NirJ, a radical SAM family member of the d₁ heme biogenesis cluster

NirJ is involved in the transformation of precorrin-2 into heme d₁, although its precise role in the process has not been established. The purified protein was found to contain a 4Fe-4S centre, in line with the prediction that it belongs to the radical SAM class of enzymes. This was further confirmed by binding of S-adenosyl-l-methionine (SAM) to dithionite-reduced NirJ, which resulted in a decrease in the signal intensity and in a shift to higher field of the [4Fe-4S]¹⁺ EPR signal. Significantly, though, this approach also led to the appearance of a small but reproducible organic radical signal that was associated with about 2% of the NirJ molecules and was affected by the incorporation of SAM deuterated at the 5' adenosyl group.