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

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

Mitochondria are genetically controlled by both nuclear DNA and the mitochondrial genome [1, 4]. A wide range of molecular defects have been identified in the human mitochondrial genome [4–9]. Diseases due to mutations in the mitochondrial genome are clinically, genetically, and biochemically diverse [1, 2, 4, 6, 10]. Similarly, deficiencies in mitochondrial genes encoded by nuclear genome can also lead various mitochondrial disorders and a wide range of cellular perturbations such as undue reactive oxygen species and distracted apoptosis, aberrant calcium homeostasis, and deficient energy production. This in turn leads failure to meet the requirements of numerous organs, especially those with high energy needs. Hence, various pathological conditions appears due to impaired mitochondrial function in human body involving different cell types, tissues, and organs including heart and brain. Such multiorgan manifestations are all mitochondria related and these diseases varies from epilepsy to

**3. Mitochondria and genetics of mitochondria-related diseases**

The mitochondrial genome is a multicopy, double-stranded circular DNA molecule, which is 16.6 kb in human [11]. This genome encodes 13 essential proteins for the OXPHOS system and 24 components of the RNA machinery: 2 rRNAs and 22 tRNAs [11]. It is intronless and the only noncoding region is the displacement region (D-Loop), a region of 1.1 kb. It contains both the replication origins and the transcriptional promoters. Although mitochondria are genetically controlled by both mitochondrial and nuclear genomes, mtDNA is only maternally inherited [3]. Mitochondrial genetics differ greatly from Mendelian genetics in size, number of encoded genes, number of DNA molecules per cell, lack of introns, gene density, replication, transcription, recombination, and mode of inheritance. The 13 proteins include 7 subunits of NADH

of this organelle.

238 Mitochondrial Diseases

**2. Mitochondria**

cardiac myopathies.

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 machinery [12].

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 proteins involved in DNA metabolism and genome maintenance).

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 iron-sulfur scaffold protein. The changes in this gene result in severe myopathy and lactic acidosis ("ISCU Fe-S Cluster Assembly Enzyme [*Homo sapiens* (Human)] - Gene - NCBI") Complexes 1, 2, and 3 contain Fe-S clusters. They function in electron transport by transfer of one electron in redox processes [16]. The assembly of the clusters is recently studied in Yeast. In photosynthetic organisms, the iron-sulfur clusters play role in chloroplast processes and are important for plastid functioning [17].

Yeast frataxin, Isu1, and Nfs1 (cysteine desulfurase) take part in *de novo* synthesis of ISC. Many genes encode ISC assembly factors such as *BOLA3*, *NFU1*, *GLRX5*, *NUBPL*, *LYRM4*, *IBA57*, *ISCA1*, and *ISCA2*. These molecules have significant role in mitochondria. They are essential cofactors in the assembly of cluster. Deficiency of these genes leads to different diseases, for instance, GLRX5 deficiency causes sideroblastic anemia, whereas NUBPL mutations lead to respiratory chain complex 1 deficiency. On the other hand, some of these deficiencies are classified under a unique category such as MMDS.
