H. Deng et al., SOURCE OF CATALYSIS IN THE LACTATE-DEHYDROGENASE SYSTEM - GROUND-STATE INTERACTIONS IN THE ENZYME-SUBSTRATE COMPLEX, Biochemistry, 33(8), 1994, pp. 2297-2305
The Ramam spectra of both the NAd-pyruvate and the pyridine aldehyde a
denine dinucleotide (PAAD)-pyruvate bound to pig heart, pig muscle, an
d bacillus stearothermophilus lactate dehydrogenases were measured and
are nearly the same, which is consistent with the conserved shell of
residues surrounding the active8site cavity in these enzymes. The symm
etrical stretching mode of the pyruvate carboxylate group, found at 13
98 cm(-1), is shifted only slightly when complexed to these enzymes, w
hich shows that the group remains ionized in the ion pair complex with
Arg-171 on the enzyme. The vibrational mode for the carbonyl stretch
of the bound pyruvate moiety is shifted about 35 cm(-1) to a lower fre
quency than observed for the carbonyl of unliganded pyruvate in the ba
cterial enzyme because of polarization of the carbonyl bond. Thus, the
bacterial enzyme shows the same substrate activation because of the C
+-O- charge separation that was seen previously with the mammalian enz
ymes. On the basis of an empirical Badger-Bauer relationship between f
requency shift and interaction enthalpy, this shift in frequency is eq
uivalent to an approximately -14 to -17 kcal/mol interaction between t
he enzyme and the adduct C=O coordinate, a substantial part of which i
s an electrostatic interaction (hydrogen bond) between the C=O and the
protonated His-195. Thus, while the C=O bond is polarized on the enzy
me (which requires energy), the overall ground-state enthalpy of the c
arbonyl imidazolium part of the reaction coordinate is stabilized subs
tantially relative to its value in solution, and this is the dominant
enthalpic effect on the entire reaction coordinate since the other int
ernal coordinates for the hydride transfer are not much affected durin
g formation of the ternary complex. The total enthalpy of binding for
pyruvate analogs to lactate dehydrogenase is nearly the same as the su
m of local enthalpies for interactions between pyruvates C=O and the p
rotein. Thus, even though ligand binding may cause any number of large
, mostley compensating effects. The Raman spectra of the PAAD-pyruvate
adduct bound to two different sets of mutant forms of the bacterial e
nzyme also were measured. Mutation of Arg-109, which normally hydrogen
bonds to the pyruvate C=O, to Gln-109, reduces the extent of C+-O- ch
arge separation by about 4 kcal/mol. Similarly, mutation of Asp-168 or
Ala-168 also reduces charge separation but to a somewhat greater degr
ee, 4 and 10 kcal/mol, respectively. The ground state for the carbonyl
imidazolium interactions of the mutant complexes is thus destabilized
relative to the wild-type enzyme, and yet, the height of the transitio
n-state barrier must increase, which clearly indicates the height of t
he barrier must increase faster than the ground state is destabilized.
This view is analyzed from a plot of the log of the hydride transfer
step versus the change in frequency of the C=O stretch (or ground-stat
e interaction enthalpy).