D-peptide

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A D-peptide is a small sequence of D-amino acids. Since ribosomes are specific to L-amino acids, D-peptides rarely occur naturally in organisms and are not easily digested or degraded. D-peptide peptidomimetics are D-peptides designed to mimic natural L-peptides that commonly have therapeutic properties. A peptide with secondary structure cannot be mimicked by its retro-inverse, as linking in the reverse order breaks many backbone interactions essential for the secondary structure. [1] An approach to mimicking these peptides is by searching for similar (sidechain) structures in a mirrored copy of the Protein Data Bank for the structured elements, and then linking the sections by retro-inversed versions of the loops found in the original protein. [2]

Contents

Figure 2. D-peptides assume the mirror image conformation of their L-peptide analogues. Many D-proteins and other D-peptides when placed in a nonchiral solvent like water, assume the mirror image conformation of their L-peptide counterpart. Pictured is an L-peptide (1) fragment with the sequence Asp-Val-Ser and the D-peptide Asp-Val-Ser (2) shown from C-terminus to N-terminus. L-Peptide-D-PeptideMirrorImages.png
Figure 2. D-peptides assume the mirror image conformation of their L-peptide analogues. Many D-proteins and other D-peptides when placed in a nonchiral solvent like water, assume the mirror image conformation of their L-peptide counterpart. Pictured is an L-peptide (1) fragment with the sequence Asp-Val-Ser and the D-peptide Asp-Val-Ser (2) shown from C-terminus to N-terminus.

When placed in a nonchiral solvent like water, D-peptides, as well as the larger polypeptide D-proteins, have similar but mirrored properties to the L-peptides and L-proteins with identical sequences. If an L-protein does not require a chaperone or a structural cofactor to fold, its D-enantiomer protein should have a mirror image conformation with respect to the L-protein (Figure 2). A D-enzyme should act on substrates of reverse chirality compared to the L-enzyme with the same sequence. Similarly, if an L-peptide binds to an L-protein, their D-peptide and D-protein counterparts should bind together in a mirrored way. [3]

D-peptides also have properties that make them attractive as drugs. D-peptides are less susceptible to be degraded in the stomach or inside cells by proteolysis. D-peptide drugs can, therefore, be taken orally and are effective for a longer period of time. D-peptides are easy to synthesize when compared to many other drugs. In some cases, D-peptides can have a low immunogenic response. [4]

Ret design

An L-peptide has three analogue sequences (Figure 3) built from L and D amino acids: the D-enantiomer or inverso-peptide with the same sequence, but composed of D-amino acids and a mirror conformation; the retro-peptide, consisting of the same sequence of L amino acids but in reverse order; and the retro-inverso or D-retro-enantiomer peptide, consisting of D-amino acids in the reversed sequence. [5] [6]

While the L-peptide and its D-enantiomer are mirror structures of each other, the L-retro-peptide is the mirror image of the D-retro-inverso-peptide. On the other hand, the L-peptide and the D-retro-inverso-peptide share a similar arrangement of side-chains, although their carboxyl and amino groups point in opposing directions. For small peptides that do not depend on a secondary structure for binding, an L-peptide and its D-retro-inverso-peptide is likely to have a similar binding affinity with a target L-protein.

Figure 3. An L-peptide and its analogues. An L-peptide (1) sequence has three analogues: the D-enantiomer (3) with the same sequence, the retro L-peptide (4) with the inverted sequence, and the retro-inverso D-peptide(2), with all D-amino acids and the inverted sequence. In this image (1) and (3) are shown from C-terminus on the left to N-terminus on the right, while (2) and (4) are shown from N-terminus to C-terminus. Note that (1) and (2) have similar side chain positions; one is the retro-inverso sequence of the other. The same applies to (3) and (4). L-peptideD-peptideAnalogues.png
Figure 3. An L-peptide and its analogues. An L-peptide (1) sequence has three analogues: the D-enantiomer (3) with the same sequence, the retro L-peptide (4) with the inverted sequence, and the retro-inverso D-peptide(2), with all D-amino acids and the inverted sequence. In this image (1) and (3) are shown from C-terminus on the left to N-terminus on the right, while (2) and (4) are shown from N-terminus to C-terminus. Note that (1) and (2) have similar side chain positions; one is the retro-inverso sequence of the other. The same applies to (3) and (4).

Mirror-image phage display

Phage display is a technique to screen large libraries of peptides for binding to a target protein. In phage display, the DNA sequence that codes the potential drug-peptide is fused to the gene of the protein coat of bacteriophages and introduced into a vector. Diversity can be introduced to the peptide by mutagenesis. The protein coats peptides are then expressed and purified, and applied to a surface of immobilized protein targets. The surface is then washed away to remove non-binding peptides, while the remaining binding peptides are eluted. [7]

Mirror-image phage display is a similar method that can be used to screen large libraries of D-peptides that bind to target L-proteins. More precisely, since D-peptides can not be expressed in bacteriophages, mirror-image phage display screens L-peptides that bind to immobilized D-proteins that are previously chemically synthesized. Because of the mirror properties of D-peptides, the D-enantiomer of an L-peptide that binds to a D-protein will bind to the L-protein.

Mirror-image phage display, however, has two disadvantages when compared to phage display. Target D-proteins must be chemically synthesized, which is normally an expensive and time-consuming process. Also, the target protein must not require a cofactor or a chaperone to fold, otherwise the chemically synthesized D-protein will not fold to the target, mirror structure.

Figure 4. Mirror image phage display. (A) A target L-amino acid protein (L-protein) for which an L-peptide inhibitor might be available is selected. The D-enantiomer protein (D-protein) is chemically synthesized from the same sequence using D-amino acids. If the target L-protein does not require a chaperone or co-factor to fold, the D-protein will mirror the conformation and properties of the L-protein, but the L-peptide inhibitor will most likely have little binding affinity towards it. (B) The synthesized D-proteins are fixed to a surface and exposed to phage displaying many different L-peptides. (C) L-peptides that do not bind well to the surface are washed away. The remaining L-peptides are sequenced. (D) The D-enantiomer peptides (D-peptides) of the binding L-peptides are synthesized using the same sequence and tested against the L-proteins. The D-peptide will bind the L-protein but most likely not bind the D-protein mirror. MirrorImagePhageDisplay.png
Figure 4. Mirror image phage display. (A) A target L-amino acid protein (L-protein) for which an L-peptide inhibitor might be available is selected. The D-enantiomer protein (D-protein) is chemically synthesized from the same sequence using D-amino acids. If the target L-protein does not require a chaperone or co-factor to fold, the D-protein will mirror the conformation and properties of the L-protein, but the L-peptide inhibitor will most likely have little binding affinity towards it. (B) The synthesized D-proteins are fixed to a surface and exposed to phage displaying many different L-peptides. (C) L-peptides that do not bind well to the surface are washed away. The remaining L-peptides are sequenced. (D) The D-enantiomer peptides (D-peptides) of the binding L-peptides are synthesized using the same sequence and tested against the L-proteins. The D-peptide will bind the L-protein but most likely not bind the D-protein mirror.

Related Research Articles

<span class="mw-page-title-main">Protein</span> Biomolecule consisting of chains of amino acid residues

Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, and transporting molecules from one location to another. Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in protein folding into a specific 3D structure that determines its activity.

<span class="mw-page-title-main">Zinc finger</span> Small structural protein motif found mostly in transcriptional proteins

A zinc finger is a small protein structural motif that is characterized by the coordination of one or more zinc ions (Zn2+) which stabilizes the fold. It was originally coined to describe the finger-like appearance of a hypothesized structure from the African clawed frog (Xenopus laevis) transcription factor IIIA. However, it has been found to encompass a wide variety of differing protein structures in eukaryotic cells. Xenopus laevis TFIIIA was originally demonstrated to contain zinc and require the metal for function in 1983, the first such reported zinc requirement for a gene regulatory protein followed soon thereafter by the Krüppel factor in Drosophila. It often appears as a metal-binding domain in multi-domain proteins.

<span class="mw-page-title-main">Peptidomimetic</span> Class of compounds designed to mimic features of peptides

A peptidomimetic is a small protein-like chain designed to mimic a peptide. They typically arise either from modification of an existing peptide, or by designing similar systems that mimic peptides, such as peptoids and β-peptides. Irrespective of the approach, the altered chemical structure is designed to advantageously adjust the molecular properties such as stability or biological activity. This can have a role in the development of drug-like compounds from existing peptides. Peptidomimetics can be prepared by cyclization of linear peptides or coupling of stable unnatural amino acids. These modifications involve changes to the peptide that will not occur naturally. Unnatural amino acids can be generated from their native analogs via modifications such as amine alkylation, side chain substitution, structural bond extension cyclization, and isosteric replacements within the amino acid backbone. Based on their similarity with the precursor peptide, peptidomimetics can be grouped into four classes where A features the most and D the least similarities. Classes A and B involve peptide-like scaffolds, while classes C and D include small molecules.

<span class="mw-page-title-main">Phage display</span> Biological technique to evolve proteins using bacteriophages

Phage display is a laboratory technique for the study of protein–protein, protein–peptide, and protein–DNA interactions that uses bacteriophages to connect proteins with the genetic information that encodes them. In this technique, a gene encoding a protein of interest is inserted into a phage coat protein gene, causing the phage to "display" the protein on its outside while containing the gene for the protein on its inside, resulting in a connection between genotype and phenotype. The proteins that the phages are displaying can then be screened against other proteins, peptides or DNA sequences, in order to detect interaction between the displayed protein and those of other molecules. In this way, large libraries of proteins can be screened and amplified in a process called in vitro selection, which is analogous to natural selection.

<span class="mw-page-title-main">Dermorphin</span> Opioid agonist peptide compound

Dermorphin is a hepta-peptide first isolated from the skin of South American frogs belonging to the genus Phyllomedusa. The peptide is a natural opioid that binds as an agonist with high potency and selectivity to mu opioid receptors. Dermorphin is about 30–40 times more potent than morphine, but theoretically may be less likely to produce drug tolerance and addiction due to its high potency. The amino acid sequence of dermorphin is H-Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2.

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<span class="mw-page-title-main">Aptamer</span> Oligonucleotide or peptide molecules that bind specific targets

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<span class="mw-page-title-main">Epitope mapping</span> Identifying the binding site of an antibody on its target antigen

In immunology, epitope mapping is the process of experimentally identifying the binding site, or epitope, of an antibody on its target antigen. Identification and characterization of antibody binding sites aid in the discovery and development of new therapeutics, vaccines, and diagnostics. Epitope characterization can also help elucidate the binding mechanism of an antibody and can strengthen intellectual property (patent) protection. Experimental epitope mapping data can be incorporated into robust algorithms to facilitate in silico prediction of B-cell epitopes based on sequence and/or structural data.

<i>trp</i> operon Operon that codes for the components for production of tryptophan

The trp operon is a group of genes that are transcribed together, encoding the enzymes that produce the amino acid tryptophan in bacteria. The trp operon was first characterized in Escherichia coli, and it has since been discovered in many other bacteria. The operon is regulated so that, when tryptophan is present in the environment, the genes for tryptophan synthesis are repressed.

mRNA display

mRNA display is a display technique used for in vitro protein, and/or peptide evolution to create molecules that can bind to a desired target. The process results in translated peptides or proteins that are associated with their mRNA progenitor via a puromycin linkage. The complex then binds to an immobilized target in a selection step. The mRNA-protein fusions that bind well are then reverse transcribed to cDNA and their sequence amplified via a polymerase chain reaction. The result is a nucleotide sequence that encodes a peptide with high affinity for the molecule of interest.

<span class="mw-page-title-main">Lysin</span>

Lysins, also known as endolysins or murein hydrolases, are hydrolytic enzymes produced by bacteriophages in order to cleave the host's cell wall during the final stage of the lytic cycle. Lysins are highly evolved enzymes that are able to target one of the five bonds in peptidoglycan (murein), the main component of bacterial cell walls, which allows the release of progeny virions from the lysed cell. Cell-wall-containing Archaea are also lysed by specialized pseudomurein-cleaving lysins, while most archaeal viruses employ alternative mechanisms. Similarly, not all bacteriophages synthesize lysins: some small single-stranded DNA and RNA phages produce membrane proteins that activate the host's autolytic mechanisms such as autolysins.

Zinc finger protein chimera are chimeric proteins composed of a DNA-binding zinc finger protein domain and another domain through which the protein exerts its effect. The effector domain may be a transcriptional activator (A) or repressor (R), a methylation domain (M) or a nuclease (N).

Imperatoxin I (IpTx) is a peptide toxin derived from the venom of the African scorpion Pandinus imperator.

<span class="mw-page-title-main">Racemic crystallography</span>

Racemic crystallography is a technique used in structural biology where crystals of a protein molecule are developed from an equimolar mixture of an L-protein molecule of natural chirality and its D-protein mirror image. L-protein molecules consist of 'left-handed' L-amino acids and the achiral amino acid glycine, whereas the mirror image D-protein molecules consist of 'right-handed' D-amino acids and glycine. Typically, both the L-protein and the D-protein are prepared by total chemical synthesis.

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<small>L</small>-Ribonucleic acid aptamer RNA-like molecule

An L-ribonucleic acid aptamer is an RNA-like molecule built from L-ribose units. It is an artificial oligonucleotide named for being a mirror image of natural oligonucleotides. L-RNA aptamers are a form of aptamers. Due to their L-nucleotides, they are highly resistant to degradation by nucleases. L-RNA aptamers are considered potential drugs and are currently being tested in clinical trials.

<span class="mw-page-title-main">Collagen hybridizing peptide</span> Type of synthetic peptide

A collagen hybridizing peptide (CHP) is a synthetic peptide sequence with typically 6 to 10 repeating units of the Gly-Xaa-Yaa amino acid triplet, which mimics the hallmark sequence of natural collagens. A CHP peptide usually possesses a high content of proline and hydroxyproline in the Xaa and Yaa positions, which confers it a strong propensity to form the collagen's unique triple helix conformation. In the single-stranded (monomeric) status, the peptide can recognize denatured collagen strands in tissues by forming a hybridized triple helix with the collagen strands. This occurs via the triple helical chain assembly and inter-chain hydrogen bonding, in a manner similar to primers binding to melted DNA strands during PCR. The binding does not depend on a specific sequence or epitope on collagen, enabling CHPs to target denatured collagen chains of different types.

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