Conspectus
Mammalian aconitases (mitochondrial and cytosolic
isoenzymes) are
unique iron–sulfur cluster-containing proteins in which the
metallic center participates in the catalysis of a non-redox reaction.
Within the cubane iron–sulfur cluster of aconitases only three
of the four iron ions have cysteine thiolate ligands; the fourth iron
ion (Feα) is solvent exposed within the active-site
pocket and bound to oxygen atoms from either water or substrates to
be dehydrated. The catalyzed reaction is the reversible isomerization
of citrate to isocitrate with an intermediate metabolite, cis-aconitate. The cytosolic isoform of aconitase is a moonlighting
enzyme; when intracellular iron is scarce, the complete disassembly
of the iron–sulfur cluster occurs and apo-aconitase acquires
the function of an iron responsive protein and regulates the translation
of proteins involved in iron metabolism.
In the late 1980s and
during the 1990s, cumulative experimental
evidence pointed out that aconitases are main targets of reactive
oxygen and nitrogen species such as superoxide radical (O2
•–), hydrogen peroxide (H2O2), nitric oxide (•NO), and peroxynitrite
(ONOO–). These intermediates are capable of oxidizing
the cluster, which leads to iron release and consequent loss of the
catalytic activity of aconitase. As the reaction of the Fe–S
cluster with O2
•– is fast (∼107 M–1 s–1), quite specific,
and reversible in vivo, quantification of active
aconitase has been used to evaluate O2
•– formation in cells. While •NO is modestly reactive
with aconitase, its reaction with O2
•– yields ONOO–, a strong oxidant that readily leads
to the disruption of the Fe–S cluster. In the case of cytosolic
aconitase, it has been seen that H2O2 and •NO promote activation of iron responsive protein activity
in cells. Proteomic advances in the 2000s confirmed that aconitases
are main targets of reactive species in cellular models and in vivo, and other post-translational oxidative modifications
such as protein nitration and carbonylation have been detected.
Herein, we (1) outline the particular structural features of aconitase
that make these proteins specific targets of reactive species, (2)
characterize the reactions of O2
•–, H2O2, •NO, and ONOO– and related species with aconitases, (3) discuss how
different oxidative post-translational modifications of aconitase
impact the different functions of aconitases, and (4) argue how these
proteins might function as redox sensors within different cellular
compartments, regulating citrate concentration and efflux from mitochondria,
iron availability in the cytosol, and cellular oxidant production.
The SPATA5 gene encodes a 892 amino-acids long protein that has a putative mitochondrial targeting sequence and has been proposed to function in maintenance of mitochondrial function and integrity during mouse spermatogenesis. Several studies have associated homozygous or compound heterozygous mutations in SPATA5 gene to microcephaly, intellectual disability, seizures and hearing loss. This suggests a role of the SPATA5 gene also in neuronal development. Recently, our group presented results validating the use of blood cells for the assessment of mitochondrial function for diagnosis and follow-up of mitochondrial disease, minimizing the need for invasive procedures such as muscle biopsy. In this study, we were able to diagnose a patient with epileptogenic encephalopathy using next generation sequencing. We found two novel compound heterozygous variants in SPATA5 that are most likely causative. To analyze the impact of SPATA5 mutations on mitochondrial functional studies directly on the patients' mononuclear cells and platelets were undertaken. Oxygen consumption rates in platelets and PBMCs were impaired in the patient when compared to a healthy control. Also, a decrease in mitochondrial mass was observed in the patient monocytes with respect to the control. This suggests a true pathogenic effect of the mutations in mitochondrial function, especially in energy production and possibly biogenesis, leading to the observed phenotype.
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