Arsenic and the origin of life

[SteveF]

Interesting speculation:

Wolfe-Simon, F. et al. (2009) Did nature also choose arsenic? International Journal of Astrobiology, advance online

All known life requires phosphorus (P) in the form of inorganic phosphate (PO43− or Pi) and phosphate-containing organic molecules. Pi serves 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.

you can read the paper here:

http://precedings.nature.com/documents/1482/version/1

[Monad]

Cool - that was one of the main points I was making re the possibility of more than one extant tree of life.

[llanitedave]

I would guess that a primary reason that phosphorus is successfully used in biology rather than arsenic is because of the relative abundance of the two. Whatever life could use, it will be more abundant if it uses the more abundant resource.

[premjan]

Nitrogen and phosphorus are both used. Sulphur is used, but Arsenic isn't. Probably Arsenic is heavier / less reactive than phosphorus and also not as chemically active as Sulphur. I'm surprised that Lithium, Beryllium and Boron aren't used but they are probably locked up in minerals.

[Steviepinhead]

Ah, interesting.

But I thought it was, hmm, abundantly clear that the whole thing was kicked off by the judicious application of a dollop of that rare substance, Creomoronium.

[RAFH]

I think the real reason phosphorous won out is because of the extreme rarity of Old Lace back then.

[Jobar]

A year or so ago there was a cover article in Scientific American concerning the possibility of various 'alien' lifeforms on Earth. Arsenical life was one of them (along with left-handed DNA, silicon life, and life using amino acids other than the 20 used by our sort.)

I think that llanitedave hits the nail on the head; if arsenic was more common on Earth than phosphorus, life would probably use it instead. (Then the play would be Phosphorus and Old Lace. )

[Calilasseia]

But haven't some organisms already co-opted some less abundant, and from the standpoint of organic chemistry, more exotic elements into their biochemistry?

Don't we humans have the facility to press Selenium into service via Selenocysteine?

I just did a brief cursory check, and it seems that this facility was an adjunct to the genetic code, because there doesn't exist a codon sequence directly coding for Selenocysteine. Apparently what happens (for those like me who are new to the mechanism) is that the UGA stop codon is pressed into service as a marker, which instead of causing translation to stop when a special mRNA structure called a SECIS element is present, causes the translation to continue, with a Selenocysteine being inserted into the amino acid chain. Exactly HOW this all works is something I'm going to have to spend long hours studying in order to understand it properly, but it would appear that Selenium (as bound in Selenocysteine) is sufficiently useful to a wide range of organisms that a special adjunct to the genetic code has arisen to facilitate its integration into our molecular biology.

Then, of course, there are all those porphyrins, that co-opt metal ions (Fe ions in the case of Haemoglobin, Cu ions in the case of Haemocyanin in crustaceans, Mg in the case of Chlorophyll), and those Zinc finger proteins, along with Cobalamin (vitamin B12), which contains a Cobalt atom.

[llanitedave]

Exotic elements can be used, but it appears that wherever that's the case the use is specialized enough so that only the trace amounts are needed. It's the more common elements that will be primary structural components.

[premjan]

All starts with a basic chemical cycle of I guess, RNA replication.

[llanitedave]

Maybe. I'm putting bets out that when E.T. is actually discovered, whether bacterial or advanced, it will use something other than DNA for inheritance. And maybe something other than RNA as well.

I've got no data or theory to back it up, just a gut feeling, but there it is.