Chapter 10: Sulfur Respiration

Summary

Certain anaerobic prokaryotes grow by oxidative phosphorylation with elemental sulfur as the terminal electron acceptor instead of O₂ (sulfur respiration or anaerobic respiration with sulfur). The mechanism of sulfur respiration has been investigated in great detail only inWolinella (W.) succinogenes. This ε-proteobacterium uses, as terminal electron acceptor, polysulfide which is formed abiotically from elemental sulfur and sulfide. Polysulfide reduction is facilitated by a periplasmic sulfur-binding protein whose structure was determined. Polysulfide respiration with H₂ or formate as electron donor is catalyzed by the membrane ofW.succinogenes and is coupled to apparent proton translocation across the membrane. The eletrochemical proton potential (Δp) thus generated and the corresponding H⁺/e^– ratio were determined to be 170 mV (negative inside) and 0.5, respectively. The electron transport chain catalyzing polysulfide respiration by H₂ or formate consists of polysulfide reductase, methyl-menaquinone and hydrogenase or formate dehydrogenase. Each enzyme consists of two different hydrophilic and a hydrophobic subunit which anchors the enzyme in the membrane. The membrane anchors of hydrogenase and formate dehydrogenase are similar di-heme cytochromesb which carry the site of quinone reduction. Methyl-menaquinone is thought to be bound to the membrane anchor of polylsulfide reductase and to accept electrons from the cytochromesb of the dehydrogenases. Proteoliposomes containing polysulfide reductase, methy-menaquinone, and formate dehydrogenase isolated fromw.succinogenes catalyze polysulfide reduction by formate which is coupled to Δp generation. The coupling mechanism is discussed.