title: Thermodynamic, conformational and functional properties of the human C1q globular heads in the intact C1q molecule in solution.
abstract: Thermodynamic. conformational and functional properties of the human C1q globular heads (hgC1q) were studied with the experimental approaches, which allow investigating these properties in the intact hC1q molecule in solution. Surprisingly, the scanning calorimetry data reveal a low level of cooperativity of interactions between the hgC1q A, B and C domains even at a neutral pH area. Ionization of His residues due to acidification of the medium at the pH range from 6 to 5 or the chemical modification of His residues completely abolishes the cooperative interactions between the domains without significant effect on their conformation. The thermodynamic data provide evidence that the hgC1q module is composed of three structurally independent A, B and C globular domains characterized by the practically identical thermal stability and very similar enthalpy of melting. The spectroscopic studies and modification with 2-oxy-5-nitrobenzylbromide (ONBB) indicate that Trp residues in the hgC1q A and C domains are accessible to the solvent that has been confirmed by the hgC1q crystal structure solved and refined to 1.9 A. The modification of Trp residues significantly affects the complement-dependent cytotoxicity without noticeable effect on the hC1q conformation. These data provide evidence that Trp residues are the components of immunoglobulin-binding sites both in the hgC1q A and C domains.
authors.affiliation: The National Center for Biodefense, George Mason University, 10900 University Blvd., MS 4F3 Manassas, VA 20110, USA.
authors.forename: V M
authors.initials: VM
authors.lastname: Tischenko
authors.forename: G A
authors.initials: GA
authors.lastname: Zav'yalova
authors.forename: O P
authors.initials: OP
authors.lastname: Bliznyukov
authors.forename: V P
authors.initials: VP
authors.lastname: Zav'yalov
mesh_terms: D002152:Calorimetry, Differential Scanning; D002942:Circular Dichroism; D015922:Complement C1q / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics
chemical_list: D015922:Complement C1q