C O MMU N I C A T I O N S
Table 1. First-Order Rate Constants Estimated from Reported
Values for Spontaneous Decarboxylation of Carboxylate Ions and
the pKa Values of Carbon Acids Generated by Their
Decarboxylation, 25 °C
Enzymes that catalyze amino acid decarboxylation use cofactors
such as PLP to stabilize the carbanion generated by elimination of
CO2. In contrast, orotidine 5′-phosphate (OMP) decarboxylase acts
purely as a protein catalyst. This enzyme converts OMP to UMP
with a rate constant (kcat) of 20 s-1, whereas the spontaneous
decarboxylation of OMP occurs with a rate constant of 3 × 10-16
s-1 in the absence of enzyme.13 The anion produced by decarboxyl-
ation of OMP is extremely unstable, with an estimated pKC-H value
of 34 for the 6-CH group of product UMP.14 In effect, this enzyme
finds a way of stabilizing the 6-carbanion of UMP through binding
interactions that are not yet fully understood. If the kcat value for
yeast ODCase is placed on the Brønsted plot in Figure 2, one is
led to infer that the “effective” pKC-H value of UMP, at the active
site of yeast ODCase, is 9.5. The results of point mutation
experiments suggest that the anion of UMP may be stabilized by
interaction with a basic residue (Lys-93 in the yeast enzyme).15 It
would be of interest to know the extent to which the pKa value of
that residue may be perturbed from its value in solution, in the
central complexes that arise during catalysis.
carboxylate ion
k 25°C, (s-1
0.0316
)
∼t1/2
22 s
18 days
8 days
pK
C-H
iminomalonate
acetoacetate
aminomalonate
trichloroacetate
malonate monoanion
cyanoacetate
141
4.5 × 10-7 17
1.1 × 10-6 16
9 × 10-9 19,20
9.5 × 10-11 4,5
3 × 10-12 20
3 × 10-16 13
19.1618
213
2 years
230 years
7000 years
78,000,000 years
2521
25.622
28.911
3414
1-methylorotate
Acknowledgment. This work was supported by NIH Grant GM-
18325.
References
(1) Rios, A.; Crugeiras, J.; Amyes, T. L.; Richard. J. P. J. Am. Chem. Soc.
2001, 123, 7949.
(2) Bailey, G. B.; Chotamangsa, O.; Vuttivej, K. Biochemistry 1970, 9, 3243.
(3) Rios, A.; Amyes, T. L.; Richard J. P. J. Am. Chem. Soc. 2000, 122, 9373.
(4) Fairclough, R. A. J. Chem. Soc. 1938, 1186.
(5) Hall, G. A. J. Am. Chem. Soc. 1949, 71, 269, 12693.
(6) Snider, M. J.; Wolfenden, R. J. Am. Chem. Soc. 2000, 122, 11507.
(7) Thanassi, J. W. Biochemistry 1970, 9, 525.
Figure 2. First-order rate constants estimated for decarboxylation of
carboxylate ions plotted as function of product C-H acidity (Table 1).
substituent effect of -6.1 kcal/mol on the free energy of activation
for decarboxylation, somewhat less pronounced than the imminium
effect of -9.5 kcal/mol on the carbon acidity of glycine.1 That effect
is consistent with the development of a substantial amount of
negative charge at the R-carbon atom in the transition state for
decarboxylation. A Brønsted plot of rate constants for decarboxyl-
ation of aminomalonate, iminomalonate, and other carboxylic acids
(Table 1), as a function of the carbon acidities of the products
generated by their decarboxylation, exhibits a slope (âlg) of -0.7
(Figure 2).
(8) The aldimine produced by reaction of 2-methylaminomalonate with PLP
undergoes decarboxylation in water with a rate constant of 0.01 s-1 at
pH 5, 25 °C (Zabinski, R. F.; Toney, M. D. J. Am. Chem. Soc. 2001, 123,
193). If that value is adjusted for the fraction of 2-methylaminomalonate
present in the zwitterionic form and compared with a value of 2 × 10-7
s-1 for the corresponding spontaneous reaction in water (Callahan, B. P.,
Wolfenden, R., unpublished), we estimate that the rate enhancement
produced by PLP exceeds that produced by acetone by a factor of 102.
(9) Streitwieser, A., Jr.; Boerth, D. W. J. Am. Chem. Soc. 1978, 100, 755.
(10) Aqueous benzoic acid has been reported to undergo measurable conversion
to benzene after incubation at 350 °C for 6 h (Katritzky, A. R.;
Balasubramanian, M.; Siskin, M. Energy Fuels 1990, 4, 499).
(11) Richard, J. P.; Williams, G.; Gao J. J. Am. Chem. Soc. 1999, 121, 715.
(12) Jorgensen, W. L.; Briggs, J. M.; Gao, J. J. Am. Chem. Soc. 1987, 109,
6857.
(13) Radzicka A.; Wolfenden R. Science 1995, 267, 90.
(14) Sievers A.; Wolfenden R. J. Am. Chem. Soc. 2002, 124, 13986.
(15) Miller, B. G.; Wolfenden, R. Annu. ReV. Biochem. 2002, 71, 847.
(16) This work.
(17) Guthrie, J. P.; Jordan F. J. Am. Chem. Soc. 1972, 94, 9136.
(18) Chiang, Y.; Kresge, A. J.; Tang, Y. S.; Wirz, J. J. Am. Chem. Soc. 1984,
106, 460.
If the pKC-H value of benzene is 43,9 extrapolation of the plot
in Figure 2 indicates that the benzoate ion should undergo
decarboxylation in water with a rate constant of ∼10-22 s-1 at
25 °C. If we assume that ∆Sq for benzoate decarboxylation is the
same as the average value observed for the other acids in Table 1
(0.010 kcal K-1 mol-1), then the decarboxylation of the aqueous
benzoate ion should be detectable at temperatures below the critical
point of water. We verified that prediction experimentally by
observing the accumulation of benzene in aqueous solutions of
potassium benzoate (0.05 M) at 344 °C, indicating an apparent rate
(19) Verhoek, F. H. J. Am. Chem. Soc. 1934, 56, 571.
constant of 4 × 10-6 s-1 10
. By the same criterion, the decarboxyl-
(20) Belsky, A. J.; Maiella, P. G.; Brill, T. B. J. Phys. Chem. A 1999, 103,
ation of the acetate ion (methane pKC-H 49)11,12 is expected to be
slower than that of benzoate by a factor of 10 000. We found that
solutions of sodium acetate (0.1 M), showed no trace of decar-
boxylation after incubation at 344 °C for 15 days.
4253.
(21) Ege, S. Organic Chemistry; Heath and Co.: Lexington, MA, 1989; p 100.
(22) Amyes, T. L.; Richard, J. P. J. Am. Chem. Soc. 1996, 118, 3129.
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