**1. Introduction**

Mitochondria are double membrane-bound cellular organelles surrounded by outer and inner membranes [1, 2]. The organelle is considered cell's powerhouse generating adenosine

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

triphosphate (ATP) during cellular respiration; hence, facilitating energy conversion in eukaryotes. Uniquely, each mitochondrion has its own DNA and encodes mitochondrial genes; hence, contributing the cell's proteome independently. The inheritance of the mitochondrial genome differs from nuclear genome since the donor of mitochondrial DNA (mtDNA) is the egg rather than sperm whose mitochondria are marked for obliteration upon entering the egg [3]. Hence, the organelle's DNA is inherited through females known as "maternal inheritance." Since these organelles generate energy, most biochemical reactions in the eukaryotic cells occur in the mitochondria. These reactions include pyruvate oxidation, citric acid cycle, electron transport, and oxidative phosphorylation (OXPHOS) all needed for energy production. Mitochondria also have an important role in calcium signaling, regulation of cellular metabolism, heme synthesis, steroid synthesis, apoptosis, and the biosynthesis of iron-sulfur (IS) clusters (ISC). The high number of human diseases caused by the malfunction of the mitochondrial proteins—encoded by nuclear or mtDNA—drew attention to the importance of this organelle.

Dehydrogenase (complex I: ND1, ND2, ND3, ND4, ND4L, ND5 and ND6), Cytochrome b (subunit of complex III), 3 subunits of Cytochrome c oxidase or complex IV (COI, COII and COIII), and 2 subunits of F0F1 ATPase (ATPase 6 and ATPase 8). They are all encoded by mtDNA and synthesized in the organelle. While, complex II (Succinate Dehydrogenase) and the remaining subunits of complexes I, III, IV, and V are entirely encoded by the nuclear genome. These nuclear-encoded proteins are synthesized on cytosolic ribosomes and subse-

Hereditary Disorders and Human Mutations of Iron-Sulfur Assembly Genes

http://dx.doi.org/10.5772/intechopen.78006

239

ISCs are evolutionarily ancient cofactors consisting of Fe (iron) and S (sulfur) associated to the cysteine sulfurs of proteins. The clusters are found in variety of organisms including archaea, protists, prokaryotes, and eukaryotes. In a eukaryotic cell, they can be found in the mitochondria, cytosol, and nucleus where they perform diverse functions [12]. ISCs play a critical role in many fundamental molecular processes and have roles in electron transfer, structural stabilization, gene regulation, enzymatic catalysis, metabolic regulation, and sensing environmental signals [13]. Almost 30 proteins in the mitochondria and the cytosol are involved in synthesizing and assembling these clusters. ISC have two most common forms [2Fe-2S] and [4Fe-4S] clusters. ISC-related proteins of the electron transport chain in the mitochondrion are mainly located in the inner membrane. Moreover, some of these proteins are also found in the mitochondrial matrix in the organelles. For the cluster assembly, two machineries are required, the mitochondrial ISC assembly machinery and the cytosolic IS protein assembly

Eukaryotic IS proteins are located in mitochondria, cytosol, and nucleus, where they perform diverse functions in cellular metabolism and regulation. The mitochondrial ISC assembly machinery matures all organellar IS proteins, and additionally contributes to the biogenesis of cytosolic and nuclear IS proteins by producing an unknown sulfur-containing compound (X-S) that is exported to the cytosol and used by the cytosolic IS protein assembly machinery. Hence, mitochondria are directly responsible for the essential functions (e.g., of nuclear IS

Mitochondria forms iron-sulfur clusters of significant proteins such as DNA polymerase and DNA helicases, and, therefore, plays a significant role in survival. There are 17 different proteins forming iron-sulfur cluster machinery that places the clusters into the Apo proteins. The mechanism of formation of iron-sulfur clusters can be divided into three steps. First, it is synthesized on a scaffold protein. Second, it is bound to transfer protein after dislocation from scaffold protein. Third, the transfer protein, the cluster and the specific ISC targeting factor place the cluster into the Apo protein. The changes in the first two steps inhibit the maturation of extra mitochondrial Fe/S proteins and disturb the iron homeostasis [14]. Assembly of Fe-S cluster also takes place by NIF, SUF, and CIA machineries. Cysteine desulfurase is an enzyme that unites Fe-S assembly machineries. It is encoded by NFS 1 which functions to deliver sulfur to ISCU [15]. ISCU is an iron-sulfur cluster assembly enzyme; encodes component of

proteins involved in DNA metabolism and genome maintenance).

quently transported into the mitochondria.

**4. Fe-S clusters (ISCs)**

machinery [12].
