Cytochrome-P450-Cytochrome-b5 Interaction in a Membrane Environment Changes 15N Chemical Shift Anisotropy Tensors

It has been well realized that the dependence of chemical shift anisotropy (CSA) tensors on the amino acid sequence, secondary structure, dynamics, and electrostatic interactions can be utilized in the structural and dynamic studies of proteins by NMR spectroscopy. In addition, CSA tensors could also be utilized to measure the structural interactions between proteins in a protein protein complex. amide-N-15 CSA tensors for a membrane-bound 16.7 kDa full-length rabbit cytochrome-b(5) (cytb(5)), in complexation with a 55.8 kDa microsomal rabbit cytochrome P450 2B4 (cytP4502B4). The N-15-CSAs, determined using the N-15 CSA/N-15-H-1 dipolar coupling transverse cross-correlated rates, for free cytb(5) are compared with those for the cytb(5) bound to cytP4502B4. An overall increase in backbone amide-N-15 transverse cross-correlated rates for the cytb(5) residues in the cytb(5)-cytP450 complex is observed as compared to the free cytb, residues. Due to fast spin spin relaxation (T-2) and subsequent broadening of the signals in the complex, we could measure amide-N-15 CSAs only for 48 residues of cytb(5) as compared to 84 residues of free cytb(5). We observed a change in N-15 CSA for most residues of cytbs in the complex, as compared to free cytbs, suggesting a dynamic interaction between the oppositely charged surfaces of anionic cytb(5) and cationic cytP450. The mean values of N-15 CSA determined for residues in helical, sheet, and turn regions of cytb(5) in the complex are -184.5, -146.8, and -146.2 ppm, respectively, with an overall average value of -165.5 ppm (excluding the values from residues in more flexible termini). The measured CSA value for residues in helical conformation is slightly larger as compared to previously reported values. This may be attributed to the paramagnetic effect from Fe(III) of the heme in cytb(5), which is similar to our previously reported values for the free cytb(5).