Photosynthesis is considered the most important life process because it harvests energy from sunlight to carry out redox reactions that can drive other metabolic processes. Heliomicrobium modesticaldum is a unique photosynthetic organism in that its reaction center is devoid of quinones and iron-sulfur clusters FA and FB. Instead, it contains seven soluble dicluster ferredoxins that have the potential to accept electrons from FX. Previous work involved the characterization of some of these ferredoxins and it was determined that the H. modesticaldum reaction center can reduce ferredoxins PshB1, PshB2, and HM1_2505, but not fdxB. Here, we investigate alternative pathways for the reduction of H. modesticaldum ferredoxins involving a putative ferredoxin-NADP+ oxidoreductase (FNR) named HM1_0289. After cloning, expression, and purification of the putative FNR and ferredoxins, steady state kinetic assays were conducted to assess the enzymatic activity of the putative FNR, and it was discovered it is active only in the presence of NADH, not NADPH. Based on the selectivity for NADH, it is not likely this enzyme is an FNR. Further work identified the enzyme as a possible dihydroorotate dehydrogenase, which contains two flavin groups and a [2Fe-2S] cluster, as well as an NAD+ binding site.

Interactions of the Heliomicrobium modesticaldum reaction center with redox partner proteins

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