Role of the nonheme Fe(II) center in the biosynthesis of the plant hormone ethylene

The final step of ethylene biosynthesis in plants is catalyzed by the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACCO). In addition to ACC, Fe(II), O(2), CO(2), and ascorbate are required for in vitro enzyme activity. Direct evidence for the role of the Fe(II) center in the recombinant avocado ACCO has now been obtained through formation of enzyme⋅(substrate or cofactor)⋅NO complexes. These NO adducts convert the normally EPR-silent ACCO complexes into EPR-active species with structural properties similar to those of the corresponding O(2) complexes. It is shown here that the ternary Fe(II)ACCO⋅ACC⋅NO complex is readily formed, but no Fe(II)ACCO⋅ascorbate⋅NO complex could be observed, suggesting that ascorbate and NO are mutually exclusive in the active site. The binding modes of ACC and the structural analog alanine specifically labeled with (15)N or (17)O were examined by using Q-band electron nuclear double resonance (ENDOR). The data indicate that these molecules bind directly to the iron through both the α-amino and α-carboxylate groups. These observations are inconsistent with the currently favored mechanism for ACCO, in which it is proposed that both ascorbate and O(2) bind to the iron as a step in O(2) activation. We propose a different mechanism in which the iron serves instead to simultaneously bind ACC and O(2), thereby fixing their relative orientations and promoting electron transfer between them to initiate catalysis.