http://scienceblogs.com/pharyngula/2010 ... ote]Posted by: As-hole Skeptic | December 3, 2010 10:24 PM
Things to worry about:
1) The best As:P ratio they got was 7.3:1 in dry cell weight. They are using media with phosphate contaminants (~3 uM). The extremely slow growth rate (20-fold in six days; compared to E. coli roughly 20-fold in 90 min) suggests limited growth that is occurring from phosphate salvage.
2) Their As measurements are unconvincing for ICP-MS. The dry weight of As was measured at 0.19% +/- 0.25 % over eight measurements. The error is bigger than the data. There are numerical fubars obvious in the paper – the ratio of As to P calculated from 0.19/0.019 (Table 1) should not be 7.3.
3) There is no evidence that As is incorporated into functional DNA or RNA and that such As-nucleotide is competent in replication/translation. They have evidence that As is incorporated into nucleic acids. That’s a major leap from there to functionally competent DNA/RNA.
4) Arsenate diesters are unstable in water. The hydrolysis rates for arsenate esters are 10,000 – 1,000,000 times faster than the corresponding phosphate esters. No stability; no genetic information. The notion that water is kept away is curious at best and the hallmark of pathological science at worst.
5) The available redox potentials for As can cause problems as As(III) and As(V) can cycle under physiological conditions… for example, anoxic lake bed sediment. Only As(V) has the tetrahedral geometry needed to mimic phosphate.
6) It’s been known that arseno-ADP, the ATP analog, is not stable in water. Hydrolysis rates have been estimated at 70 min-1. To put that into context, the study of enzyme kinetics using arseno-ADP is challenging as the straightforward water hydrolysis reaction is far faster than any enzymic reaction. How do you get to arseno-DNA without arsenic analogs of ATP?
7) Arsenic accumulation by plants and bacteria has been known for a long time. The organisms have been genetically engineered to sequester high levels of arsenic with the hopes that they can be used in bioremediation. Bacteria are known to generate polymeric material to sequester arsenic.
It would be poor of me to bash this work so harshly without giving an alternate theory, so here goes. There is no As incorporation into canonical, functional DNA. The organism is generating garbage nucleic acid to sequester the arsenic and avoid the toxicity. The garbage nucleic acid is being partitioned into vacuoles and, thus, the cells get bigger. This theory fits the data presented without rewriting any biochemistry. Much as I would love this result to be true I can’t just throw out a whole bunch of very well established chemistry. In support of the authors, the Science paper is fairly muted and only really claims As incorporation into ‘biomolecules.’ The press conference, the media frenzy and the breathless acceptance of As-based life on the other hand… stink.
#129
Posted by: steverino63 | December 4, 2010 3:06 AM
@As-hole 128: Thanks for the additional detail on things like As diesters' and ATP analog's problems with water. I had read general info on that elsewhere, but had not sceen details.
Can you tell Greg Laden, still frothing at the mouth over this, Point No. 1?[/quote]http://networkedblogs.com/bjVUk
Update: John Sutherland from the MRC Laboratory of Molecular Biology at Cambridge adds to the skepticism. He notes that arsenic-based compounds are “not sufficiently stable in water for the phosphorus to arsenic substitution implied in this paper to be functional” and the arsenic-phosphorus swap hasn’t been demonstrated by the study’s experiments in a “chemically rigorous manner”. For Sutherland, the acid test would be actually synthesising a double helix of arsenic-based DNA and characterising its structure in detail. You could then use the data from that analysis “as a reference point” to examine the DNA from the Lake Mono bacteria. “This has not been done,” he notes, and even if it were, the existing evidence suggests that the molecule would break apart when it’s exposed to water.
Annyira érdekes - ha igaz - , hogy szerintem nem kellene ez a hamis felhajtás mellé - akkor is tényleges figyelmet keltene. Én ezért nem vagyok még meggyőzve.> Érdekes a felfedezés, de azért messze nem annyira, mint ahogy beharangozták. Ez azért még vissza fog ütni.
Bizony visszaüthet, bár az is lecseng ... - s jó az Ed féle kifigurázás is.
Köszi a kommentárt is. Már az Ed link alapján olvastam. Én nem értek
ezeknek kísérleti technikáknak a megbízhatóságához. Azért is kezdtem a
szerzőlista nacionáléjának utánanézni. No meg a kémia, biokémia ismereteim is rendkívül szegényesek. Sajnálom, hogy profi, a foszfor és arzén kémiában, DNS szerkezetben jártas szakember még nem kommentált semmit. Van-e egyáltalán módjuk olyan adatokhoz jutni, ami kommentálható? Furcsállom, hogy csak arzéntartalmat méregettek, csak arról számolnak be, s semmi arzénvegyületet nem említenek - például semmi NMR mérést nem láttam. Ha az 1940-es évek óta eltelt hat-hét évtized. Meg kellene kérdezni a kvantumkémikusokat, hol állnak a DNS lánc számolgatásával - s mi a helyzet, ha a foszfor helyett az arzént tennék oda - helyenként x %-ban? De hát ezt a NASA-nál ne tudnák? Biztos, csak ennek a csapatnak az idegen életformákra kihegyezett publicitás kell. Azt azért nem gondolnám, hogy Sokál féle átverés esete forogna fenn - de ...
Egyszóval szerintem ez még egyáltalán nem ment át a normál tudományos
kísérlet verifikálás szűrőjén.
HAsO2 + 2H2O +0,560
