Human Frataxin Activates Fe鈥揝 Cluster Biosynthesis by Facilitating Sulfur Transfer Chemistry

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• 作者：Jennifer Bridwell-Rabb ; Nicholas G. Fox ; Chi-Lin Tsai ; Andrew M. Winn ; David P. Barondeau
• 刊名：Biochemistry
• 出版年：2014
• 出版时间：August 5, 2014
• 年：2014
• 卷：53
• 期：30
• 页码：4904-4913
• 全文大小：419K
• ISSN：1520-4995

文摘

Iron鈥搒ulfur clusters are ubiquitous protein cofactors with critical cellular functions. The mitochondrial Fe鈥揝 assembly complex, which consists of the cysteine desulfurase NFS1 and its accessory protein (ISD11), the Fe鈥揝 assembly protein (ISCU2), and frataxin (FXN), converts substrates pan class="smallcaps">lpan>-cysteine, ferrous iron, and electrons into Fe鈥揝 clusters. The physiological function of FXN has received a tremendous amount of attention since the discovery that its loss is directly linked to the neurodegenerative disease Friedreich鈥檚 ataxia. Previous in vitro results revealed a role for human FXN in activating the cysteine desulfurase and Fe鈥揝 cluster biosynthesis activities of the Fe鈥揝 assembly complex. Here we present radiolabeling experiments that indicate FXN accelerates the accumulation of sulfur on ISCU2 and that the resulting persulfide species is viable in the subsequent synthesis of Fe鈥揝 clusters. Additional mutagenesis, enzyme kinetic, UV鈥搗isible, and circular dichroism spectroscopic studies suggest conserved ISCU2 residue C104 is critical for FXN activation, whereas C35, C61, and C104 are all essential for Fe鈥揝 cluster formation on the assembly complex. These results cannot be fully explained by the hypothesis that FXN functions as an iron donor for Fe鈥揝 cluster biosynthesis, and further support an allosteric regulator role for FXN. Together, these results lead to an activation model in which FXN accelerates persulfide formation on NFS1 and favors a helix-to-coil interconversion on ISCU2 that facilitates the transfer of sulfur from NFS1 to ISCU2 as an initial step in Fe鈥揝 cluster biosynthesis.