2
AVANT- PROPOS This special issue of Biochimie was prepared for the first meeting of the (( Groupe Th~ma- tique )) : (( Bioenergetics and Membranes )) of the French Biochemical Society (Grenoble, June 29, 1978). The ability of certain bacteria to produce H2 and to use this gas as an energy source has long been recognised and has given rise to a far reaching field of research: this meeting encompasses a wide spectrum of biological interests and is devoted to the study of H2 metabolism in strict anaerobes (e.g. Clostridia, Desulfovibrio), aerobes (e.g. Alcaligenes, Azo- tobacter, Rhizobia), photosynthetic bacteria (e.g. Rhodopseudomonas) and in blue-green algae (e.g. Anabaena). The collection of papers contained in this special issue of Biochimie gives a general view of the current state of knowledge and of the various approaches used for the study of H= metabolism in bacteria and blue-green algae, with special emphasis on nitrogenase-hydrogenase interrelationships. Hydrogenase and nitrogenase are the two main enzymes involved in bacterial H2 meta- bolism. The enzyme hydrogenase enables bac- teria to use H~ as an electron donor ; it is gene- rally a membrane-bound enzyme. However, a soluble hydrogenase has been found in anae- robic (e.g. Desulfovibrio vulgaris), aerobic (e.g. A Icaligenes eutrophus) and photosynthetic (e.g. Thiocapsa roseopersicina) bacteria. Both soluble and membrane-bound hydrogenases are reversible and may catalyze H~ production with suitable electron donors. Nitrogen-fixing bacteria possess the nitroge- nase enzyme complex. As well as enabling con- version of nitrogen to ammonia, nitrogenase may also catalyse an ATP-dependent H~ evo- lution. Nitrogenase is found in anaerobes (e.g. Desulfovibrio), aerobes (e.g. Azotobacter), in cyanobacteria (e.g. Anabaena) and in photo- synthetic bacteria (e.g. Rhodopseudomonas). Generally the N2-fixing bacteria also contain hydrogenase which catalyses a H= uptake and allows the recycling of H2 (( wasted )) by nitro- genase. Studies of the synthesis of hydrogenase and nitrogenase in different physiological condi- tions, of the factors affecting their activity in vitro or in vivo and of the interplay of the two enzymes in the hydrogen cycle are des- cribed in several papers of this volume. Other contributions report on studies of electron transfer at the molecular level. It is also shown how physical techniques such as Electron Paramagnetic Resonance and Nuclear Magnetic Resonance spectrometry can be used in this field of research. The renewal of interest in H= metabolism is linked to the possible use of biological mate- rial for solar energy conversion into H2. Although this was not the theme of the meeting, possible avenues of research are briefly men- tioned in some reports. The attraction of biological systems, and of bacteria in particular, is their flexibility in their ability to adapt to environmental condi- 16

Avant-propos

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A V A N T - P R O P O S

This special issue of Biochimie was prepared for the first meeting of the (( Groupe Th~ma- tique )) : (( Bioenergetics and Membranes )) of the French Biochemical Society (Grenoble, June 29, 1978).

The abil i ty of certain bacteria to produce H2 and to use this gas as an energy source has long been recognised and has given rise to a far reaching field of research: this meeting encompasses a wide spectrum of biological interests and is devoted to the study of H2 metabolism in strict anaerobes (e.g. Clostridia, Desulfovibrio), aerobes (e.g. Alcaligenes, Azo- tobacter, Rhizobia), photosynthetic bacteria (e.g. Rhodopseudomonas) and in blue-green algae (e.g. Anabaena). The collection of papers contained in this special issue of Biochimie gives a general view of the current state of knowledge and of the various approaches used for the study of H= metabolism in bacteria and blue-green algae, with special emphasis on nitrogenase-hydrogena se interrelationships.

Hydrogenase and nitrogenase are the two main enzymes involved in bacterial H2 meta- bolism. The enzyme hydrogenase enables bac- teria to use H~ as an electron donor ; it is gene- rally a membrane-bound enzyme. However, a soluble hydrogenase has been found in anae- robic (e.g. Desulfovibrio vulgaris), aerobic (e.g. A Icaligenes eutrophus) and photosynthetic (e.g. Thiocapsa roseopersicina) bacteria. Both soluble and membrane-bound hydrogenases are reversible and may catalyze H~ production with suitable electron donors.

Nitrogen-fixing bacteria possess the nitroge- nase enzyme complex. As well as enabling con- version of nitrogen to ammonia, nitrogenase may also catalyse an ATP-dependent H~ evo- lution. Nitrogenase is found in anaerobes (e.g. Desulfovibrio), aerobes (e.g. Azotobacter), in cyanobacteria (e.g. Anabaena) and in photo- synthetic bacteria (e.g. Rhodopseudomonas). Generally the N2-fixing bacteria also contain hydrogenase which catalyses a H= uptake and allows the recycling of H2 (( wasted )) by nitro- genase.

Studies of the synthesis of hydrogenase and nitrogenase in different physiological condi- tions, of the factors affecting their activity in vitro or in vivo and of the interplay of the two enzymes in the hydrogen cycle are des- cribed in several papers of this volume. Other contributions report on studies of electron transfer at the molecular level. It is also shown how physical techniques such as Electron Paramagnetic Resonance and Nuclear Magnetic Resonance spectrometry can be used in this field of research.

The renewal of interest in H= metabolism is linked to the possible use of biological mate- rial for solar energy conversion into H2. Although this was not the theme of the meeting, possible avenues of research are briefly men- tioned in some reports.

The attraction of biological systems, and of bacteria in particular, is their f lexibi l i ty in their abil ity to adapt to environmental condi-

16

Page 2: Avant-propos

2 1 8 Biochimie.

tions and to use waste products or solar energy as an energy source. Their rapid growth and ease of handling make bacteria well suited for utilization on an industrial scale.

As biological solar energy converting sys- tems, intact bacteria offer the advantage of being both self-regenerating and self-repairing. Mutant strains further extend the range of possibilities. However cells are subject to regu- latory mechanisms which may lead to H2 con- sumption for cellular energy balance. To favour H~ production, reconstituted systems with isolated enzymes may be more suitable, but the weakness of such reconstituted systems

is their lability. Chemical model systems mi- micking the active sites of hydrogenase have been synthesized and may prove to be more stable. This is one of the reasons why physico- chemical studies of the active site of hydro- genase and nitrogenase, together with che- mical studies of Fe-S clusters and Mo-Fe com- plexes, are of a great interest. However, a clear picture of the hydrogenase and nitrogenase reactions will not only require the elucidation of the active sites, but also a good under- standing of the interaction between the metal- lic cluster cores and the polypeptide chains of the enzyme.

P. M.V.

BIOCHIMIE, 1978 , 60, n ° 3.