Divergence in nonspecific hydrophobic packing interactions in the Apo State, and its possible role in functional specialization of mitochondrial and microsomal cytochrome b5

Document Type


Publication Date



The outer mitochondrial membrane isoform of mammalian cytochrome b 5 (OM b5) is distinguished from the microsomal isoform (Mc b5) by its considerably greater stability. In contrast, OM and Mc apocytochrome b5 (apo-b5) exhibit similar thermodynamic stability. Contributing substantially to the greater stability of OM b 5 relative to that of Mc b5 is the presence of Leu at position 71. Replacing Leu-71 in OM b5 with the corresponding Mc b5 residue (Ser) not only diminishes holoprotein stability but also markedly compromises apoprotein stability. The studies reported herein were undertaken to clarify the role played by Leu-71 in stabilizing OM b5 relative to Mc b5s and were motivated by the possibility that stability is related to other differences in OM and Mc b5 properties that are important for their specialized subcellular roles. The results of these studies show that Leu-71 plays an essential role in maintaining the structural integrity of the heme-independent folding core of OM apo-b5 (core 2), despite its location in the disordered empty heme-binding pocket (core 1). The conformational integrity of core 2 in Mc apo-b5s is not similarly dependent on the presence of a hydrophobic residue at position 71, providing new evidence for evolution of compensating structural features not present in OM b5s. We propose that Leu-71 achieves its effect on OM apo-b 5 core 2 structure by participating in a nonspecific hydrophobic collapse of disordered core 1, templated by more conformationally restricted side chains of residues in the β-sheet that separates the two cores. We hypothesize that this has the added effect of maintaining core 1 of OM apo-b5s in a state more compact than that which occurs in Mc apo-b5s, possibly contributing to stronger heme binding by limiting the number of non-native conformations that the empty heme-binding pocket can populate. © 2005 American Chemical Society.

Publication Source (Journal or Book title)


First Page


Last Page


This document is currently not available here.