Improved 1H ENDOR data from the SEPR1 intermediate formed during turnover of the nitrogenase α-195Gln MoFe protein with C21,2H2 in 1,2H2O buffers, taken in context with the recent study of the intermediate formed from propargyl alcohol, indicate that SEPR1 is a product complex, likely with C2H4 bound as a ferracycle to a single Fe of the FeMo-cofactor active site. 35 GHz CW and Mims pulsed 57Fe ENDOR of 57Fe-enriched SEPR1 cofactor indicates that it exhibits the same valencies as those of the CO-bound cofactor of the lo-CO intermediate formed during turnover with CO, [Mo4+, Fe3+, Fe 62+, S92-(d43)] +1, reduced by m = 2 electrons relative to the resting-state cofactor. Consideration of 57Fe hyperfine coupling in S EPR1 and lo-CO leads to a picture in which CO bridges two Fe of lo-CO, while the C2H4 of SEPR1 binds to one of these. To correlate these and other intermediates with Lowe-Thorneley (LT) kinetic schemes for substrate reduction, we introduce the concept of an "electron inventory". It partitions the number of electrons a MoFe protein intermediate has accepted from the Fe protein (n) into the number transmitted to the substrate (s), the number that remain on the intermediate cofactor (m), and the additional number delivered to the cofactor from the P clusters (p): n = m + s - p (with p = 0 here). The cofactors of lo-CO and S EPR1 both are reduced by m = 2 electrons, but the intermediates are not at the same LT reduction stage (En): (n = 2; m = 2, s = 0) for lo-CO; (n = 4; s = 2, m = 2) for SEPR1 This is the first proposed correlation of an LT En kinetic state with a well-defined chemical state of the enzyme.
ASJC Scopus subject areas
- Colloid and Surface Chemistry