A complete list of all the genes predicted to be involved in the hydrogen production pathway is listed in Table 5. The whole selleck chem inhibitor genome information of the organism suggests that hydrogen production in Enterobacter sp. IIT-BT 08 is carried out through the formate hydrogen lyase (FHL) complex which consists of formate dehydrogenase (FDH-H), hydrogenase (Hyd-3) and the electron transfer mediators [30]. Table 5 Preliminary genes involved in the hydrogen production pathway according to the MIGS recommendations [8] However, in the future the hypothetical pathway must be verified with wet lab experiments. Based on the previous reported literature it may be that formate dehydrogenase and hydrogenase 3 together form a membrane protein complex that is responsible for hydrogen production in facultative anaerobes [30-32].
Rossmann et al. suggested that in facultative anaerobes hydrogen production was determined by the concentration of formate in the cell, which in turn determined the formation of the FHL complex [32]. A putative model (Figure 3) has been suggested based on the biochemistry of the reactions involved in the pathway [34]. Formate dehydrogenase is suggested to catalyze the oxidation of formate into carbon dioxide. The electrons released in the process are transferred to Hyd3 encoded by hycABCDEFGH to generate molecular hydrogen under anaerobic conditions [33]. The model suggests a plausible scheme of electron transfer from FdhF to the catalytic subunit of hycE via hycBCFG subunits. Among these, hycB and hycF have been determined to be [4Fe-4S] ferredoxin type electron transfer proteins [35].
On the other hand, hycE and hycG shares homology with NADH ubiquinone oxidoreductase (NUO) subunits of the mitochondria and chloroplast [35]. In the model, hycC and hycD have been suggested to act as transmembrane proteins. Figure 3 Putative mechanism of hydrogen production by Enterobacter sp. IIT-BT 08 based on the genes identified in the genome. Figure is adapted from [33]. Electron acceptors, like oxygen or nitrate, generally inhibit the expression of the FHL complex, whereas its biosynthesis is controlled by the concentration of formate in the cell [32]. Further, it has been suggested that the micro elements selenium and molybdenum are involved at the active site of FDH-H, while nickel is a component of the Hyd-3 active site [30,36].
Accordingly, Brefeldin_A it has been suggested that the FHL complex can be induced by regulating the presence of formate and metal ions in slightly acidic pH under anaerobic conditions. Transcription of the FHL complex is under the control of several genes, including fhlA, which codes for the FHL activator protein FHLA, a tetramer that binds to the upstream region of the DNA encoding the FHL complex and promotes the transcription of the FHL complex [34,37].