NYMC Faculty Publications
Zinc Switch in Pig Heart Lipoamide Dehydrogenase: Steady-State and Transient Kinetic Studies of the Diaphorase Reaction
DOI
10.1134/S0006297920080064
Journal Title
Biochemistry. Biokhimiia
First Page
908
Last Page
919
Document Type
Article
Publication Date
8-2020
Department
Cell Biology and Anatomy
Abstract
Elevation of intracellular Zn2+ following ischemia contributes to cell death by affecting mitochondrial function. Zn2+ is a differential regulator of the mitochondrial enzyme lipoamide dehydrogenase (LADH) at physiological concentrations (Ka = 0.1 µM free zinc), inhibiting lipoamide and accelerating NADH dehydrogenase activities. These differential effects have been attributed to coordination of Zn2+ by LADH active-site cysteines. A detailed kinetic mechanism has now been developed for the diaphorase (NADH-dehydrogenase) reaction catalyzed by pig heart LADH using 2,6-dichlorophenol-indophenol (DCPIP) as a model quinone electron acceptor. Anaerobic stopped-flow experiments show that two-electron reduced LADH is 15-25-fold less active towards DCPIP reduction than four-electron reduced enzyme, or Zn2+-modified reduced LADH (the corresponding values of the rate constants are (6.5 ± 1.5) × 103 M-1·s-1, (9 ± 2) × 104 M-1·s-1, and (1.6 ± 0.5) × 105 M-1·s-1, respectively). Steady-state kinetic studies with different diaphorase substrates show that Zn2+ accelerates reaction rates exclusively for two-electron acceptors (duroquinone, DCPIP), but not for one-electron acceptors (benzoquinone, ubiquinone, ferricyanide). This implies that the two-electron reduced form of LADH, prevalent at low NADH levels, is a poor two-electron donor compared to the four-electron reduced or Zn2+-modified reduced LADH forms. These data suggest that zinc binding to the active-site thiols switches the enzyme from one- to two-electron donor mode. This zinc-activated switch has the potential to alter the ratio of superoxide and H2O2 generated by the LADH oxidase activity.
Recommended Citation
Gazaryan, I. G., Shchedrina, V. A., Klyachko, N., Zakhariants, A. A., Kazakov, S. V., & Brown, A. M. (2020). Zinc Switch in Pig Heart Lipoamide Dehydrogenase: Steady-State and Transient Kinetic Studies of the Diaphorase Reaction. Biochemistry. Biokhimiia, 85 (8), 908-919. https://doi.org/10.1134/S0006297920080064
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