title: Cooperative metal binding and helical folding in model peptides of treble-clef zinc fingers.
abstract: Two peptides, L(TC) and L(TC)(T) have been synthesised to model the treble-clef zinc fingers encountered in many Zn(Cys)(4)-site-containing proteins. Both are cyclic peptides with a linear tail grafted on a glutamate side chain of the cycle. They differ by the length of this tail, which lacks five amino acids in L(TC)(T) compared to L(TC). Both peptides bind Zn(2+) and Co(2+) in 1:1 metal/peptide ratio and the structure of these complexes have been characterised by NMR, UV/Vis and CD spectroscopy. Both peptides fold the same way around the metal ion and they fully reproduce the classical fold of treble-clef zinc fingers and display an extended hydrogen-bond network around the coordinating sulfur atoms. The structures of the ML(TC) complexes reveal that the linear tail forms a short two-turn alpha-helix, present in the metallated form only. The formation of this helix constitutes a rare example of metal-induced folding. The second turn of this helix is composed of the five amino acids that are absent in L(TC)(T). The study of the pH-dependence of the Zn(2+) binding constants shows that the metal ion is bound by four cysteinates above pH 5.2 and the binding constants are the highest reported so far. Interestingly, the binding constant of Zn x L(TC) is about tenfold higher than that of Zn x L(TC)(T). This difference clearly indicates that the helix, present in Zn x L(TC) only, stabilises the Zn(2+) complex by about 1.2 kcal mol(-1). The origin of this stabilisation is ascribed to an electrostatic interaction between the [ZnS(4)](2-) centre and the helix. This reveals a cooperative effect: zinc binding allows the folding of the tail into a helix which, in turn, strengthens the zinc complex.
authors.affiliation: Laboratoire de Chimie et Biologie des Métaux, CNRS UMR 5249, 17, rue des Martyrs, 38054 Grenoble, France. olivier.seneque@cea.fr
authors.forename: Olivier
authors.initials: O
authors.lastname: Sénèque
authors.forename: Estelle
authors.initials: E
authors.lastname: Bonnet
authors.forename: France Lyne
authors.initials: FL
authors.lastname: Joumas
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Latour
mesh_terms: D001665:Binding Sites; D002942:Circular Dichroism; D003035:Cobalt / Q000737:chemistry / Q000378:metabolism*; D003545:Cysteine / Q000737:chemistry / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D010456:Peptides, Cyclic / Q000138:chemical synthesis* / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D013455:Sulfur / Q000737:chemistry; D015032:Zinc / Q000737:chemistry / Q000378:metabolism*; D016335:Zinc Fingers
chemical_list: D010456:Peptides, Cyclic; D003035:Cobalt; D013455:Sulfur; D015032:Zinc; D003545:Cysteine