Et dna

[BenTheMan]

http://www.technologyreview.com/blog/arxiv/23309/

We know that amino acids are common in our solar system and beyond. Various first experiments to recreate the conditions in the Earth's early atmosphere have produced 10 of the amino acids found in proteins. Curiously, analyses of meteorite samples have found exactly these same 10 amino acids. Various researchers have noted this link but none have explained it.

[Researchers] have ranked the amino acids found in proteins according to the thermodynamic likelihood of them forming. This turns out to match the observed abundances in meteorites and in early Earth simulations, more or less exactly.

The paper is here: http://arxiv.org/abs/0904.0402

Is anyone familiar with this work? I'll read the paper later today, and try to summarize the main ideas. Anyone else is welcomed/encouraged to follow along.

 

[Walter L. Wagner]

Wow. Chemistry works the same in our solar system as the chemistry in other solar systems! Who would have thought. So ET will be based on a protein system using proteins that can be made by natural chemistry.

 

[leopold]

the really interesting part to all of this is chirality.
nearly all amino acids found in proteins are left handed and the opposite type can be toxic to life.
all polymers of carbohydrates are right handed.
i find that just a bit much to cede to pure coincidence for a "natural" origin for life.
on top of that, life has never been observed coming from non life.
the fact that "et dna" yields the same amino acids as earth based experiments makes "seeding" from space highly unlikely.

 

[Sciencelovah]

As the amino acids in the meteorites match those in the early earth simulations (Miller?), maybe a new life is currently developing up there beyond our solar system. :shrug: Can't give further comment as I have no enough basic in either thermodynamics or origin of genetic code.

 

[iceaura]

Confusing the amino acid likelihood with the genetic code will lead us pretty badly astray.

If the implications of the paper bear out, ET is apparently likely to be built of similar proteins as we are - if built of proteins at all. His genetic code, on the other hand, remains almost pure speculation.

 

[ElectricFetus]

but chirality is still a mysterious, the amino acids produce in the lab or found inside meteors is race mixed, so how did we get our L amino acids and a D sugars? And what to stop aliens from having opposite chirality: biochemically we would be completely incompatible, toxic to each other.

 

[Nasor]

but chirality is still a mysterious, the amino acids produce in the lab or found inside meteors is race mixed, so how did we get our L amino acids and a D sugars? And what to stop aliens from having opposite chirality: biochemically we would be completely incompatible, toxic to each other.

It seems perfectly reasonable that at some point early in life's history an organism (or proto-organism, or whatever) came up with an enzyme that improved amino acid and/or sugar synthesis and that happened to be chirally selective, which gave it a massive competitive advantage.

 

[Nasor]

the really interesting part to all of this is chirality.
nearly all amino acids found in proteins are left handed and the opposite type can be toxic to life.
all polymers of carbohydrates are right handed.
i find that just a bit much to cede to pure coincidence for a "natural" origin for life.

The fact that's it's not a coincidence doesn't mean that it doesn't have a natural explanation. Nothing about a natural explanation required it to be a coincidence. Of course, it could be a coincidence that all the sugars ended up being R and all proteins ended up being S, rather than the other way around (or both being R, or both being S).

 

[ElectricFetus]

It seems perfectly reasonable that at some point early in life's history an organism (or proto-organism, or whatever) came up with an enzyme that improved amino acid and/or sugar synthesis and that happened to be chirally selective, which gave it a massive competitive advantage.

Its perfectly unreasonable because chirality was likely necessary even to get up to the proto-life stage, so there had to be a non-living mechanism that preset chirality.

 

[Roman]

Its perfectly unreasonable because chirality was likely necessary even to get up to the proto-life stage, so there had to be a non-living mechanism that preset chirality.

Why?

The first proto-proteins could have been put together in such a manner as to serve as a chiral environment. Have a site that only lets L bind, but not R.

I don't understand why everything needs explanations. Why can't there be accidents?

 

[ElectricFetus]

Why?

The first proto-proteins could have been put together in such a manner as to serve as a chiral environment. Have a site that only lets L bind, but not R.

I don't understand why everything needs explanations. Why can't there be accidents?

To select chirality requires pre-existing chirality, so a protein or RNA that selects only the correct chirality must have been chirality pure to begin with. Sure it could have been accidental. The biochemical research on extraterrestrials would verify if its accidental or not, if its not accidental then we would see a only one chirality type, if accidental we will find aliens of opposite chirality.

 

[Roman]

Chirality doesn't require a chiral environment- only optically active substances, right? If I've got 5 different molecules that I randomly put together, I'm going to get chiral substances. It's just that they'll all be in large enough proportions as to not polarize light passed through it in solution.

 

[ElectricFetus]

Chirality doesn't require a chiral environment- only optically active substances, right? If I've got 5 different molecules that I randomly put together, I'm going to get chiral substances. It's just that they'll all be in large enough proportions as to not polarize light passed through it in solution.

To replicate the molecules most be able to select all of it components. RNA in the presents of racemic nucleic acids cannot form double strands, it could not replicate, no RNA world. Racemic amino acid can halt protein polymerization. These are the reasons why we are chiral to begine with.

 

[Roman]

Oh, yeah.

 

[Nasor]

These are the reasons why we are chiral to begine with.

There are reasons why chirality is important to the biochemistry that goes on now, but that wasn't necessarily true for the proto-biochemistry that was going on 4 billion years ago. The scenario would be something like this:

1. Early proto-life appears and starts evolving, but its chemistry is achiral.
2. Eventually some bit of very early life develops some enzymes (or proto-enzyme catalysts, or whatever you want to call them) that happen to be chiral, giving it a huge advantage and allowing it to out-compete everything else and take over. Its biochemistry can use biomolecules of either chirality, but it's mostly only getting molecules of one chirality thanks to its new catalysts.
3. Eventually the organism (or proto-organism, or whatever) adapts its biochemistry to the fact that it's only getting amino acids or sugars or whatever of a particular chirality, and loses the ability to work with the other chirality.