REACTIVE OXYGEN SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 133

TABLE 4.2. Individual Rate Constants in the Kinetic Mechanism of Different
MnSODs and Their Mutants

k 26 ′ (/s) MnSOD k 24 (/nM/s) k 25 (/nM/s) k 26 (/nM/s) k 27 (/s)

Human 1.5 1.1 1.1 - 120

y34A 0.25 <0.02 0.38 1600 330

y34N 0.14 <0.02 0.15 850 200

y34H 0.07 <0.02 0.04 250 61

y34V 0.15 0.15 - 1000

<0.02

y34F 0.55 <0.02 0.46 - 52

H30N 0.21 0.40 0.68 - 480

H30Q 0.57 0.79 0.79 - 200

H30V 0.005 0.03 0.16 - 0.7

E162D 0.36 0.13 0.21 - 40

E162A 0.06 0.05 0.09 - 30

F66A 0.6 0.5 0.7 - 82

F66L 0.7 0.8 0.2 - 40

W123F 0.76 <0.02 0.64 - 79

y166F 0.2 0.2 0.2 - 270

W161A 0.08 <0.01 0.37 - 180

W161F 0.3 <0.01 0.46 - 33

W161V na na 0.27 - 265

W161y na na 0.20 - 130

W161H na na 0.29 - 136

H30F/y166F 0.1 0.1 0.1 - 440

y34F/W123F 0.55 <0.22 0.46 - 52

E. coli 1.1 0.9 0.2 - 60

Deinococcus
radiodurans

1.2 1.1 0.07 - 30

y34F 0.9 0.9 0.5 - 30

Data were taken from Abreu and cabelli [31] with the permission of the Elsevier Inc.
na, not available.

Table 4.2 displays the variation of rate constants of reactions (4.24)-(4.27)
among the various species [31]. Reaction (4.25) represents an outer-sphere
mechanism, while reactions (4.26) and (4.27) are of inner-sphere mechanism
pathways. The ratios of k 25 / k 26 can thus distinguish outer-sphere and inner-
sphere mechanisms for the reactions of O •− with MnSODs. In reaction (4.27),
dissociation of the complex resulted in H 2 O 2 , and hence, the value of k 27
reflected the rate for the protonation of O •− in the complex. Both reactions
(4.25) and (4.27) were highly affected by the residue mutations involved in the
hydrogen-bounded network (Table 4.2). The mutation of y34 to histidine,
glutamine, phenylalanine, and valine resulted in large variations in k 27 (see
Table 4.2). Both E. coli and D. radiodurans had two and four times slower k 27
than for human enzymes, respectively. The change in histidine 30 in H30N and