Laetyl-CoA dehydratase consists of two enzymes, E1 and E2, and requires catalytic quantities of ATP for activity [Kuchta, R. D., & Abeles, R. H. (1985) J. Biol. Chem. 260, 13181-13189]. In contrast to E1, which contains no Fe, E2 contains 8.20 ± 0.04 mol of Fe/mol of E2, one of which can be removed by 1,10-phenanthroline. E2 also contains 7.33 ± 0.68 mol of inorganic sulfur/mol of E2, indicating that at least seven of the Fe atoms are present as Fe-S clusters. E1 and E2 contain <0.14 mol of Cu, Co, Zn, Mn, and Ni/mol of E1 or E2. Both reduced and oxidized E1 are EPR silent over a 10 000-G scan range at 4 K, while two signals in E2 are observable at 4 K. Identical spectra were obtained with E2 containing either seven or eight Fe atoms, and both signals were only observable at T < 30 K. Signal 1 has axial symmetry with g⊥ = 2.0232 and g∥ = 2.0006. Signal 2 is orthorhombic with g1 = 1.982, g2 = 1.995, and g3 = 2.019. Computer simulation of these spectra with a S = 1/2 spin Hamiltonian was used to extract the g matrices. The intensity of both signals decreases when E2 is reduced with Na2S2O4. We propose that signal 1 is due to an unusual [4Fe-4S] cluster and signal 2 to a [3Fe-3/4S] cluster. Addition of either acrylyl-CoA or lactyl-CoA dramatically alters signal 2. Thus, substrates bind to E2 and alter the environment of the [3Fe-3/4S] cluster. Neither ATP nor E1 alters the spectrum of E2. O2 inactivates E2 and causes loss of flavin and the [3Fe-3/4S] cluster. Inclusion of fast protein liquid chromatography during purification of E2 causes partial loss of flavin and alters E2 such that the Fe-S centers become more difficult to oxidize or reduce, but has much less effect upon catalytic activity. The role of the redox-active centers on E2 during the dehydration of lactyl-CoA is not known. © 1986 American Chemical Society.