Why are only L-amino acids found in living beings (very rare occurrence of D-amino's)?

Question

PLEASE ONLY OBJECTIVE, SCIENTIFIC ANSWERS. I am not interested in any philisophical or religious takes on this issue. It would be great if I could start a dialogue amoung Chemists / Biochemists on this issue. (I am a Chemistry major at Ohio State and I just finished the third of three organic chem classes.)

Since the inter-molecular reactions for the D-amino would be very different (due to the stereochemical hinderances of the functional groups, possibilities of different H-bonds, etc) would it have been possible for a different type to life to have evolved from D-amino's? The best known ways to create amino acids in the lab yield racemic mixtures (D and L). My assumption is the way amino acids were originally created is still unknown, and that natural way yields a primary product of L-amino's. What where the product yields in the Miller Urey experiment? Were they a racemized amino mixture?

Answer 1

Optical centers tend to be linked with the chemical method(s) of synthesis and disassembly of the optically active molecules.

Enzymes are proteins which act as catalysts and have a three dimensional structure. If primarily L-amino acids are used in the structure of the proteins (enzymes) which assemble and disassemble other proteins, it stands to reason that L-amino acids will be more inclined to "fit".

The smallest amino acid, Glycine, is not optically active because it has no optical center. Most of the other 40 something amino acids found in life forms do have the L-configuration. Biologically, this seems to be linked to the absolute configuration of a common biological starting material: L-glyceraldehyde. This just brings up the question Why is L-glyceraldehyde so prevalent and not D-glyceraldehyde?

D-amino acids found in nature are often part of polypeptides which have antibiotic properties. D-amino acids are not part of proteins. There are no known proteins, natural or artificial, of any size which are made up entirely of D-amino acids. There is hope that researching the structure of such a protein and comparing this to the structure of a similar L-amino acid protein might shed some light on this topic.

Answer 2

This is still an open question among scientists. One possibility is that the chirality of amino acids was defined by the template they were made on. I don't remember the details but there were some experiments done in the past that showed that when synthesized on zeolites (a common mineral on Earth), L-amino acids were made in excess. It is a very likely hypothesis as we know a lot of chiral catalist these days. Chemical and biological evolution is directionless, so it's really hard to say what exactly happened, but the fact is that L-amino acids later became more prevalent (maybe they acted as auto-catalysts and even more L-amino acids were made). A similar question can be asked about D-carbohydrates (D-glucose etc). Why not L-carbohydrates?

As far as the explanation using enzymes - amino acids were first, enzymes came later. This hypothesis cannot explain why L-amino acids were made in the first place.

Answer 3

Actually, the enzymes in our body are very specific and u can imagine them to be having fixed places for fixed groups or parts of molecules to be attached.So, an enzyme can produce a protein with only and only one type of substrate, not many.(D- and L- isomers are different substrates) During the course of evolution, those humans survived who had enzymes transforming into L-amino compatible types. (transformation during evolution) There's no reason behind it, i mean, that the humans with enzymes transforming into D-amino compatible types could hv survived equally well, but due to other reasons, they slowly vanished.(it doesnt mean that they died coz of that, but it means that the humans with L-amino compatible enzymes grew more in number, and finally, only they were found)

So, the root reason behind it is that enzymes are not compatible with D-aminos, hence cannot form proteins with them. ENZYMES ARE VERY SPECIFIC.

Answer 4

Very interesting. I suppose the best response would be that it just happened because whatever "life" or organisms that were created with the L-isomer survived, and the organisms with the D- didn't.