28 November 2007

L or D or L+D?

The majority of the peptide-building amino acids and most of the monosaccharides (glucose, ribose, etc.) in all earthly life forms consist of the "left-handed", or "L" and "right-handed", or "D" isomers, respectively. Of course, this doesn't mean that the amino acids and monosaccharides of every known organism have been checked and found to conform this rule. What it means that no exceptions have been found in those instances when such tests were made. So it is assumed that all life forms contain mostly L-amino acids and mostly D-monosaccharides. This is probably a safe assumption, because once the isomeric configuration of a group of molecules, for example, amino acids, has been fixed during evolution, everything else would then be complementary, for example, the configurations of substrates that bind to active sites of proteins and therefore, switching from one stereo isomer to another would require a whole set of simultaneously occurring changes, and ultimately, mutations. That is not a very likely thing to happen for obvious reasons.

But that doesn't mean that 2 separate lineages of organisms, one lineage using one stereo isomer of say, amino acids and the other the other stereo isomer can't evolve right from the beginning.

In a recent post, Pascal at Research at a snail's pace, mentioned that if amino acids of only one chiral form (L or D) are found on another planet or comet or some other extraterrestrial body, this will be a strong evidence that they were produced by alien life forms. I should add here that ordinary chemical reactions normally produce a racemic mixture of amino acids or monosaccharides consisting of about equal amounts of each stereo isomer.

Although Pascal is not claiming to be the originator of this idea, I should nevertheless mention, to prevent any misunderstandings that I may be thought to be criticizing him, that the hypothesis that the presence of only one stereo isomer of amino acids on an another planet may be an indication of extraterrestrial life has been around for a long time.

But how strong a hypothesis is it?

1. There could be some inorganic chemical or physical mechanism that we, earth-bound misfits, haven't discovered yet and which concentrates one stereo isomer preferentially over the other.

This means that the detection of only one stereo isomer of amino acids on, say, a meteorite, doesn't necessarily mean that they were produced by extraterrestrial life forms.

2. As suggested above, 2 independent lineages of life forms using different stereo isomers of essential building blocks of their bodies could evolve and be present simultaneously on the same planet. Just because this didn't happen on earth doesn't mean that it couldn't have happened elsewhere.

This means that the detection of a racemic mixture of amino acids on a comet or a planet doesn't necessarily rule out the presence of life there.


Anonymous said...

Racemic mixture of amino acids also exists on earth, but not necessarily in the proteins of its higher life forms. Lower life forms might actually use these D-amino acids as antibiotics.
There is also the issue of alpha vs. beta amino acids. The amino acids you mentioned are all alpha. Why not a life form based on beta? We already know that asparagine in proteins can undergo a transformation and link through its side chain.

pascal said...

Another way to look at it might be, "are there any NON-biological processes that concentrate amino acids of one particular chirality?"

But yeah, I've thought the idea is interesting, but what is the possibility of type 1 or 2 errors with the life = L-aminos as your hypothesis?


One argument I read for why beta-amino acids may not have been selected is that a polypeptide chain composed of them would be too flexible to fold into a stable 3-D shape.

The most sensible approach to detecting life elsewhere in the universe would be not to rely on 1 test but to use multiple tests. But then one has to make sure that several of them give colloborative + results, while keeping in mind that if one tested for many variables, 1 or 2 could give + results just by chance.

pascal said...

There's a group at UW-Madison looking at Fe isotopes as another potential biomarker. Given how difficult it would be to keep alien life in a lab to study it, I wonder how we would know "for sure" if we were looking at alien biological processes.