Role of glutamate-104 in generating a transition state analogue inhibitor at the active site of cytidine deaminase.
The 19F-NMR resonance of 5-[19F]fluoropyrimidin-2-one ribonucleoside moves upfield when it is bound by wild-type cytidine deaminase from Escherichia coli, in agreement with UV and X-ray spectroscopic indications that this inhibitor is bound as the rate 3,4-hydrated species 5-fluoro-3,4-dihydrouridine, a transition state analogue inhibitor resembling an intermediate in direct water attack on 5-fluorocytidine. Comparison of pKa values of model compounds indicates that the equilibrium constant for 3,4-hydration of this inhibitor in free solution is 3.5 x 10(-4) M, so that the corrected dissociation constant of 5-fluoro-3,4-dihydrouridine from the wild-type enzyme is 3.9 x 10(-11) M. Very different behavior is observed for a mutant enzyme in which alanine replaces Glu-104 at the active site, and kcat has been reduced by a factor of 10(8). 5-[19F]Fluoropyrimidin-2-one ribonucleoside is strongly fluorescent, making it possible to observe that the mutant enzyme binds this inhibitor even more tightly (Kd = 4.4 x 10(-8) M) than does the native enzyme (Kd = 1.1 x 10(-7) M). 19F-NMR indicates, however, that the E104A mutant enzyme binds the inhibitor without modification, in a form that resembles the substrate in the ground state. These results are consistent with a major role for Glu-104, not only in stabilizing the ES++ complex in the transition state, but also in destabilizing the ES complex in the ground state.
Carlow
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Department of Biochemistry and Biophysics, University of North Carolina, Chapel Hill 27599-7260, USA.
Short
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Wolfenden
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D001665:Binding Sites; D003564:Cytidine Deaminase / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000378:metabolism*; D005453:Fluorescence; D018698:Glutamic Acid; D009682:Magnetic Resonance Spectroscopy; D013056:Spectrophotometry, Ultraviolet
D004791:Enzyme Inhibitors; D018698:Glutamic Acid; D003564:Cytidine Deaminase