Circulin

The three-dimensional solution structure of circulin A, a 30 residue polypeptide from the African plant Chassalia parvifolia, has been determined using two-dimensional 1H-NMR spectroscopy. Circulin A was originally identified based upon its inhibition of the cytopathic effects and replication of the human immunodeficiency virus. Structural restraints consisting of 369 interproton distances inferred from nuclear Overhauser effects, and 21 backbone dihedral and nine chi1 angle restraints from spin-spin coupling constants were used as input for simulated annealing calculations and energy minimisation in the program X-PLOR. The final set of 12 structures had mean pairwise rms differences over the whole molecule of 0.91 A for the backbone atom, and 1.68 A for all heavy atoms. For the well-defined region encompassing residues 2-12 and 18-27, the corresponding values were 0.71 and 1.66 A, respectively. Circulin A adopts a compact structure consisting of beta-turns and a distorted segment of triple-stranded beta-sheet. Fluorescence spectroscopy provided additional evidence for a solvent-exposed Trp residue. The molecule is stabilised by three disulfide bonds, two of which form an embedded loop completed by the backbone fragments connecting the cysteine residues. A third disulfide bond threads through the centre of this loop to form a "cystine-knot" motif. This motif is present in a range of other biologically active proteins, including omega-contoxin GVIA and Cucurbita maxima trypsin inhibitor. Circulin A belongs to a novel class of macrocyclic peptides which have been isolated from plants in the Rubiaceae family. The global fold of circulin A is similar to kalata B1, the only member of this class for which a structure has previously been determined.

Computational methods have refined the mode of peptide drug designing to a new plateau recently. Circulin, a 30 residue natural plant polypeptide acts as a plant defensive peptide. Additional to its antimicrobial activity it also possesses an inhibitory cytopathic effect on the replication of human immunodeficiency virus (HIV). Stable Circulin can be a functionally able template for scaffolding peptide based drugs. Hence, structural stability of Chassalia parvifolia, Circulin A (1BH4), and Circulin B (2ERI) was computationally investigated. From this analysis, the stability favored toward Circulin B which was supported by various parameters such as intra-molecular interactions (61), secondary structure, hydrophobicity (67.34%), root mean square deviation (2.64Å), root mean square fluctuation (0.08Å), radius of gyration (8.96Å), ovality (3.49), angular deviation (73.6%), surface area (both polar and non-polar), hydrogen bond distribution (11.94), and disulphide bond distances. Further, the functional activity calculated in terms of membrane associated free energy (-4.10 kcal/mol) also favored Circulin B. Hence, Circulin B could be proposed as the best template for scaffolding antimicrobial as well as antiviral (HIV) peptide based drug design. The obtained computational data can aid experimental biologists to successfully produce stable therapeutic peptides from natural resources reducing erroneous wastage of monetary sources and time.

Four new macrocyclic polypeptides were isolated and identified from an extract of the tropical tree Chassalia parvifolia. Circulins C-F are 29-30 amino acid cyclic peptides in which the entire primary amino acid chain is covalently cyclized via peptide bonds. Their structures were deduced from a combination of FABMS analyses, N-terminal Edman degradation, endoproteinase digestion, and amino acid analyses. All the peptides share a high degree of sequence homology and contain six cysteine residues forming three intramolecular disulfide bridges. Circulins C-F inhibited the cytopathic effects of in vitro HIV-1 infection with EC(50) values of 50-275 nM.