Cyanide resistant respiration and the alternative oxidase pathway: A journey from plants to mammals

Mitochondrial alternative oxidase across the tree of life: Presence, absence, and putative cases of lateral gene transfer

The alternative oxidase (AOX) is a terminal oxidase in the electron transport system that plays a role in mitochondrial bioenergetics. The past 20 years of research shows AOX has a wide yet patchy distribution across the tree of life. AOX has been suggested to have a role in stress tolerance, growth, and development in plants, but less is known about its function in other groups, including animals. In this study, we analyzed the taxonomic distribution of AOX across >2800 species representatives from prokaryotes and eukaryotes and developed a standardized workflow for finding and verifying the authenticity of AOX sequences. We found that AOX is limited to proteobacteria among prokaryotes, but is widely distributed in eukaryotes, with the highest prevalence in plants, fungi, and protists. AOX is present in many invertebrates, but is absent in others including most arthropods, and is absent from vertebrates. We found aberrant AOX sequences associated with some animal groups. Some of these aberrant AOXs were contaminants, but we also found putative cases of lateral gene transfer of AOX from fungi and protists to nematodes, springtails, fungus gnats, and rotifers. Our findings provide a robust and detailed analysis of the distribution of AOX and a method for identifying and verifying putative AOX sequences, which will be useful as more sequence data becomes available on public repositories.

Xenotopic expression of alternative oxidase (AOX) to study mechanisms of mitochondrial disease

The mitochondrial respiratory chain or electron transport chain (ETC) facilitates redox reactions which ultimately lead to the reduction of oxygen to water (respiration). Energy released by this process is used to establish a proton electrochemical gradient which drives ATP formation (oxidative phosphorylation, OXPHOS). It also plays an important role in vital processes beyond ATP formation and cellular metabolism, such as heat production, redox and ion homeostasis. Dysfunction of the ETC can thus impair cellular and organismal viability and is thought to be the underlying cause of a heterogeneous group of so-called mitochondrial diseases. Plants, yeasts, and many lower organisms, but not insects and vertebrates, possess an enzymatic mechanism that confers resistance to respiratory stress conditions, i.e., the alternative oxidase (AOX). Even in cells that naturally lack AOX, it is autonomously imported into the mitochondrial compartment upon xenotopic expression, where it refolds and becomes catalytically engaged when the cytochrome segment of the ETC is blocked. AOX was therefore proposed as a tool to study disease etiologies. To this end, AOX has been xenotopically expressed in mammalian cells and disease models of the fruit fly and mouse. Surprisingly, AOX showed remarkable rescue effects in some cases, whilst in others it had no effect or even exacerbated a condition. Here we summarize what has been learnt from the use of AOX in various disease models and discuss issues which still need to be addressed in order to understand the role of the ETC in health and disease.

Recent advances in bioremediation of heavy metals and persistent organic pollutants: A review

Heavy metals and persistent organic pollutants are causing detrimental effects on the environment. The seepage of heavy metals through untreated industrial waste destroys the crops and lands. Moreover, incineration and combustion of several products are responsible for primary and secondary emissions of pollutants. This review has gathered the remediation strategies, current bioremediation technologies, and their primary use in both in situ and ex situ methods, followed by a detailed explanation for bioremediation over other techniques. However, an amalgam of bioremediation techniques and nanotechnology could be a breakthrough in cleaning the environment by degrading heavy metals and persistant organic pollutants.