Loss of Electrostatic Interactions Causes Increase
of Dynamics within the Plastocyanin–Cytochrome <i>f</i> Complex
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Abstract
Recent studies on the electron transfer
complex formed by cytochrome <i>f</i> and plastocyanin from <i>Nostoc</i> revealed
that both hydrophobic and electrostatic interactions play a role in
the process of complex formation. To study the balance between these
two types of interactions in the encounter and the final state, the
complex between plastocyanin from <i>Phormidium laminosum</i> and cytochrome <i>f</i> from <i>Nostoc</i> sp.
PCC 7119 was investigated using NMR spectroscopy and Monte Carlo docking.
Cytochrome <i>f</i> has a highly negative charge. <i>Phormidium</i> plastocyanin is similar to that from <i>Nostoc</i>, but the net charge of the protein is negative rather
than positive. NMR titrations of Zn-substituted <i>Phormidium</i> plastocyanin and <i>Nostoc</i> cytochrome <i>f</i> indicated that a complex with an affinity intermediate between those
of the <i>Nostoc</i> and <i>Phormidium</i> complexes
is formed. Plastocyanin was found in a head-on orientation, as determined
using pseudocontact shifts, similar to that in the <i>Phormidium</i> complex, in which the hydrophobic patch represents the main site
of interaction on plastocyanin. However, the interaction in the cross-complex
is dependent on electrostatics, similar to that in the <i>Nostoc</i> complex. The negative charge of plastocyanin decreases, but not
abolishes, the attraction to cytochrome <i>f</i>, resulting
in the formation of a more diffuse encounter complex than in the <i>Nostoc</i> case, as could be determined using paramagnetic relaxation
spectroscopy. This work illustrates the subtle interplay of electrostatic
and hydrophobic interactions in the formation of transient protein
complexes. The results are discussed in the context of a model for
association on the basis of hydrophobic contacts in the encounter
state