Did nature also choose arsenic?

Abstract

All known life requires phosphorus (P) in the form of inorganic phosphate (PO43−or Pi) and phosphate-containing organic molecules. Piserves as the backbone of the nucleic acids that constitute genetic material and as the major repository of chemical energy for metabolism in polyphosphate bonds. Arsenic (As) lies directly below P on the periodic table and so the two elements share many chemical properties, although their chemistries are sufficiently dissimilar that As cannot directly replace P in modern biochemistry. Arsenic is toxic because As and P are similar enough that organisms attempt this substitution. We hypothesize that ancient biochemical systems, analogous to but distinct from those known today, could have utilized arsenate in the equivalent biological role as phosphate. Organisms utilizing such ‘weird life’ biochemical pathways may have supported a ‘shadow biosphere’ at the time of the origin and early evolution of life on Earth or on other planets. Such organisms may even persist on Earth today, undetected, in unusual niches.

A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus

“…Arsenic possesses a similar atomic radius, as well as near identical electronegativity to P (5). The most common form of P in biology is phosphate (PO 4 3-), which behaves similarly to arsenate (AsO 4 3-) over the range of biologically relevant pH and redox gradients (6). The physicochemical similarity between AsO 4 3-and PO 4 3-contributes to the biological toxicity of AsO 4 3-because metabolic pathways intended for PO 4 3-cannot distinguish between the two molecules (7) and AsO 4 3-may be incorporated into some early steps in the pathways [(6) and references therein].…”

“…These downstream biochemical pathways may require the more chemically stable P-based metabolites; the lifetimes of more easily hydrolyzed As-bearing analogs are thought to be too short. However, given the similarities of As and P-and by analogy with trace element substitutions-we hypothesized that AsO 4 3-could specifically substitute for PO 4 3-in an organism possessing mechanisms to cope with the inherent instability of AsO 4 3-compounds (6). Here, we experimentally tested this hypothesis by using AsO 4 3-, combined with no added PO 4 3-, to select for and isolate a microbe capable of accomplishing this substitution.…”

Can Adenosine Triarsenate Role as an Energy Carrier?

“…If Asbased organism is possible, they will possess adenosine triarsenate (ATA) instead of ATP since there will be no or little P content to form ATP. Even forming the formerly discovered adenosine diphosphate arsenate (ADP-Asi) 2,9,10 will be unlikely under a harsh condition without any P in the environment. If the organism has adopted As as a brick of forming various biomolecules and/or as mortar for gluing them, ATA should (a) be structurally similar with ATP so that the relevant enzymes can adopt a similar mechanism as in the ATP case, (b) be stable enough to function regularly in biochemical systems, and (c) release a large enough amount of free energy during the hydrolysis reaction.…”

Comment on “A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus”

“…Extrapolation of this to the pH of Mono Lake (pH ≈ 10) gives a pseudo first-order rate constant ≈ 10 s (1) is, in fact, arsenate-linked DNA, then either (i) that DNA contains very few arsenate linkages, (ii) the kinetic data reported in (5) [and elsewhere; see review in (6)] are incorrect, (iii) the extrapolation fails by many orders of magnitude, (iv) the analogy between model arsenate esters and arseno-DNA is similarly imperfect, or (v) the band must be associated with additional compounds that stabilize arsenate diester linkages. Explanation (iv) runs afoul of measurements made for many other arsenate esters (6). Explanation (v) would require those additional compounds to remain associated with the hypothetical arseno-DNA through extractions by phenol and chloroform, ethanol precipitation, and the time required to prepare the sample and run the gel electrophoresis experiment (1).…”