Electron inventory, kinetic assignment (En), structure, end bonding of nitrogenase turnover intermediates with C2H2 and CO

Improved H-1 ENDOR data from the S-EPR1 intermediate formed during turnover of the nitrogenase alpha-195(Gln) MoFe protein with (C2H2)-H-1,2 in (H2O)-H-1,2 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 Fe-57 ENDOR of Fe-57-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-6(2+), S-9(2-)(d(43))](+1), reduced by m = 2 electrons relative to the resting-state cofactor. Consideration of Fe-57 hyperfine coupling in SEPR1 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 SEPR1 both are reduced by m = 2 electrons, but the intermediates are not at the same LT reduction stage (E): (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 E kinetic state with a well-defined chemical state of the enzyme.