Dynamic structure of the mitrochondrial cytochrome bc₁ complex

We have crystallized complex ill (the < vtochrome bc.i complex) in hexagona.l(P6₅22) crystals from cow and rabbit. monoclinir(P2] ) crystals from cow. and orthorhornbic(P2₁2₁2₁} cry-, t als from chicken. The chicken crystals diffract to 3.0 A. The orthorhoinbic and inonoclinic crystals each have a dimor of the bc'i complex in ilio asymmetric unit. Thii allows us to address questions about the symmetry of t lie dimnc lie, compiex Structural evidence .supports t.lie hypothfMh tliat tlic reaction mechanism involves movement of the Riosko irorisulfur protein bei ween a proximal position, in which t he cluster üganri H 161 hydrogen bonds with, and ato-pis UK electron from, qtiinol at center Q,: and a distal position in which eleci ron transfer wiih cytorhronie CT takes place. Cytochrorne c₁ core strmture similar to Ambler's Class L cytochrornes. without the loop corresponding to residues 41-58 in cvtochrome c. I his nx pooses the hene propionaies allowing the cytochrome to be reduced by the Rieskc protein tiironeli this side of the heme. As in oilier Class I cylochromes. the C rorrnof the hemeis exposed. This faces away from the membrane and scnnii likely to be ihe binding s te for cvlnchroine <. Klectron densitv in cocrvstais of the chicken bcj complex wiili hoi> heart cytochrome c support this position for cyochiome however the cytochromee is somewhat disordered in these crystals and the orientation cannto be determined rigorously at the current stage of analysis.