title: Key role of glutamic acid for the cytotoxic activity of the cyclotide cycloviolacin O2.
abstract: Cyclotides are cyclic plant proteins with potent cytotoxic effects. Here we systematically probed the importance of surface-exposed charged amino acid residues of the cyclotide cycloviolacin O2, using a strategy involving chemical modifications. We show that the single glutamic acid plays a key role for the cytotoxicity: methylation of this residue produced a 48-fold decrease in potency. Virtually no change in potency was observed when masking the single arginine residue using 1,2-cyclohexanedione, while acetylation of the two lysine residues reduced the potency 3-fold. The derivative with modifications at both arginine and lysine residues showed a 7-fold loss of potency. In addition, we show that the activity is dependent on an intact disulfide network and that the short sequences between the six cysteine residues, that is, the backbone loops, are devoid of cytotoxic activity.
authors.affiliation: Division of Pharmacognosy, Department of Medicinal Chemistry, Uppsala University, BMC, Box 574, 751 23 Uppsala, Sweden.
authors.forename: A
authors.initials: A
authors.lastname: Herrmann
authors.forename: E
authors.initials: E
authors.lastname: Svangård
authors.forename: P
authors.initials: P
authors.lastname: Claeson
authors.forename: J
authors.initials: J
authors.lastname: Gullbo
authors.forename: L
authors.initials: L
authors.lastname: Bohlin
authors.forename: U
authors.initials: U
authors.lastname: Göransson
mesh_terms: D000595:Amino Acid Sequence; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D047169:Cyclotides / Q000737:chemistry* / Q000302:isolation & purification / Q000494:pharmacology*; D047168:Cystine Knot Motifs / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D018698:Glutamic Acid / Q000031:analogs & derivatives* / Q000502:physiology*; D006801:Humans; D013058:Mass Spectrometry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017433:Protein Structure, Secondary
chemical_list: D047169:Cyclotides; D018698:Glutamic Acid; C473355:cycloviolacin O2