How Microbes Evolved to Tolerate Oxygen.

Ancient microbes invented biochemical mechanisms and assembled core metabolic pathways on an anoxic Earth. Molecular oxygen appeared far later, forcing microbes to devise layers of defensive tactics that fend off the destructive actions of both reactive oxygen species (ROS) and oxygen itself. Recent work has pinpointed the enzymes that ROS attack, plus an array of clever protective strategies that abet the well known scavenging systems. Oxygen also directly damages the low-potential metal centers and radical-based mechanisms that optimize anaerobic metabolism; therefore, committed anaerobes have evolved customized tactics that defend these various enzymes from occasional oxygen exposure. Thus a more comprehensive, detailed, and surprising view of oxygen toxicity is coming into view. Reactive oxygen species (ROS) attack iron-dependent enzymes that contemporary microbes inherited from their anoxic ancestors. Microbes that adapted to live in oxic environments evolved layers of defenses against ROS, rather than discarding iron-centered metabolism. Molecular oxygen directly damages the radical species and low-potential metal centers that are critical for optimal anaerobic metabolism, and these injuries may be more important than ROS stress in constraining anaerobes to anoxic environments. Anaerobes periodically confront oxygen, and they have developed esoteric tactics to minimize injuries to some of their most vulnerable enzymes. [ABSTRACT FROM AUTHOR]