Why are amino acids in biology homochiral?

Question

Why are nearly all amino acids in organisms left-handed (exception is glycine which has no isomer) when abiotic samples typical have an even mix of left- and right-handed molecules?

Answer 1

I know that you are referring to the commonly ribosome-translated L-proteins, but I can't help but add that there are some peptides, called nonribosomal peptides, which are not dependent on the mRNA and can incorporate D-amino acids. They have very important pharmaceutical properties. I recommend this (1) review article if you are interested in the subject. It is also worth mentioning that D-alanine and D-glutamine are incorporated into the peptidoglycane of bacteria.

I read several papers (2, 3, 4) that discuss the problem of chirality but all of them conclude that there is no apparent reason why we live in the L-world. The L-amino acids should not have chemical advantages over the D-amino acids, as biocs already pointed out.

Reasons for the occurrence of the twenty coded protein amino acids (2) has an informative and interesting outline. This is the paragraph on the topic of chirality:

This is related to the question of the origin of optical activity in living organisms on which there is a very large literature (Bonner 1972; Norden 1978; Brack and Spack 1980). We do not propose to deal with this question here, except to note that arguments presented in this paper would apply to organisms constructed from either D or L amino acids.

I couldn't find any of the literature they are referring to. Perhaps you will have more luck.

It might be possible that both L and D lives were present (L/D-amino acids, L/D-enzymes recognizing L/D-substrates), but, by random chance the L-world outcompeted the D-world.

I also found the same question in a forum where one of the answers seems intriguing. I cannot comment on the reliability of the answer, but hopefully someone will have the expertise to do so:

One, our galaxy has a chiral spin and a magnetic orientation, which causes cosmic dust particles to polarize starlight as circularly polarized in one direction only. This circularly polarized light degrades D enantiomers of amino acids more than L enantiomers, and this effect is clear when analyzing the amino acids found on comets and meteors. This explains why, at least in the milky way, L enantiomers are preferred.

Two, although gravity, electromagnetism, and the strong nuclear force are achiral, the weak nuclear force (radioactive decay) is chiral. During beta decay, the emitted electrons preferentially favor one kind of spin. That's right, the parity of the universe is not conserved in nuclear decay. These chiral electrons once again preferrentially degrade D amino acids vs. L amino acids.

Thus due to the chirality of sunlight and the chirality of nuclear radiation, L amino acids are the more stable enantiomers and therefore are favored for abiogenesis.

1. BIOSYNTHESIS OF NONRIBOSOMAL PEPTIDES

2. Reasons for the occurrence of the twenty coded protein amino acids

3. Molecular Basis for Chiral Selection in RNA Aminoacylation

4. How nature deals with stereoisomers

Answer 2

The normal results of an attempt to assemble proteins with mixed chiral amino acids is a protein that fails to fold.

The general assumption due to this result is a choice has to be made very early on to use all right-handed or all left-handed amino acids. There doesn't seem to be any particular reason to choose one way over the other except for prevalence.

Answer 3

As far as I know, it is unknown why we only see left-handed and not right–handed amino acids. A recent article speculates that the weak force could be responsible for a tiny asymmetry in energy levels between the stereo-isomers. However, if the effect is tiny, its hard to see why it should have biological implications. In 2004, Tamura and Schimmel showed that RNA has a preference for L-amino acids, whereas mirrored RNA has a preference for D-amino acids. They conclude:

These results suggest the possibility that the selection of L-amino acids for proteins was determined by the stereochemistry of RNA.

So the next question is: Why do we observe only one kind of RNA? It could just be by chance, that a polymer of one RNA configuration became self-replicating.

Answer 4

The ribosome holds the peptide-bound tRNA and aminoacyl-tRNA in the right orientation to catalyze the peptidyltransferase reaction.

http://www.pnas.org/content/103/36/13327/F1.expansion.html

If the incoming aminoacyl-tRNA was the other enantiomer, the amino acid moiety would not fit properly into the ribosome active site. In other words, the shape of the ribosome selects for specific amino acid enantiomers. In abiotic mixes, the creation of amino acids and their polymerization is non-catalytic, and so there is no specificity or selection for certain enantiomers.

If you're asking the "biogenesis" question, then I think the answer is that we don't know the original selection, and it may just be chance. But once biochemistry began making and using them, of course they were all the same. But frankly "why not D-amino acids" makes about as much sense as "why not 22 amino acids, or 23, or 24, or 25?" Because that's what happened.