: NMR spectroscopy of a crystalline serine protease.
The hydrogen-bonding status of His57 in the catalytic triad (Asp-His-Ser) of serine protease has important mechanistic implications for this class of enzymes. Recent nitrogen-15 nuclear magnetic resonance (NMR) studies of alpha-lytic protease find His57 and Ser195 to be strongly hydrogen-bonded, a result that conflicts with the corresponding crystallographic studies, thereby suggesting that the crystal and solution structures may differ. This discrepancy is addressed and resolved in a nitrogen-15 NMR study of the enzyme in the crystalline state. The results show that the His-Ser and Asp-His interactions are identical in crystals and solutions, but that in crystals His57 titrates with a pKa of 7.9, nearly one pKa unit higher than in solution. This elevated pKa accounts for the absence of the His-Ser hydrogen bond in previous x-ray studies.
Smith
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SO
Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge 02139.
Farr-Jones
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Griffin
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Bachovchin
W W
WW
D003460:Crystallization; D006639:Histidine; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D012694:Serine; D012697:Serine Endopeptidases; D012996:Solutions; D014961:X-Ray Diffraction
D012996:Solutions; D012694:Serine; D006639:Histidine; D012697:Serine Endopeptidases