Protein - Protein Interactions
by Dr. J. Hazzard

• The interaction between two proteins is a complex process. At large distances, electrostatic interactions play a predominate role, helping to properly orient and attract two proteins.  While electrostatic interactions are also significant in the transient colliosional complex formed between two proteins, hydrophobic forces also help stabilize the protein-protein complex, enabling a chemical reaction to occur.  In the photosynthetic pathway, ferredoxin-NADP⁺ oxido-reductase (FNR), containing a FAD, oxidizes the FeS protein, ferredoxin (Fd).   Under physiological conditions, a strong (however, transient) complex (K_d = 3 mM) is formed. 

The space-filling representation of Fd shows a large number of acidic sidechains (pI ~ 3.5) clustered around the periphery of the FeS prosthetic group.  However, only one acidic sidechain, Glu-94, is absolutely required for salt bridge interaction with FNR.  The other acidic residues presumably simply create a net negative electrostatic potential.  The hydrophobic residue, Phe-65, is also directly involved with binding to FNR.

The distribution of electrostatic potential on the surface of Fd is shown in this figure (Red = -, Blue = +, White = neutral).  The FeS center is buried within an electrostatically neutral “nose” which is surrounded by negative charges.

The Fd binding region of FNR has the FAD group in the middle.  Electron transfer between Fd and FNR occurs at the exposed edge of the flavin ring of FAD.  Three FNR sidechains, which have been shown to be essential for binding Fd, are shown:  Lys-75, Leu-76, and Leu-78, the former being basic while the latter two are hydrophobic.  Interestingly, an acidic patch, located to the left of the FAD also contributes significantly to the proper orientation of Fd, when bound to FNR, repelling the Fd away from this region of the FNR surface due to electrostatic repulsion.  The electrostatic potential map of FNR at clearly shows the dipolar nature of FNR’s surface. In fact, the topology of FNR is much like a cupped hand with the important residues lying on one extended finger.

In order to bring the FeS and FAD prosthetic groups into close proximity to allow facile electron transfer, complex formation between FNR (white) and Fd (green) involves both electrostatic and hydrophobic interactions.  The docked complex shown is based upon kinetic data obtained by Drs. John Hurley and Gordon Tollin (U of A), with the key interactions highlighted.  Key features are: Electrostatic attraction between FNR K75 and Fd E94; Electrostatic repulsion between FNR’s acidic patch and the acidic Fd; Hydrophobic interactions between FNR L76, L78 and Fd F65. The nature of this complex is such that the electron transfer reaction may proceed, however, the two proteins can readily dissociate.  Recent crystallographic data supports this proposed model.