gregxy
Hazard to Others
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Probability of life??
I'm always amazed when I see all the elaborate biochemical reaction that people have figured out.
What seems even more amazing it that if can all work (and so reliabily).
Consided creating a simple protien from 100 amino acids, that is a 100 step synthesis (not to mention all of the reactions to translate the DNA to RNA
etc etc.)
To get a 50% overall yield requires 99.3% yield on each of the 100 steps. Getting a yield like this in the lab for simple
reactions with pure reactants is very difficult. In the cell there
are hundreds of incorrect molecules floating around for
each correct amino acid that is to be added.
Of course enzymes in the cell direct each of the steps, but what is the difference in energy between the "correct" reaction and each of the hundreds
of incorrect ones?
I guess it works or else we wouldn't be here, yet if someone
were to propose such a complex machine I would think the
probability that it could work would be zero.
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chemoleo
Biochemicus Energeticus
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Proteins and enzymes are more specific than this. Unlike general organic reactions, proteins make use of large interaction surfaces, as well as
spacially positioned reactive groups (of amino acids) that *just* favour binding of one particular target molecule. Organic chemistry doesn't work
that way - it rarely uses specific catalysts like that, and it usually is a direct funciton of rate constant of conversion, where a large activation
barrier impedes the reaction and leads to side products. Enzyme catalysis, by binding directly its targets, brings it close to the catalytic center,
which hugely increases reaction rates, efficiency and specificity. That's the difference between organic and biochemical reactions. Thus you have
virtually no error during translation. I never heard of a 50% efficiency figure, I was under the impression that protein translation efficiency is
close to 100 %. Not so for DNA replication and synthesis of course - but this may be desired because a small mutation rate enhances the ability of the
species to survive, particularly to changing conditions. And the environment of any species is certainly exposed to changing conditions at all times!
The efficiency of organic reactions could be similarly enhanced that way, if we could evolve energy landscapes/surfaces on i.e. proteins that favour
binding of a specific target molecule whose reactive group directly faces the active site of conversion.
Check catalytic antibodies ( a hugely fascinating concept, but sadly it does not work too well in reality) and directed evolution.
Also, don't forget, any reactions in biology that require high efficiency but cannot provide it are mercilessly wiped out in the process of natural
selection, leaving only those that actually do provide higher efficiencies. And there were ~4 billion years of improvement.
Not surprisingly, the most crucial molecules to cells are usually those involved in metabolism, and their DNA (and respective protein) sequences are
unbelievably conserved, from man to fish to fly to bacterium. Goes to show that despite of billions of years of selective pressure, the metabolic
proteins we have at work in our bodies *right now* are pretty much perfect.
[Edited on 20-10-2006 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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unionised
International Hazard
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Just for the record, getting a 100 residue chain isn't necessarily a 100 step reaction.
The last step is coupling 2 chains of 50 residues, the second last steps are there coupling of 2 25 acid chains and so on.
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coach
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The atmospheric and other environmental conditions were a bit different 3 and a half billion years ago when, what can really be viewed as a single
cascade reaction, all got started from what we see when we head out the front door today. Things have really calmed down a bit. The conditions, so
goes the prevailing theory for the moment, were much more favorable for the spontanious formation of the complex organic compounds that were needed.
Then, and again the local weather was helping by consisting primarily of intense electrical storms, it was really just a matter of a couple of hundred
million years before the practically inevitable proper assemblage got together to become self replicating. Well, it probably just expanded and broke
into more expandable parts at first, but I don't here anyone complaining. Years ago, and their names escape me at the moment because I think I drank
them, a couple of guys did a pretty good job of reproducing what conditions were probably like back then. And wouldn't you know it, when they applied
a charge to simulate the electrical discharge of a lightning strike very complex organic molecules that could easily be the precursors to proteins and
nucleic acids began to spontaniously form. "Spontaniously" is not really the proper term for it considering the amount of energy the system required
to counter entropy and all, but it plays well for the gravity of the moment. If they would have tried it again in a bigger vessel, say roughly ocean
sized, and let it react for a few million years...well, let's just say that I would probably remember their names now.
In all likelyhood, considering the conditions at the time, life, or a precursor to it, probably started up and fizzled out countless times. We,
and all other life on the planet are, following this logic, meerly the current progression of a single complex reaction that was maybe a bit better
than most, but almost certainly not all, of the other attempts at immortality swimming around the primordial oceans. I'd rather be lucky than good any
day; especially on the sixth day.
Anyhoo, the probabilities run almost to certainty that this same thing has occurred, probably with varying degrees of success, plenty of times
out there and will continue to do so long after our own reaction finds its entropy. Makes it all worth it.
Eric J. Chillseyzn
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