October 4, 2012
Study Confirms Bacterium Proteins Bond To Phosphate, Not Arsenate
Lawrence LeBlond for redOrbit.com - Your Universe Online
A 2010 controversial study that discovered a certain bacterium that used arsenic instead of phosphorus to build its DNA had been heavily scrutinized by the scientific community. Other researchers, awestruck by such a revelation, set out to replicate the study, yet were unable to glean the same results.
The international team of researchers found that the microbe´s detailed chemical structure allows it to selectively choose phosphorus, even when arsenic far outweighs it. The finding confirms that GFAJ-1 follows the same genetic rule that all carbon-based life forms need phosphorus to thrive.
While the latest findings undermine the 2010 study findings, it was not clear how the bacterium discriminated between similar arsenic and phosphorus molecules.
To find an answer, Dan Tawfik, Mikael Elias and Alon Wellner of the Weizmann Institute of Science in Israel studied the mechanisms by which some bacterial proteins bind to phosphate and not arsenate. Publishing their work in the journal Nature, the team suggests that just one chemical bond holds the key, and shows that the ℠arsenic-life´ bacterium actually has a strong preference for phosphorus. In essence, the bacterium´s periplasmic phosphate-binding proteins (PBPs) can detect the very tiny difference in the thermochemical radius between the arsenate and phosphate molecules, thanks to reactions imposed on hydrogen bonds in the protein.
“PBPs therefore seem to have evolved a unique mode of binding that is capable of distinguishing between the highly similar phosphate and arsenate,” the authors wrote in their paper.
“This work provides in a sense an answer to how GFAJ-1 (and related bacteria) can thrive in very high arsenic concentrations,” said Tobias Erb and Julia Vorholt of the Swiss Federal Institute of Technology in Zurich, co-authors of the latest paper; Erb and Vorholt were also research coauthors on a follow-up paper that originally cast doubt on the 2010 study claim.
As part of the latest study, the research team looked at five types of phosphate-binding protein from four separate species of bacteria, two of which were sensitive to arsenate and two were resistant to it. To determine how effective PBPs were at discriminating between phosphate and arsenate, the bacteria were placed in solution with a set amount of phosphate and different concentrations of arsenate for 24 hours, and then checked which of the molecules the PBPs would bind to.
While all five proteins of the bacteria were able to preferentially bind to phosphate--even in solutions containing 500-fold more arsenate than phosphate--the research team found that one protein, from the Mono Lake bacterium, could do so at an even greater 4,500-fold more arsenate than phosphate concentration.
Tawfik said he was dumbfounded by how well the proteins were at discriminating between essential phosphate and poisonous arsenate. He noted that this does not mean that arsenate does not get into the bacteria, “it just shows that this bacterium has evolved to extract phosphate under almost all circumstances.”
This study shows that the “arsenic monster” GFAJ-1 goes to extremes to avoid arsenate, said Wolfgang Nitschke from the Mediterranean Institute of Microbiology in Marseilles, France, coauthor of a commentary piece questioning the 2010 study conclusion that GFAJ-1 could replace phosphate with arsenate. “This shows clearly that life doesn´t like arsenate in cytoplasm,” he said.
Felisa Wolfe-Simon, lead author on the original Science paper and now at Lawrence Berkeley National Laboratory in Berkeley, California, said the results represent the type of “careful and interesting studies that aid the community.”
“They have helped us understand molecular level discrimination between arsenate and phosphate in GFAJ-1 and other microbes. We agree, the ecological connection and differences in phylogenetic relationships between the two PBP was quite interesting,” she told Popsci´s Rebecca Boyle in an email.
However, she pointed out that this work “doesn´t necessarily rule out an entirely novel mechanism” for arsenate getting into cells. “There´s still a lot of interesting open questions."