Abstract
Hydrogenases are key enzymes of the energy metabolism of many microorganisms. Especially in anoxic habitats where molecular hydrogen (H2) is an important intermediate, these enzymes are used to expel excess reducing power by reducing protons or they are used for the oxidation of H2 as energy and electron source. Despite the fact that hydrogenases catalyze the simplest chemical reaction of reducing two protons with two electrons it turned out that they are often parts of multimeric enzyme complexes catalyzing complex chemical reactions with a multitude of functions in the metabolism. Recent findings revealed multimeric hydrogenases with so far unknown functions particularly in bacteria from the class Clostridia. The discovery of [FeFe] hydrogenases coupled to electron bifurcating subunits solved the enigma of how the otherwise highly endergonic reduction of the electron carrier ferredoxin can be carried out and how H2 production from NADH is possible. Complexes of [FeFe] hydrogenases with formate dehydrogenases revealed a novel enzymatic coupling of the two electron carriers H2 and formate. These novel hydrogenase enzyme complex could also contribute to biotechnological H2 production and H2 storage, both processes essential for an envisaged economy based on H2 as energy carrier.
Highlights
Molecular hydrogen (H2) is only present in trace concentrations (550 parts per billion) in the Earth’s lower atmosphere (Novelli et al, 1999)
Soon after the discovery of flavin-based electron bifurcation (FBEB) in Clostridia, the first hydrogenase was reported that utilizes the mechanism of FBEB, in the physiological context in the reverse direction (Schut and Adams, 2009)
This mode of metabolism leaves 2 NADH and 2 reduced ferredoxin that are converted to H2 by FBEB hydrogenase
Summary
Molecular hydrogen (H2) is only present in trace concentrations (550 parts per billion) in the Earth’s lower atmosphere (Novelli et al, 1999). The auxiliary subunits follow a very modular structure and add multiple functions to the core subunit These functions can include electron transfer to soluble electron carriers, coupling of H2 oxidation/production to other chemical reactions, coupling to energy conservation by coupling the electron transfer to the generation of a transmembrane ion potential, or utilization of the novel energetic coupling mechanism of flavin-based electron bifurcation (FBEB) to overcome energetic limitations of the electron transfer (Buckel and Thauer, 2018a,b; Müller et al, 2018). We review the recent findings of novel complex multimeric hydrogenases and especially their function in the microbial H2 metabolism These include enzymes using the novel energy coupling mechanism of FBEB enabling otherwise endergonic H2 production from NADH or the otherwise endergonic electron transfer from H2 to the iron–sulfur protein ferredoxin. Both hydrogenase types will be discussed in the context of their biotechnological potential for the H2 economy
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