Iron-sulfur center of biotin synthase and lipoate synthase

S Ollagnier-de Choudens, Y Sanakis, KS Hewitson… - Biochemistry, 2000 - ACS Publications
S Ollagnier-de Choudens, Y Sanakis, KS Hewitson, P Roach, JE Baldwin, E Münck…
Biochemistry, 2000ACS Publications
Biotin synthase and lipoate synthase are homodimers that are required for the C− S bond
formation at nonactivated carbon in the biosynthesis of biotin and lipoic acid, respectively.
Aerobically isolated monomers were previously shown to contain a (2Fe-2S) cluster,
however, after incubation with dithionite one (4Fe-4S) cluster per dimer was obtained,
suggesting that two (2Fe-2S) clusters had combined at the interface of the subunits to form
the (4Fe-4S) cluster. Here we report Mössbauer studies of 57Fe-reconstituted biotin …
Biotin synthase and lipoate synthase are homodimers that are required for the C−S bond formation at nonactivated carbon in the biosynthesis of biotin and lipoic acid, respectively. Aerobically isolated monomers were previously shown to contain a (2Fe-2S) cluster, however, after incubation with dithionite one (4Fe-4S) cluster per dimer was obtained, suggesting that two (2Fe-2S) clusters had combined at the interface of the subunits to form the (4Fe-4S) cluster. Here we report Mössbauer studies of 57Fe-reconstituted biotin synthase showing that anaerobically prepared enzyme can accommodate two (4Fe-4S) clusters per dimer. The (4Fe-4S) cluster is quantitatively converted into a (2Fe-2S)2+ cluster upon exposure to air. Reduction of the air-exposed enzyme with dithionite or photoreduced deazaflavin yields again (4Fe-4S) clusters. The (4Fe-4S) cluster is stable in both the 2+ and 1+ oxidation states. The Mössbauer and EPR parameters were ΔEq = 1.13 mm/s and δ = 0.44 mm/s for the diamagnetic (4Fe-4S)2+ and ΔEq = 0.51 mm/s, δ = 0.85 mm/s, gpar = 2.035, and gperp = 1.93 for the S = 1/2 state of (4Fe-4S)1+. Considering that we find two (4Fe-4S) clusters per dimer, our studies argue against the early proposal that the enzyme contains one (4Fe-4S) cluster bridging the two subunits. Our study of lipoate synthase gave results similar to those obtained for BS:  under strict anaerobiosis, lipoate synthase can accommodate a (4Fe-4S) cluster per subunit [ΔEq = 1.20 mm/s and δ = 0.44 mm/s for the diamagnetic (4Fe-4S)2+ and gpar = 2.039 and gperp = 1.93 for the S = 1/2 state of (4Fe-4S)1+], which reacts with oxygen to generate a (2Fe-2S)2+ center.
ACS Publications