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The MutT enzyme catalyzes the hydrolysis of nucleoside triphosphates to
nucleoside monophosphates and pyrophosphate by substitution at the rarely
attacked beta-phosphorus. Nucleotides containing bulky substituents at the 8
position of the purine ring are preferentially hydrolyzed. The conformation of
the MutT-bound nonhydrolyzable substrate analog Mg(2+)-AMPCPP, determined by 10
intramolecular NOEs and molecular dynamics refinement using a full relaxation
matrix analysis with back-calculation of the NOESY intensities, is high anti
(chi = 53 +/- 9 degrees), with a C2'-exo, O1'-endo sugar pucker. Similarly, the
product of dGTP hydrolysis, dGMP, also binds MutT in a high anti (chi = 73 +/- 9
degrees) C1'-endo conformation based on seven intramolecular NOEs. Such high
anti rotations of the base would allow MutT to accommodate nucleotides
substituted at the C-8 position with no intramolecular clashes. Changes in
chemical shifts in the 1H-15N spectra of the enzyme induced by Mg2+ and Mg2+
AMPCPP suggest that the metal activator and nucleotide interact with residues in
loop I, at the carboxyl end of helix I, loop II, loop III, and beta-strands A
and B of the secondary structure of MutT. The displacement of Mg2+ by Mn2+
causes the selective disappearance due to paramagnetic broadening of 1H-15N
cross peaks from G37, G38, and K39 in loop I and E57 in helix I. Eleven
intermolecular NOEs between Mg2+AMPCPP and hydrophobic residues of MutT are
found, three of which are tentatively assigned to L67 in loop II and three to
L54 in helix I. Similarly, seven intermolecular NOEs between dGMP and
hydrophobic residues of the enzyme are found, four of which are tentatively
assigned to L54 and two to V58, both in helix I. These interactions indicate
that the loop I-helix I-loop II motif contributes significantly to the active
site of MutT in accord with mutagenesis studies and with sequence homologies
among MutT-like NTP pyrophosphohydrolases.
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