Date: Thu Nov 26 1992 08:59:52 To: Richard Rearden Subj: abiogenesis Attr: science Rick, e
Date: Thu Nov 26 1992 08:59:52
From: Jeff Otto
To: Richard Rearden
Subj: abiogenesis
Attr:
science -------------------------------
Rick, earlier you have questioned the probability of life spontaneously
evolving. In other messages, I have posted extensively on the RNA world
hypothesis. In the following messages, I address the chemistry involved
in the synthesis of these precursor molecules,
I posted the following messages back last august in the SCIENCE echo.
Recently, it has been requested that the material be posted once again.
I have not kept up with the field as much as I would have liked (my own
thesis work taking more and more of my time these days), and
consequently, have little to add to the following text. The one
addition is that further evidence is accumulating that the functional
parts of snrps (small nuclear riboproteins), is in fact the RNA. I got
this information second hand from a biochemist (Stephen Monroe), who
atttended an RNA meeting late last summer. If anyone can update what I
have here, or address the role of lipid formation in early biological
systems, I would be greatly indebted.
The bottom line remains the same. The following does not prove that
this is the way that life first evolved. Rather it indicates a possible
scenario where life might have evolved. From this, even with its
problems it is clear that the chemistry of the compounds of which we are
made is such that abiogenesis could occur. A further distinction should
also be made. The following has no bearing on evolutionary theory.
Evolutionary theory would predict the same regardless of: special
creation, space men, abiogenesis or anything else. It only predicts
change in the frequency with which certain alleles exist in a population
as environmental forces act upon that population.
Jeff Otto
3-30-92
====================================================================
Chuck Maier has requested that I post a detailed message involving a
possible biochemistry of abiogenesis. The following messages contain my
comments on the matter.
The following are various organic/biochemical reactions that may have
occurred on primitive earth. The reactions are taken directly from the
text Biochemistry by Geoffrey Zubay, the second edition, 1988. To be
honest, I though this text was more comprehensive that it appears to be.
In order to address abiogenesis, one first must decide what would be
required for a primitive "living" system. Based on the studies of
Thomas Cech, Norman Pace, Sidney Altman, and Alan Weiner, I would
suggest that a membrane encapsulated system containing RNA or an RNA
like molecule would be sufficient. This is based upon experiments which
have demonstrated that RNA can perform the following:
1) act as a polymerase and direct template specific synthesis of RNA
2) act as a site specific nuclease to cleave RNA
3) act as polymerase and direct template independent synthesis of RNA
The result of these reactions is a molecule that under different ionic
conditions can replicate, and release the products of replication via
cleavage. To my way of thinking, in order to optimize the
concentrations, and allow for somewhat adequete conditions for a self
replicating system, it should be self contained, thus a membrane would
be important if not required for our first "living" organism. It is
quite possible that the earliest life forms performed these required
reactions by nucleating in pockets of salt water saturated clays.
Eventually however, a membrane is required. You should not from the
above discussion assume that proteins are not required for this most primitive
of scenarios.
Beyond this, there is circumstantial evidence that would support RNA's
role in primitive life. First of all, it is completely ubiquitous and
absolutely required for life of all known systems. No known biological
systems can survive without RNA. DNA viruses have to go through an RNA
intermediate. Not all RNA viruses require a DNA intermediate. This is
an important distinction. Secondly, increasing evidence has
demonstrated that it is the RNA in ribosomes that is critical for
protein synthesis, not the proteins. It appears that the proteins are
more of a scaffolding, while the RNA performs the catalytic function.
Thus we have evidence of yet another role for RNA - that for polypeptide
synthesis. Furthermore, RNA has been implicated in maintenance of
telomeres, which is important to prevent loss of genetic information in
each round of replication. Other groups have also implicated RNA as a
catalyst involved in carbohydrate metabolism. From these examples it is
clear that no other molecule is nearly as wide reaching in its
biological implications as RNA.
Now, what is required to form an RNA molecule, and is it reasonable to
expect that these molecules may have formed spontaneosly on primitive
earth?
To answer the first part, you need bases, a sugar and phosphates. To
answer the second part, the answer is yes, and no. Although the
arguments are certainly not definitive, they are currently the best ones
that I am aware of, although it is entirely possible that I have missed
important research in this area in the last few years. The next
message(s) will detail these reactions and my comments on them. Much to
my regret, the text that I have does not supply the reactions for lipid
synthesis or sugar synthesis. The lipid reactions I have completely
forgotten and will have to ignore. The sugar reactions, I remember a
bit more of, and will try to recount what I can.
First, I will discuss the biochemistry required for synthesis of the
purine bases adenine and guanine. Under conditions postulated to have
occurred on primitive earth, all of these reactions have been shown to
occur, and the resulting end products are major products of the
precursors.
H2N CN This is diaminoaleonitrile, a
\ / relatively simple product, easily
HCN ---> C synthesized from hydrogen cyanide
||
C
/ \
H2N CN
|
| Now add a little ionizing radiation
| and another molecule of HCN and we
V get:
NC N
\ / \ \ A mess. Organic molecules do
C not lend themselves well to this
|| C media. Seriously though, you get
C / 5-aminoimidazole-4-carbonitrile
/ \ N which is a direct precursor of
H2N adenine. Just add HCN
|
| HCN
V
NH2
| N
N // \/ \\
| || C
\\ / \ /
N N
By adding H2O to 5-aminoinudazole-4 carbonitrile you get a
precursor of guanine
|
| H2O
V
O Is it my imagination or are
|| N my drawings getting better?
/ \ / \\ anyways, now just add a little
H2N || C cyanogen and voila!
/ \ /
H2N N
H
|
| (CN)2
|
V
O
|| N
HN/ \ / \\ Here is guanine. So the purines
| || C seem easy enough to make. Lets
/\\ / \ / try some pyrimidines now.
NH2 N N
H
Fortunately at least one pathway for pyrimidine synthesis is a bit less
complicated than for the purines. For the sake of brevity I will post
it here, if you are genuinely curious, you can find all of this in the
text cited in the first message.
NH2 O
HC | ||
||| NCO- //\ H2O / \
C ------> N C -----> HN C
C | || | ||
N //\ / //\ /
O N O N
H H
Cytosine Uracil
So now we have four bases. The next step is the sugar. To me, this is
the biggest problem of the whole thing. Not because sugars would not
form spontaneously under these circumstances, but because of the
exponential nature of stereoisomers that can form with each additional
carbon atom. The number of separate 5 carbon sugars is high enough to
make the selection of ribose seem prohibitive. Some researchers think
that glycerol or another similar sugar may have evolved first,
simulating the structure that would later be achieved through ribose.
Such a structure might look like:
O Base
\ |
C
H H / |
C - C H
OH OH
Where as ribose looks like:
*
HOCH O Base
\ / \ |
C C
/\ H H / |
H C - C H
OH OH
* **
* denotes carbons involved in forming nucleotide polymers
** denotes hydroxyl groups required for RNA catalytic activity.
As can be seen in the above diagrams glycerol supplies the critical
catalytic hydroxyl, but lacks the carbons required for polymerization.
To me, this is critical, and needs to be resolved, but until such a time
it is the most current thinking. As for the phosphates, suffice it to
say that they are added fairly easily. I will look for the lipid
reactions, and if I can find them, I will post them along with the
phosphate reactions. I hope everyone has found this interesting and
informative.
Jeff
E-Mail Fredric L. Rice / The Skeptic Tank