Different Effects of Carboxy-Terminal Deletion in the Adrenodoxin Molecule on Cytochrome c and Acetylated Cytochrome c Reductions.

Abstract

In immunoblotting analysis using a rabbit antibody to bovine adrenodoxin, the total proteins of the bovine adrenal cortex gave two bands, suggesting the presence of two forms of adrenodoxin in vivo: full-length and carboxy-terminal deleted adrenodoxins. To examine the effect of the carboxy-terminal deletion of adrenodoxin on its activity, cDNAs for Arg115stop mutant adrenodoxin and for Asp113stop mutant adrenodoxin were constructed. The wild type [Ad(2-128)] and carboxy-terminal deleted [Ad(2-114) and Ad(2-112)] recombinant adrenodoxins expressed in Escherichia coli were purified to give a single band on SDS-PAGE. They showed an A414/A276 value of 0.92. In an NADPH-cytochrome c reduction assay, the Km values for cytochrome c in the reconstituted system with AD(2-128), Ad(2-114) and Ad(2-112) were 39, 235 and 618 mM, respectively. The Vmax values were 638, 700 and 898 mol/min/mol flavin, respectively. In an NADPH-acetylated cytochrome c reduction assay, the maximum activity of Ad(2-128) was obtained at 50 mM NaCl, while the maximum activities of Ad(2-114) and Ad(2-112) were obtained at 100 mM NaCl; the latter values were 4-times higher than that of Ad(2-128). In the presence of 100 mM NaCl, the Km values for acetylated cytochrome c in the system reconstituted with Ad(2-128), Ad(2-114) and Ad(2-112) were 220, 33 and 22 microM, respectively. The Vmax values were 352, 305 and 382 mol/min/mol flavin, respectively. These results indicate that the effects of the carboxy-terminal deletion of adrenodoxin on NADPH-cytochrome c and acetylated cytochrome c reductions are different; the carboxy-terminal region (residues 113-128) of adrenodoxin largely contributes to the binding with cytochrome c but disturbs the binding with acetylated cytochrome c.

New aspects of electron transfer revealed by the crystal structure of a truncated bovine adrenodoxin, Adx(4–108)

“…Deletion of the C-terminal tail enhances the efficiency of electron transport to Cyt P450 and Cyt c as long as the contact sites of AR and Cyt P450 are left intact. This observation is in agreement with the improved binding to Cyt P450, but decreased affinity for Cyt c [31,42,43]. Whether this behavior is triggered by electrostatic or steric effects can only be decided from the structures of native Adx and its complexes with redox partners.…”