The Alternative Oxidase: in vivo Regulation and Function

F. F. Millenaar; H. Lambers

doi:10.1055/s-2003-37974

Plant Biology, Inhaltsverzeichnis

Plant Biol (Stuttg) 2003; 5(1): 2-15
DOI: 10.1055/s-2003-37974

Review Article

Georg Thieme Verlag Stuttgart ·New York

The Alternative Oxidase: in vivo Regulation and Function

F. F. Millenaar ¹ , H. Lambers ^1,2

¹ Plant Ecophysiology, Utrecht University, Utrecht, The Netherlands
² School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

Abstract

This review focuses on the biochemical regulation and function of the alternative oxidase in vivo. About 10 years ago, two activation mechanisms were discovered in isolated mitochondria, namely activation by reducing sulfur bonds in the protein and activation by an allosteric effect of pyruvate. It was proposed that plants would have a regulatory mechanism to modify alternative oxidase activity in vivo. However, more recent studies have shown that these two activation mechanisms may not play such an important role in regulation of alternative oxidase activity in vivo after all. Pyruvate and reduction of the sulfide bonds in the protein are definitely required for alternative oxidase activity, but they do not appear to be regulating the activity in vivo.

Despite the energy wasting nature of the alternative oxidase, there was no obvious physiological function for the pathway for many years. It is now more clear that the alternative oxidase can prevent the production of excess reactive oxygen species radicals by stabilizing the redox state of the mitochondrial ubiquinone pool, while allowing continued activity of the citric acid cycle. This may be important under conditions when the NADH supply is relatively high (reductant overflow), or when the cytochrome pathway is restrained. The cytochrome pathway might be inhibited by naturally occurring cyanide, nitric oxide, sulfide, high concentrations of CO₂, low temperatures, or by limited phosphate supply.

Abbreviations

AOX: alternative oxidase

BHAM: benzhydroxamic acid

SHAM: salicylhyroxamic acid

Q: ubiquinone

Q_ox: oxidized ubiquinone

Q_r: reduced ubiquinone

Q_t: total pool of ubiquinone

Q_r/Q_t: relative reduction state of the ubiquinone pool

Key words

Alternative oxidase - respiration - ubiquinone - oxygen free radicals - pyruvate

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F. F. Millenaar

Plant Ecophysiology
Utrecht University

Sorbonnelaan 16
3584 CA Utrecht
The Netherlands

eMail: f.f.millenaar@bio.uu.nl

Section Editor: L. A. C. J. Voesenek