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Cyclic peptides in plants are synthesized via a two-step process; the translation of a linear peptide chain, and its subsequent formation into a cyclic structure through activities of a protease-like enzyme or other ways. [6] [7] [8] Some peptides, such as cyclotides, are gene-coded products obtained by the processing of larger precursor proteins.
Cyclotides have a well-defined three-dimensional structure due to their interlocking disulfide bonds and cyclic peptide backbone. Backbone loops and selected residues are labeled on the structure to help orientation. The amino acid sequence (single-letter amino acid representation) for this peptide is indicated on the sequence diagram to the right.
A number of natural antimicrobial peptides are cyclic, and the products of SICLOPPS are "increasingly viewed as ideal backbones for modulation of protein-protein interactions." [3] Circular peptides tend to be resistant to protease activity, and may be suitable for use as orally administered drugs.
Lin and her lab use computational chemistry to provide information on the solution structures of cyclic peptides. [2] They recently successfully used molecular dynamics simulation with enhanced sampling methods to design well-structured cyclic peptides. [12] [13]
Crystal structure of an extracellular segment of integrin alphaVbeta3 complexed with a cyclic peptide containing the arginyl-glycyl-aspartic acid (RGD) sequence. RGD is shown in maroon. CEND-1, also known as iRGD, is a cyclic peptide that homes to tumors via binding to integrin alpha V receptors. [22]
The ICK motif is a very stable protein structure which is resistant to heat denaturation and proteolysis. [6] CK peptide components of venoms target voltage-gated ion channels but members of the family also act as antibacterial and haemolytic agents. [7] Plant ICK proteins are often protease inhibitors.
[1] [2] 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 ...
Each nonribosomal peptide synthetase can synthesize only one type of peptide. Nonribosomal peptides often have cyclic and/or branched structures, can contain non-proteinogenic amino acids including D-amino acids, carry modifications like N-methyl and N-formyl groups, or are glycosylated, acylated, halogenated, or hydroxylated.