How to synthesize D proteins
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Amino acids are chiral molecules that exist in nature in a D- and an L-form. In natural proteins, however, only the L-amino acids occur, which is why the D-amino acids are also counted among the non-proteinogenic amino acids.
Nevertheless, D-amino acids perform some important tasks in biological systems. For example, bacterial cell walls contain D-amino acids (e.g. D-alanine) in addition to L-amino acids. These amino acids protect the bacterium from attack by peptidases, which are produced by many other organisms to digest bacterial cell walls, since these enzymes can usually only attack between L-amino acids.
D-amino acids in mushrooms: antibiotics
D-amino acids are also often found in antibiotics, which are synthesized by many fungi or bacteria as defense substances. The antibiotic valinomycin is a cyclic depsipeptide that consists of a sequence of (L-Val) - (D-Hydroxyisovaleric acid) - (D-Val) - (L-lactate).
Other uses for D-amino acids
Interestingly, the L- and D-enantiomers of the amino acids can partly be differentiated by taste. For example, D-amino acids are often added to yoghurt or haddock dishes to improve the taste of these foods. The D-shape basically seems to be sweeter or less bitter for humans than the respective L-shape.
D-amino acids are also of interest for forensic medicine. The radiocarbon method is often used for dating biological material, but this fails in relatively short periods of time. The analysis of the racemization of Asp, Ser or (for older samples) Leu helps here. D-Asp created by racemization can be determined, for example, in the lens protein of the eye. At a racemization rate of 0.14% per year, a 30 year old person has accumulated 4.2% D-Asp in this particular protein. Even old paintings can be dated relatively precisely in this way - at least if the colors contained proteins!
|Doyle, D. A .; Wallace, B. A. (1997): Crystal structure of the gramicidin / potassium thiocyanate complex. In: J. Mol. Biol.. 266 , 963-977|
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