| dc.description.abstract | NADH cytochrome b5 oxidoreductase (Ncb5or) is a multi-domain redox enzyme found in all animal tissues and associated with the endoplasmic reticulum (ER). Ncb5or contains (from N-terminus to C terminus) a novel N-terminal region, the b5 domain (Ncb5or-b5), the CS domain, and the b5R domain (Ncb5or-b5R). Ncb5or-b5, the heme binding domain, is homologous to microsomal cytochrome b5 (Cyb5A) and belongs to cytochrome b5 superfamily. Ncb5or-b5R, the FAD (flavin adenine dinucleotide) binding domain, is homologous to cytochrome b5 reductase (Cyb5R3) and belongs to ferredoxin NADP+ reductase superfamily. Both superfamilies are of great biological significance whose members have important functions. The CS domain can be assigned into the heat shock protein 20 (HSP20, or p23) family, whose members are known to mediate protein-protein interactions. Ncb5or is unique in containing these domains and employing a novel sequence as the N-terminal region. Cyb5A and Cyb5R3 have been shown to form a complex to donate electrons to stearoyl-CoA desaturase (SCD) in fatty acid desaturation in vitro. Monounsaturated fatty acids are preferentially used in triglyceride synthesis for lipid storage. Mice with hepatic deletion of Cyb5A show no significant defect in lipid metabolism, whereas Ncb5or null mice exhibit diabetes, lipoatrophy and impaired SCD activity. We hypothesize that Ncb5or, as a fusion of both Cyb5A and Cyb5R3-like domains, serves as an alternative electron donor to SCD in vivo. Ncb5or-b5 shows unique structural and functional features, such as, low sequence homology to other family members, displacement of the second heme-ligating His residue, a decrease in the number of surface-charged residues, and a much lower redox potential at the heme center compared with Cyb5A. In order to understand how these differences between Ncb5or and Cyb5A contribute to their unique structural and functional properties in electron transfer, we solved the crystal structure of Ncb5or-b5 and performed kinetic assays by using various domain combinations. Our 1.25 Å crystal structure shows that Ncb5or-b5 has a general b5 fold. However, Ncb5or-b5 has a unique heme environment, namely the two heme-ligating His residues are nearly perpendicular to each other. The latter feature agrees with a large gmax value in electron paramagnetic resonance (EPR) spectra. This makes Ncb5or the first example with a tetragonally distorted heme environment in the cytochrome b5 superfamily. Lower density of charge on the surface of Ncb5or-b5 suggests weaker ionic interaction between Ncb5or-b5 and Ncb5or-b5R than that between Cyb5A and Cyb5R3. The lack of Ncb5or-b5/Ncb5or-b5R complex formation and lower rates of electron transfer than Cyb5A/Cyb5R3 are consistent with this hypothesis. Low affinity between Ncb5or-b5 and Ncb5or-b5R prompted a search for other factors that facilitate the inter-domain electron transfer. Specifically, the role of the N-terminal region, a region rich in random coils but with little homology to known proteins, was explored. A reductionist approach was used to test various domains of Ncb5or with kinetic assays and spectral analyses, such as circular dichroism and nuclear magnetic resonance. I have unveiled the role of the N-terminal region for the first time: the N-terminal region facilitates inter-domain electron transfer by cooperatively interacting with the CS domain and the heme center of Ncb5or-b5. The formation of tertiary structure in the N-terminal region is governed by the region from Gly22 through Trp37, especially Trp37, and this region is essential for electron transfer. This feature suggests a potential mechanism to facilitate the inter-domain electron transfer in Ncb5or. | |
