The Distal Heme Center in Bacillus subtilis Succinate:Quinone Reductase Is Crucial for Electron Transfer to Menaquinone

• Published in: Biochemistry (Easton) 2000-07, Vol.39 (29), p.8617-8624
• Main Authors: Matsson, Mikael, Tolstoy, Daria, Aasa, Roland, Hederstedt, Lars
• Format: Article
• Language: English
• Subjects: Bacillus subtilis; Bacillus subtilis - enzymology; Bacillus subtilis - genetics; Base Sequence; Binding Sites - genetics; DNA Primers - genetics; Electron Transport; Electron Transport Complex II; Heme - chemistry; Kinetics; menaquinones; Models, Molecular; Multienzyme Complexes - chemistry; Multienzyme Complexes - genetics; Multienzyme Complexes - metabolism; Mutagenesis, Site-Directed; Oxidoreductases - chemistry; Oxidoreductases - genetics; Oxidoreductases - metabolism; Spectrophotometry; Substrate Specificity; Succinate Dehydrogenase - chemistry; Succinate Dehydrogenase - genetics; Succinate Dehydrogenase - metabolism; succinate:quinone reductase; Vitamin K - chemistry; Vitamin K - metabolism
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi000271m

Succinate:quinone reductases are membrane-bound enzymes that catalyze electron transfer from succinate to quinone. Some enzymes in vivo reduce ubiquinone (exergonic reaction) whereas others reduce menaquinone (endergonic reaction). The succinate:menaquinone reductases all contain two heme groups in the membrane anchor of the enzyme:  a proximal heme (heme b P) located close to the negative side of the membrane and a distal heme (heme b D) located close to the positive side of the membrane. Heme b D is a distinctive feature of the succinate:menaquinone reductases, but the role of this heme in electron transfer to quinone has not previously been analyzed. His28 and His113 are the axial ligands to heme b D in Bacillus subtilis succinate:menaquinone reductase. We have individually replaced these His residues with Leu and Met, respectively, resulting in assembled membrane-bound enzymes. The H28L mutant enzyme lacks succinate:quinone reductase activity probably due to a defective quinone binding site. The H113M mutant enzyme contains heme b D with raised midpoint potential and is impaired in electron transfer to menaquinone. Our combined experimental data show that the heme b D center, into which we include a quinone binding site, is crucial for succinate:menaquinone reductase activity. The results support a model in which menaquinone is reduced on the positive side of the membrane and the transmembrane electrochemical potential provides driving force for electron transfer from succinate via heme b P and heme b D to menaquinone.