The complex of cytochrome f and plastocyanin from Nostoc sp. PCC 7119 is highly dynamic.
Bottom Line: The complex from Nostoc sp.Chemical-shift-perturbation analysis showed that the presence of spin labels on the surface of Cyt f does not significantly affect the binding of Pc.The paramagnetic relaxation enhancement results are not consistent with a single orientation of Pc, thus indicating that multiple orientations must occur and suggesting that an encounter state represents a large fraction of the complex.
Affiliation: Institute of Chemistry, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands.Show MeSH
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Mentions: The fraction of free Pc (f1) is 0.76, and that ofbound Pc (f2+f3)is 0.24. By dividing the observed PRE by 0.24, the PRE for 100 % bound Pcis obtained. These extrapolated PREs are plotted in Figure 4 (green symbols) against the Pc residue number.Strikingly, the patterns are qualitatively similar for the three spin labelpositions, thus indicating that the same patches of Pc are strongly affected.When the fraction of AB* is neglected(f2≈0), the PRE can be predicted from themodel of the final complex. By using model 1 from PDB ID: 1TU2, thePREAB values were predicted for each amide in Pc (Figure 4, blue symbols). Clearly, themodel alone cannot account for the observed PREs. Also, docking calculationswere performed with distances derived from the PREs as restraints. Apart from avan der Waals repulsion function (to avoid steric collisions) no otherinteractions were included. The ensemble of the ten best structures is shown inFigure 5 and compared with the modelbased on PCS. In both cases, Pc is bound in the region close to the haem, butthe orientation differs. However, also the PRE-based model alone cannot accountfor the observed PREs and the back calculated distances (Figure 4, red symbols in the left panel, red line in theright panel).
Affiliation: Institute of Chemistry, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands.