Mechanisms of iron and copper–frataxin interactions

Abstract

Frataxin is a mitochondrial protein whose deficiency is the cause of Friedreich's ataxia, a hereditary neurodegenerative disease. This protein plays a role in iron–sulfur cluster biosynthesis, protection against oxidative stress and iron metabolism. In an attempt to provide a better understanding of the role played by metals in its metabolic functions, the mechanisms of mitochondrial metal binding to frataxin in vitro have been investigated. A purified recombinant yeast frataxin homolog Yfh1 binds two Cu(II) ions with a K_d1(Cu^II) of 1.3 × 10⁻⁷ M and a K_d2(Cu^II) of 3.1 × 10⁻⁴ M and a single Cu(I) ion with a higher affinity than for Cu(II) (K_d(Cu^I) = 3.2 × 10⁻⁸ M). Mn(II) forms two complexes with Yfh1 (K_d1(Mn^II) = 4.0 × 10⁻⁸ M; K_d2(Mn^II) = 4.0 × 10⁻⁷ M). Cu and Mn bind Yfh1 with higher affinities than Fe(II). It is established for the first time that the mechanisms of the interaction of iron and copper with frataxin are comparable and involve three kinetic steps. The first step occurs in the 50–500 ms range and corresponds to a first metal uptake. This is followed by two other kinetic processes that are related to a second metal uptake and/or to a change in the conformation leading to thermodynamic equilibrium. Frataxin deficient Δyfh1 yeast cells exhibited a marked growth defect in the presence of exogenous Cu or Mn. Mitochondria from Δyfh1 strains also accumulated higher amounts of copper, suggesting a functional role of frataxin in vivo in copper homeostasis.