Re: Why are the amino acids present in our body Laevo-rotatory?

This is really a difficult question to answer, since it gets at the fundamental chemistry of the origins of life. However, before getting into the more theoretical answers, let's look at the easier question of why all the amino acids present in the body have the same chirality (and what chirality is).

There are already several posts in the archive about chirality, so I'll simply say that it is an aspect of a three-dimensional object that makes it non-superimposable with its mirror image (this is also commonly referred to as "handedness"). In organic molecules, the rule of thumb is that any carbon atom bound to four different things is chiral. In amino acids, the central carbon atom is bound to a carboxyl group (- COOH), an amino group (-NH₂), a hydrogen (-H), and a unique side chain that determines the identity of the amino acid (-R). As long as the side chain (-R) is different from the other three groups, the amino acid is chiral (note: glycine has a hydrogen side chain (-H), so it is not a chiral molecule).

So, how is this important to the organism? Proteins are made of long chains of amino acids that are folded upon themselves to form the three-dimensional structure that gives the protein its function. The two most common structures formed in this folding are helices and sheets. In each case, the repeated nature of the structure places all of the amino acid side chains in the same orientation. This orientation is only possible if all of the amino acids have the same intrinsic three-dimensional orientation, or chirality. That is, a protein composed of both D and L amino acids would have half of its side chains going one way and half the other. Clearly without distinguishing between D and L amino acids, there would be no way to control the final structure of the protein. By using only one form of each amino acid (either D or L), every copy of the same protein will be folded the same.

It is also more convenient for all amino acids to have the same chirality. Since most of the amino acids are synthesized as derivatives of each other, requiring one amino acid to be L would force several others to be the same. Similarly, many of the enzymes that interact with proteins and amino acids have conserved binding sites that can only fit one amino acid form, so having some L-amino acids and some D-amino acids would require a separate set of enzymes for each. Thus, with all the amino acids having the same chirality, this allows much more modularity during protein synthesis.

Now for the theoretical stuff: As stated above, many of the enzymes that interact with amino acids are sensitive to the spatial orientation of the side chains, not the least of which are the ribosomes that actually form the peptide bonds that hold the amino acids in the protein together. It has been demonstrated by several labs that replacing L-amino acyl-tRNA's with D-amino acyl-tRNA's greatly reduced the rate of protein synthesis. This preference for L-amino acids appears to be intrinsic to the ribosomal RNA's themselves. With the recent evidence that rRNA contains the functional enzymatic activity of the ribosome, and the further discovery that rRNA's may represent some of the earliest complex molecules, it has been suggested that some intrinsic relationship between D-ribonucleotides and L-amino acids must have arisen in the prebiotic world that eventually gave rise to cellular life. However, this relationship is not supported by recent experiments on RNA - amino acid interactions, and still doesn't explain why D-ribonucleotides should become prevalent over L- ribonucleotides. So, the ultimate answer is, "we don't know," although several scientists are still actively trying to solve this dilemma.