WATER is essential for life as we know it, but why? A new analysis may rewrite the idea that it is solely the medium in which the reactions that drive life occur, instead viewing it as an active participant. The findings offer clues to the role that water played in the beginning of life on Earth, suggesting it may have “selected” the chemicals that now form the basis of life.
“While the importance of water in life is well known and appreciated, the involvement of water as the most reactive chemical participant in today’s biochemistry was not well appreciated,” says Moran Frenkel-Pinter at the Georgia Institute of Technology in Atlanta.
Water is often viewed as the background in which all the other chemicals, such as DNA and protein, are dissolved – in other words, the stage on which the real business of life happens. To show how active water really is, Frenkel-Pinter and her colleagues turned to a database of biochemical reactions. Out of 6500 known reactions, around 40 per cent of them either made a molecule of water or destroyed one.
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That is a conservative estimate, says team member Loren Dean Williams, also at the Georgia Institute of Technology, because the precise mechanisms of many reactions aren’t known and may depend on water in subtle ways.
The team also looked at the molecules produced during the life cycle of a well-studied bacterium called Escherichia coli. More than 99 per cent of these are water molecules, the team estimates. Each time an E. coli divides to form two new cells, every water molecule it contains is either transformed or drives a chemical reaction 3.7 times on average (Journal of Molecular Evolution, ).
“I do think there is this tendency to view water as a background actor,” says Lena Vincent at the University of Wisconsin-Madison. The study “confirmed something that we already appreciated and suspected, but didn’t fully grasp the extent of”, she says.
Before the first living cells arose, Earth was home to a vast array of non-living chemicals that were constantly interacting and changing. Somehow, this “chemical evolution” gave rise to complex and self-sustaining structures that we would recognise as living organisms.
“The basic model we have is that organic molecules were created in the atmosphere… and they snowed down on the Earth,” says Williams. There they encountered water in vast quantities, both in the seas and on land. The sheer quantity of water meant it exerted a huge influence on which chemicals survived and became part of life, and which didn’t.
“There were many molecules that did not play well with water,” says Frenkel-Pinter. “They were excluded, and the surviving molecules were the ones that were soluble in water.” That much has long been clear, but she says that as well as needing to dissolve in water, the chemicals of life also had to be able to react with it. “This is how they were selected.”
