Membrane-associated redox activities in Thermotoga neapolitana

Abstract

Elemental sulfur reduction by the hyperthermophilic bacterium Thermotoga neapolitana provides an alternative to hydrogen evolution during fermentation. Electrons are transferred from reduced cofactors (ferredoxin and NADH) to sulfur by a series of unknown steps. One enzyme that may be involved is an NADH:methyl viologen oxidoreductase (NMOR), an activity that in other fermenting organisms is associated with NADH:ferredoxin oxidoreductase. We found that 83% of NMOR activity was contained in the pellet fraction of cell extracts subjected to ultracentrifugation. This pellet fraction, presumably containing cell membranes, was required for electron transfer to NAD+ from ferredoxin-dependent pyruvate oxidation. However, the NMOR activity in this fraction used neither Thermotoga nor clostridial ferredoxins as substrates. NMOR activity was also detected in aerobically prepared vesicles. By comparison with ATPase activities, NMOR was found primarily on the cytoplasmic face of these vesicles. During these studies, an extracytoplasmic hydrogenase activity was discovered. In contrast to the soluble hydrogenase, this hydrogenase activity was completely inhibited when intact cells were treated with cupric chloride and was present on the extracytoplasmic face of vescides. In contrast to a soluble hydrogenase reported in Thermotoga maritima, this activity was air-stable and was inhibited by low concentrations of nitrite.

On the chimeric nature, thermophilic origin, and phylogenetic placement of the Thermotogales

“…This operon may encode a membrane-bound NAD:methyl viologen oxidoreductase like that observed in Tt. neapolitana (15). That enzyme did not use ferredoxin as a substrate, so the cofactor naturally paired with NAD under physiological conditions is unknown.…”

The genusThermotoga: recent developments

“…These high yields and the depletion of headspace oxygen suggested that T. neapolitana utilizes oxygen for a more energy-efficient catabolic process. Extra-cytoplasmic oxygen-resistant hydrogenase activity has been detected in T. neapolitana [37], but the physiological role is unknown and H 2 -evolving hydrogenases are generally cytoplasmic [38]. Structural modelling of the [FeFe] hydrogenase α subunits suggests that the catalytic site of T. neapolitana’s enzyme may be less accessible to oxygen than T. maritima’s enzyme, accounting for increased resistance to inactivation [39].…”

Hydrogen production in anaerobic and microaerobic Thermotoga neapolitana

“…An O 2 insensitive hydrogenase has been described in T. neapolitana (Käslin et al 1998), which may explain why microaerobic H 2 production could take place in this bacterium. Since high H 2 yields are important for the development of microbial derived H 2 as a sustainable source of 'green' energy, we have tested the hypothesis that microaerobic metabolism increase the yield of H 2 from T. neapolitana (Van Ooteghem et al 2002 by investigating the conversion of glucose and generation of H 2 in anaerobic and microaerobic cultures of this bacterium.…”