On experiments were performed at room temperature employing the vapour diffusion technique. Hanging droplets were made by mixing 2 ml protein solution (10 mg/ml) with 0.2 M sodium acetate, 0.1 M HEPES, pH 7.4 and 2 M ammoniumwhere F0 is the fluorescence of protein sample when no CPA has been added, F is the protein fluorescence at any given CPA concentration and F420 is the protein fluorescence in the presence of 3 mM of CPA. In the case of 
one ligand binding site, f follows a hyperbolic dependence upon ligand concentration given by:Binding of Fatty Acids to COMPfB free Kd z free??The dissociation constant KFA can be calculated using the value of d [FA]1/2 (the amount of fatty acid that reduces the CPA fluorescence to half its original value.where B is a constant, Kd is the dissociation constant and [L]free is the concentration of free ligand (in this case CPA). The data in Fig. 3B show a good hyperbolic correlation. Therefore, the binding of CPA to COMPcc is consistent with hyperbolic one site binding and the experimentally determined binding constant was 0.760.1 mM. The probe CPA can also be used to characterize the binding of other fatty acids to COMPcc. The addition of fatty acids (FA) to the CPA-COMPcc complex will displace CPA leading to a decrease in 
fluorescence. If the concentrations of COMPcc and CPA are kept significantly lower than the Kd value, the following dissociation constants can be defined for the CPA-COMPcc and FA-COMPcc complexes: PA OMPcc PA{COMPccResults X-Ray structures of the individual COMPcc-fatty acid complexesThe coiled-coil domain of COMP comprising residues 20?2 was obtained by recombinant expression in E. coli as described previously (see also Materials and MedChemExpress GW 0742 Methods and [8]). The individual crystal structures of the COMPcc-fatty acid complexes were solved by molecular replacement using the apo-COMPcc version (PDB code:1MZ9) as a search template (Fig. 1; see also Table 1). In the individual COMPcc-fatty acid complex structures, one molecule of the respective fatty acid is bound inside the Nterminal hydrophobic compartment in a linear, elongated conformation. The longitudinal axis of the fatty acids are parallel to the five-fold channel symmetry (Fig. 1B). Diffusion of the lipophilic ligands into the channel likely occurs LY2409021 web through the Nterminus. Additional electron density in the crystal structure of palmitic acid (C16:0) supports this assumption (see below and Fig. 2B). The fatty acids are retained in the binding pocket through (i) the electrostatic interaction between the electronegative carboxylate head group and the elaborate hydrogen bonding network formed by the Gln54 ring and (ii) the hydrophobic interaction existing between the aliphatic tail of the fatty acids and the hydrophobic cavities that exists between Leu37 and Leu51 residues of COMPcc (Figs. 1B and 2A). These hydrophobic cavities can accommodate fatty acids of different lengths within the channel by mediating interactions with the aliphatic side chains. All amino acid residues in positions a and d of the heptad repeat pattern contribute 16574785 to van der Waals contacts with the alkyl chain of the bound fatty acids. The terminal methyl groups are held in a fixed position by Thr40 (for C14:0), Leu37-Thr40 (for C16:0) and Leu37 (for C18:0). This interaction is elicited by the longitudinal extension of the fully saturated elongated fatty acids. The C20:0 fatty acid complex is well ordered up to Leu37 after which point the aliphatic tail becomes disord.On experiments were performed at room temperature employing the vapour diffusion technique. Hanging droplets were made by mixing 2 ml protein solution (10 mg/ml) with 0.2 M sodium acetate, 0.1 M HEPES, pH 7.4 and 2 M ammoniumwhere F0 is the fluorescence of protein sample when no CPA has been added, F is the protein fluorescence at any given CPA concentration and F420 is the protein fluorescence in the presence of 3 mM of CPA. In the case of one ligand binding site, f follows a hyperbolic dependence upon ligand concentration given by:Binding of Fatty Acids to COMPfB free Kd z free??The dissociation constant KFA can be calculated using the value of d [FA]1/2 (the amount of fatty acid that reduces the CPA fluorescence to half its original value.where B is a constant, Kd is the dissociation constant and [L]free is the concentration of free ligand (in this case CPA). The data in Fig. 3B show a good hyperbolic correlation. Therefore, the binding of CPA to COMPcc is consistent with hyperbolic one site binding and the experimentally determined binding constant was 0.760.1 mM. The probe CPA can also be used to characterize the binding of other fatty acids to COMPcc. The addition of fatty acids (FA) to the CPA-COMPcc complex will displace CPA leading to a decrease in fluorescence. If the concentrations of COMPcc and CPA are kept significantly lower than the Kd value, the following dissociation constants can be defined for the CPA-COMPcc and FA-COMPcc complexes: PA OMPcc PA{COMPccResults X-Ray structures of the individual COMPcc-fatty acid complexesThe coiled-coil domain of COMP comprising residues 20?2 was obtained by recombinant expression in E. coli as described previously (see also Materials and Methods and [8]). The individual crystal structures of the COMPcc-fatty acid complexes were solved by molecular replacement using the apo-COMPcc version (PDB code:1MZ9) as a search template (Fig. 1; see also Table 1). In the individual COMPcc-fatty acid complex structures, one molecule of the respective fatty acid is bound inside the Nterminal hydrophobic compartment in a linear, elongated conformation. The longitudinal axis of the fatty acids are parallel to the five-fold channel symmetry (Fig. 1B). Diffusion of the lipophilic ligands into the channel likely occurs through the Nterminus. Additional electron density in the crystal structure of palmitic acid (C16:0) supports this assumption (see below and Fig. 2B). The fatty acids are retained in the binding pocket through (i) the electrostatic interaction between the electronegative carboxylate head group and the elaborate hydrogen bonding network formed by the Gln54 ring and (ii) the hydrophobic interaction existing between the aliphatic tail of the fatty acids and the hydrophobic cavities that exists between Leu37 and Leu51 residues of COMPcc (Figs. 1B and 2A). These hydrophobic cavities can accommodate fatty acids of different lengths within the channel by mediating interactions with the aliphatic side chains. All amino acid residues in positions a and d of the heptad repeat pattern contribute 16574785 to van der Waals contacts with the alkyl chain of the bound fatty acids. The terminal methyl groups are held in a fixed position by Thr40 (for C14:0), Leu37-Thr40 (for C16:0) and Leu37 (for C18:0). This interaction is elicited by the longitudinal extension of the fully saturated elongated fatty acids. The C20:0 fatty acid complex is well ordered up to Leu37 after which point the aliphatic tail becomes disord.
