Artificial protein-protein complexation between a reconstituted myoglobin and cytochrome c

Artificial prosthetic porphyrins, 1 . Fe and 1 . Zn, in which two isophthalamide units having four carboxylates were bound to the terminal of each peripheral propionate side chain in protoporphyrin LX, were inserted into horse heart apomyoglobin to give novel myoglobins, rMb(1 . Fe) and rMb(1 . Zn), respectively. The resultant reconstituted myoglobins were designed to bind cationic cytochrome c on the protein surface via electrostatic interaction. The isoelectric point for rMb(1 . Fe) was determined to be 5.5, which is about 2 pH units lower than that of native myoglobin. The pi value suggests that eight carboxylates of prosthetic group are located on the surface of the myoglobin. A construction of a myoglobin-cytochrome c complex was probed by paramagnetic H-1 NMR and flash photolysis studies. The behavior of H-1 NMR paramagnetic shifts in the rMb(1 . FeCN)-cytochrome c complex is comparable with that in the native pairing of cytochrome c-cytochrome c peroxidase. Laser flash photolysis shows that a long-range ET from photoexcited rMb-(1 . Zn) to cytochrome c occurs within the protein-protein complex. The time-dependence of the transient spectra at 460 nm identified as the triplet excited state of rMb(1 . Zn) lends to rate constant of forward ET and affinity of the protein-protein complex; k(intra) = (2.2 +/- 0.1) x 10(3) s(-1) and K-a = (6.5 +/-: 3.0) x 10(4) M-1 at 10 mM ionic strength and k(intra) = (2.3 +/- 0.2) x 10(3) s(-1) and K-a = (1.5 +/- 0.6) x 10(4) M-1 at 20 mM ionic strength and pH 7.0. The binding affinity for cytochrome c decreases with increasing the ionic strength, indicating that the protein-protein complex is formed by electrostatic interaction. This work demonstrates that the artificial functional groups bound to the terminal of porphyrin in the reconstituted myoglobin can act as an effective recognition domain for a protein at the surface of the myoglobin.