Re: EvN2 is an interesting elemolution of life
Thu, 25 Nov 1999 16:34:15 EST

In a message dated 11/25/99 11:40:58 AM Pacific Standard Time, writes:

> Element Solar Abundance Atoms in Body
> H 2.8 x 10^10 4.22 x 10^27
> O 2.3 x 10^7 1.61 x 10^27
> C 1.0 x 10^7 8.03 x 10^26
> N 3.1 x 10^6 3.9 x 10^25

Goodness! I had no idea there's that much nitrogen about. I thought stellar processes made far more O than N (O = C + He) I assume it's mostly in the gas giants, right?

>N2 is an interesting element to look at for rarity, particulary
>from the perspective of why you don't find much on Mars or Venus
>(relative to say CO2). On Mars, I can understand losing N2 before
>CO2, but on Venus there ought to be a fair amount.

Since N is mostly atmospheric on the inner planets, one good asteroid smack can get rid of it all. CO2 will be replaced by outgassing, but not N.

>So far, I haven't come across any strong reasons why the
>functions done by heavy "metals" in our body could not
>replaced by protein only constructs.

The best reason is that after 4 billion years of evolution, about 1/2 our enzymes
depend on bound metals. This is true even for organisms evolving under metal limitation (e.g., Fe in the ocean). Clearly it's quite tough to ditch the metals.

>So even though we require heavy metals (from supernovas)
>it isn't clear that all forms of life must. I suspect that
>life gets started without using metals (other than HOCN)
>and then incorporates them for specific functions.

I think metals were key to the origin of life. Life depends on a web of oxidation-
reduction reactions; organic catalysts of such reactions are highly tuned and not plausible abiotically. Metals, with their multiple valence states, make great
less-specific catalysts. In cases where both metal and non-metallic catalysts do the same job, the metal appears to be the basal state. For example, cytochrome p450 has a Fe-S protein transfering electrons in the basal state; a few derived versions (including eukaryotic versions) use FAD(H) to transfer.

Fe-S complexes are very common in life and a very good abiotic system for catalyzing redox reactions since they can absorb mutliple electrons, one at a time, by valence and conformational shifts. Wachterhauser has proposed Fe-S abiotic catalysis as the basis of living metabolism.

>Given some
>really big computers for molecular dynamics simulations,
>I'm pretty sure we could rapidly develop proteins/enzymes
>that did not require metals.

How could anyone be "pretty sure" of this outcome? We really don't know.

>I will admit that
>you may suffer some reductions in reaction rates, but
>it isn't clear that there are reactions that simply
>"cannot" occur without the presence of metals.
>So, in solar systems where heavy metals are not as abundant
>(very early population I stars) life might certainly evolve.
>It might take longer, meaning it might take 5-6 billion years
>instead of 4 billion,

Life is a rate-based process. At its origin, the indirect catalysis of life (enzyme
makes enzyme makes ... does process) would have had to "outcompete" abiotic processes for access to thermodynamic disequilibrium in the environment. Slow enough and it would certainly lose. We have no idea what the limits are, but it's certainly possible that our own abiogenesis "barely made it".