A issue of Science journal of july 2005 reported 100 questions that span the sciences (Sciences, 2005, vol. 309 - page 86). It is expected that some will drive scientific inquire for a long time while others may soon be answered.
A very intriguing question was: "What is the origin of homochirality in Nature?"
The origin of the preference between mirror-images of natural molecules (such as amino acids and sugars) still remains a mistery.
Amino acids are individual building blocks that comprise proteins. About 500 amino acids are known and have been classified in many ways (Wagner and Musso - 1983).
The classification is done on the basis of:
  • the position of the functional groups (α, β, γ, etc. depending on the position of the amino group against the carboxylic acid)
  • the type of side chain group
  • isomerism
  • be proteinogenic or less
  • to be of natural origin or not
 All α-amino acids (AA) but glycine exist in either of the two enantiomers, which are mirror images of each other (the so called D- and L-enantiomers):




What are D-amino acids?

Amino acids exists in two orientation in the space: levorotatory (L) and dextrorotatory (D), also indicated as left-handed and right-handed. The majority of amino acids (D-AA) in living organisms are L-isomers. In the last few decades with the advancement of technology, the presence of D-amino acids was discovered in living things. This finding opened the questions related to their origin, synthesis, and physiological roles.
tRNA select the L-enantiomer?

D-amino acids do not bind tRNA molecules, thus their presence in proteins is related to posttranslational modifications (Doolittle, 1983).

Recently, it was demonstrated that the cellular translational machinery accepts D-AA-tRNAs into the ribosomial AA tRNA binding site A, uses it as a peptidyl - transfer acceptor, and traslocates the resultating peptidyl-D-AA-tRNA into the ribosomial P (peptidyl) site. Subsequently, the latter traps the ribosomal peptidyl-transferase center in a conformation that impairs peptidyl transfer, (Englander et al., 2015).
References:

  • Doolittle, R. (1983) Probability and the origin of life. In: Godfrey, L.R., ed., 1983. Scientists Confront Creationism, W.W. Norton, NY
  • Englander M.T. (2015) The Ribosome cam discriminate the chirality of amino acids within its peptidyl-transferase center. PNAS 19 (112): 6038-6043
  • Wagner I, Musso H. "New Naturally Occurring Amino Acids" (1983). Angewandte Chemie International Edition 22 (22): 816-28

Authors:

Loredano Pollegioni, Università degli Studi dell'Insubria