Homochirality

[maxplanck]

Why is homochirality such a show-stopper for macroevolutionary theory? What common arguments are used to support the evolution of homochirality? What evidence is there , pro or con?

[hopefulcynic]

Excellent question. Basically chiral objects are non-superimposable mirror images. So for example, your left hand is a mirror image of your right hand, but it is impossible to superimpose the two. A sphere on the other hand, is achiral, because a ball can be superimposed on its mirror image.

In biology, almost all molecules used by living organisms are chiral, and of a particular handedness. Amino acids are almost always left handed, and sugars are almost always right handed. You can imagine that a protein composed of both left and right handed amino acids could have wildly varying shapes. Since a protein's structure is critical for its function, cells must tightly regulate the production of amino acids. Amino acid synthesis proteins will only catalyze the production of left handed amino acids.

Of course, this leads to a chicken-and-egg problem, which is what you alluded to in the OP. Which came first: left handed amino acids or left-handed amino acid synthesis proteins (which are themselves composed of left-handed amino acids)? Unfortunately, there is no firm answer yet, but there are some interesting hypotheses.

According to this link, the presence of certain achiral molecules in a solution can actually catalyze the production of an enantiomeric excess of another molecule. Enantiomeric excess is just a measure of the relative proportions of left and right handed forms- the excess implies that more of one handedness is created than the other. Interestingly, the chemists who discovered this received the Nobel prize a few years back.

Where could these achiral molecules have come from originally? Well, one hypothesis is that they came to the earth from meteorites. Some meteorites have been found to contain an excess of left-handed amino acids. See here for more information.

[Gman]

Where could these achiral molecules have come from originally? Well, one hypothesis is that they came to the earth from meteorites. Some meteorites have been found to contain an excess of left-handed amino acids. See here for more information.

Addressed here... The Origin of Homochirality: A Major Problem for Origin of Life Theories.

"Years later, in 1969, a meteor that landed in Murchison, Australia, was shown to contain the same organic compounds and A group of 20 different kinds of small molecules that link together in long chains to form proteins. Often referred to as the "building blocks" of proteins.amino acids, in roughly the same proportion as those generated through the Miller experiments. An initial examination of the A property of a compound to exist as two optical forms (left or right-handed).chirality of the A group of 20 different kinds of small molecules that link together in long chains to form proteins. Often referred to as the "building blocks" of proteins.amino acids revealed either no A property of a compound to exist as two optical forms (left or right-handed).chiral excesses or The left-handed entantiomer form of amino acid, normally produced by living organisms.L-amino acid excesses that were probably the result of contamination by terrestrial sources. However, in 1997, a study examined A property of a compound to exist as two optical forms (left or right-handed).chiral proportions of non-terrestrial A group of 20 different kinds of small molecules that link together in long chains to form proteins. Often referred to as the "building blocks" of proteins.amino acids.2 The results showed a 7% and 9% L-Referring to either of a pair of optical compounds whose molecular structures have a mirror-image relationship to each other.enantiomeric excess within extracts of the meteorite. Although statistically significant, such as small excess of The left-handed entantiomer form of amino acid, normally produced by living organisms.L-amino acidswould not solve the problem of 100% L-Referring to either of a pair of optical compounds whose molecular structures have a mirror-image relationship to each other.enantiomeric excess required by earth's life forms.

The carbonaceous meteorite GRA 95229 was discovered in 1824 in Antarctica and appears to be relatively free of terrestrial contamination. Analysis of the organic materials revealed the presence of several A group of 20 different kinds of small molecules that link together in long chains to form proteins. Often referred to as the "building blocks" of proteins.amino acids and Organic compounds containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double-bonded to an oxygen atom (chemical formula O=CH-), is called the aldehyde group.aldehydes with L-Either of a pair of optical compounds whose molecular structures have a mirror-image relationship to each other.enantiomer excesses up to 14%.3 Again, such excesses are not enough to account for the origin of A property of an optically active compound to consist of only one enantiomer (left or right-handed form).homochirality."

[hopefulcynic]

The meteorite need not contain 100% enantiomeric excess in order to explain the origins of homochirality. A slight enantiomeric excess, coupled with asymmetric catalysis, would be sufficient.