Molecular model of hydrogen-dependent carbon dioxide reductase (HDCR) which allows some bacteria to use hydrogen as an energy source. The assembly includes the subunit HydA2 (yellow), an enzyme extracting electrons from hydrogen. Subunit FdhF (green) is an enzyme that adds these electrons to carbon dioxide to form formic acid. The remaining two subunits HycB3 and HycB4 (blue, dark magenta) connect the two enzymes, delivering electrons between the two