CASE 1 THE FIRST CELL: LIFE’S ORIGINS

Many enzymes are not just made up of protein, but also contain metal ions. These ions are one type of cofactor, a substance that associates with an enzyme and plays a key role in its function. Metallic cofactors, especially iron, magnesium, manganese, cobalt, copper, zinc, and molybdenum, bind to diverse proteins, including enzymes used in DNA synthesis and nitrogen metabolism. With this in mind, scientists have asked whether metal ions might, by themselves, catalyze chemical reactions thought to have played a role in the origin of life. They do. For example, magnesium and zinc ions added to solutions can accelerate the linking of nucleotides to form RNA and DNA molecules.

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Metallic cofactors also bind to enzymes used in the transport of electrons for cellular respiration and photosynthesis, processes that are discussed in the next two chapters. Enzymes that contain iron and sulfur clustered together are particularly important in the transport of electrons within cells. Iron–sulfur minerals, especially pyrite (or fool’s gold, FeS₂), form commonly in mid-ocean hydrothermal vent systems and other environments where oxygen is absent. It has been proposed that reactions now carried out in cells by iron–sulfur proteins are the evolutionary descendants of chemical reactions that took place spontaneously on the early Earth.

The idea that your cells preserve an evolutionary memory of ancient hydrothermal environments may seem like science fiction, but it finds support in laboratory experiments. For example, the reaction of H₂S and FeS to form pyrite has been shown to catalyze a number of plausibly pre-biotic chemical reactions, including the formation of pyruvate (a key intermediate in energy metabolism discussed in Chapter 7). Thus, the metals in enzymes help connect the chemistry of life to the chemistry of Earth.