Acide aminés, peptides et exobiologie...

1
Kuge, K., Brack, A., Fujii, N. (2007). Conformation-dependent racemization of aspartyl residues in peptides, Chem. Eur. J., 13, 5617-5621.

La cinétique de racémisation de l’acide L-aspartique a été suivie dans deux peptides modèles, (Asp-Leu)15 et (Leu-Asp-Asp-Leu)8-Asp, adoptant respectivement une structure en feuillets beta et en hélice alpha. A 37 °C, la racémisation complète requiert 122,6 années pour les feuillets beta, mais seulement 13,1 années pour l’hélice alpha. Ces résultats démontrent clairement que les feuillets beta protègent l’acide aspartique contre la racémisation.

2
Brack, A. (2007) From Interstellar Amino Acids to Prebiotic Catalytic Peptides : A Review. Chemistry & Biodiversity, 4, 665-679.

Les acides aminés étaient probablement abondants sur la Terre primitive, partiellement produits dans l’atmosphère primitive

ou dans les systèmes hydrothermaux sous marins. L’importation d’acides aminés extraterrestres

représenta un apport majeur, comme suggéré par les collectes de micrométéorites et les expériences de simulation menées dans l’espace et en laboratoire. La condensation sélective d’acides aminés en mini-protéines dans l’eau a été obtenue via les N-carboxy anydrides. Les peptides homochiraux à séquence alternée hydrophile/hydrophobe adoptent des structures en feuillets beta stéréosélectifs et thermostables. Certains feuillets beta homochiraux accélèrent fortement l’hydrolyse d’oligonucléotides. Les

peptides formant des feuillets beta protègent les acides aminés contre la réaction de racémisation.

In English:

1
Kuge, K., Brack, A., Fujii, N. (2007). Conformation-dependent racemization of aspartyl residues in peptides, Chem. Eur. J., 13, 5617-5621.

Biologically uncommon D-aspartyl (D-Asp) residues have been detected in proteins of various tissues of elderly humans. The presence of D-Asp has been explained as a result

of the racemization of L-Asp (denoted as Asp) in the protein of inert tissues. We have previously suggested that the racemization of Asp may depend on the conformation of the peptide chain. However, the nature of the peptide conformation that affects the D-Asp formation has not yet been examined. Here we report the kinetics of Asp racemization in two model peptides, (Asp-Leu)15 and (Leu-Asp-Asp-Leu)8-Asp, which form

beta-sheet structures and alpha-helical structures, respectively. For the beta-sheet structures, the activation energy of racemization of Asp residues was 27.3 kcal mol-1, the racemization rate constant at 37 °C was 2.14×10-2 per year and the time required to reach a D/L ratio of 0.99 at 37 °C was 122.6 years as estimated from the Arrhenius equation. For the alpha-helical structures, the activation energy of racemization was 18.4 kcal mol-1, the racemization rate constant 20.02×10-2 per year and the time 13.1 year. These results suggest that Asp residues inserted in alpha-helical peptides are more sensitive to racemization than Asp residues inserted in peptides adopting beta-sheet structures. The results clearly indicate that the racemization rate of Asp residues in peptides depends on the secondary structure of the host peptide.

2
Brack, A. (2007) From Interstellar Amino Acids to Prebiotic Catalytic Peptides : A Review. Chemistry & Biodiversity,

4, 665-679.

Amino acids were most likely available on the primitive Earth, produced in the primitive atmosphere or in hydrothermal vents. Import of extraterrestrial amino acids may have represented the major supply, as suggested by micrometeorite collections and simulation experiments in space and in the laboratory. Selective condensation of amino acids in water has been achieved via N-carboxy anydrides. Homochiral peptides with an alternating sequence of hydrophobic and hydrophilic amino acids adopt stereoselective and thermostable beta-pleated sheet structures. Some of the homochiral beta-sheets strongly accelerate the hydrolysis of oligoribonucleotides. The beta-sheet-forming peptides have also been shown to protect their amino acids from racemization. Even if peptides are not able to self-replicate, i.e., to replicate a complete

sequence from the mixture of amino acids, the accumulation of chemically active peptides on the primitive Earth appears plausible via thermostable and

stereoselective beta-sheets made

of alternating sequences

.