Intramolecular electron-transfer reaction within a diprotein complex of cytochrome c with ferrylmyoglobin modified with diethylenetriaminepentaacetic acid.

Horse heart metmyoglobins modified with diethylenetriaminepentaacetic acid, metMb(DTPA)n (n=1, 2, 4, and 5), were characterized by a MALDI-TOF mass spectrometry, amino-acid sequence analysis, and UV-Vis and CD spectroscopies. The DTPA-binding sites on metMb were Lys47, Lys50, Lys87, Lys145, and Lys147 for metMb(DTPA)5, Lys47, Lys87, Lys145, and Lys147 for metMb(DTPA)4, Lys87 and Lys145 for metMb(DTPA)2, and Lys87 for metMbDTPA, respectively. The modified metMb(DTPA)n showed cytochrome c peroxidase-like activity more efficiently than native metMb: metMb(DTPA)5>metMb(DTPA)4>metMb(DTPA)2> metMbDTPA approximately equals native metMb. The first-order rate constants for the reactions of ferrylMb(DTPA)n (n=2, 4, and 5) with reduced cytochrome c [cyt c(II)] were saturated with concentrations of cyt c(II), suggesting that the electron transfer (ET) occurs within a diprotein complex. The intramolecular ET rate constants in the diprotein complex increased with increasing the number of DTPA ions. The reactions of native ferrylMb and ferrylMbDTPA with cyt c(II) obeyed a second-order rate law. A possible ET mechanism is proposed; cyt c(II) binds the DTPA-linked anionic patch around Lys87, Lys145, and Lys147 region of ferrylMb(DTPA)n.