1934

Superoxide

Linus Carl Pauling (1901–1994), Rebecca Gerschman (1903–1986), Edward W. Neuman (1904–1955), Irwin Fridovich (b. 1929), Joseph McCord (b. 1945)

American chemist Linus Pauling’s thoughts on the nature of chemical bonding led him, in 1931, to propose that a century-old formula for a class of oxygen compounds was wrong. It was known that if the alkali metals were burned in oxygen (a vigorous reaction), tetroxides were formed, such as K₂O₄. But Pauling realized that the product of this reaction could contain an O₂ anion (a negatively charged ion)—an oxygen molecule that had picked up an extra electron—and that the real formulas must be KO₂, NaO₂, and so on, for the rest of the alkali metals. He proposed the name superoxide for this species, and in a 1934 publication, his coworker, American chemist Edward W. Neuman proved him right by showing that potassium superoxide had the magnetic properties of free radicals (with that extra unpaired electron).

What no one realized was that superoxide wasn’t just a curiosity of inorganic chemistry—it was an essential part of life for every oxygen-breathing creature. In 1954, American biochemist Rebecca Gershman proposed that superoxide might be active in living systems, and in the early 1960s, American biochemist Irwin Fridovich likewise proposed that superoxide was a free-floating species inside cells but was met with what he called “extreme skepticism.” Then in 1968, American biochemist Joseph McCord discovered an enzyme whose only purpose was to scavenge it. The enzyme had actually been known for thirty years, but scientists did not understand what it did or why it was so abundant. McCord renamed it superoxide dismutase, and he and Fridovich went on to establish a whole new area of cell biology around it.

Superoxide is one of the most important reactive oxygen species (ROS) in cells, along with the hydroxy radical and hydrogen peroxide. These are known to be capable of causing much damage to biomolecules, which has been proposed as a mechanism for aging. But recent work has shown that ROS are essential for normal cell function and are responsible for the beneficial effects of exercise (i.e., the small amounts of ROS damage during such stress signals the muscle cells to grow and metabolism to increase).

Superoxide salts are used in oxygen generators in closed environments like submarines.