title: Ca2+-binding stoichiometry of calbindin D28k as assessed by spectroscopic analyses of synthetic peptide fragments.
abstract: Calbindin D28k is an intracellular Ca2+-binding protein noted for its abundance and specific distribution in mammalian brain and sensory neurons. This protein contains six putative Ca2+-binding sites, referred to as EF-hands. Due to the presence of the large number of putative sites, previous studies have been unsuccessful in definitively establishing the stoichiometry of Ca2+ binding. We describe a synthetic approach to identify the number of Ca2+-binding sites in which 6 33-residue peptides, designated EF1-EF6, corresponding to the 6 EF-hand sequences of calbindin D28k, were made. The response of each peptide to Ca2+ addition was assessed by 1H NMR spectroscopy, circular dichroism (CD) spectroscopy, and agarose gel electrophoresis. The Ca2+ binding by CD experiments was performed at two peptide concentrations, 20 and 200 microM, and the NMR studies at peptide concentrations ranging from 20 to 100 microM. The CD and 1H NMR data show that five of the six peptides bind Ca2+ as isolated peptides, namely, EF1, EF3, EF4, EF5, and EF6. The EF6 peptide appears to bind Ca2+ with lower affinity than the other four functional sites. In contrast, EF2 does not appear to bind Ca2+ under any of the spectroscopic conditions tested. The data suggest that at least five of the six putative sites in the native protein bind Ca2+, although their relative affinities cannot be deduced from studies of the isolated peptides.
authors.affiliation: Department of Chemistry, Rutgers University, Camden, New Jersey 08102, USA.
authors.forename: K S
authors.initials: KS
authors.lastname: Akerfeldt
authors.forename: A N
authors.initials: AN
authors.lastname: Coyne
authors.forename: R R
authors.initials: RR
authors.lastname: Wilk
authors.forename: E
authors.initials: E
authors.lastname: Thulin
authors.forename: S
authors.initials: S
authors.lastname: Linse
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D064026:Calbindins; D002118:Calcium / Q000378:metabolism*; D002942:Circular Dichroism; D016384:Consensus Sequence; D004587:Electrophoresis, Agar Gel; D018257:Helix-Loop-Helix Motifs; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000737:chemistry / Q000378:metabolism*; D010446:Peptide Fragments / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism; D017433:Protein Structure, Secondary; D064030:S100 Calcium Binding Protein G / Q000737:chemistry / Q000378:metabolism*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
chemical_list: D064026:Calbindins; D009419:Nerve Tissue Proteins; D010446:Peptide Fragments; D064030:S100 Calcium Binding Protein G; D002118:Calcium