**1. Introduction**

Mitochondria support aerobic respiration and produce the majority of cellular ATP by oxidative phosphorylation (OXPHOS) [1]. Electrons derived from the oxidation of fatty acids, carbohydrates and amino acids are shuttled to oxygen along the respiratory chain complexes (I–IV) embedded in the inner mitochondrial membrane (IMM), producing water and releasing the energy necessary to pump protons from the mitochondrial matrix to the intermembrane space (IMS). This results in the formation of a transmembrane electrochemical gradient across the IMM, which enables the ATP synthase to produce ATP from ADP and inorganic phosphate [2]. A reverse catalytic process can occur under anoxia, a condition in which ATP synthase couples ATP hydrolysis to the generation of a transmembrane potential [3,4]. The mitochondrial OXPHOS is the only metabolic pathway that is under dual genetic control. It is therefore possible to distinguish genetic defects caused by (i) alterations in mitochondrial DNA (mtDNA), ∼15%, e.g., Neuropathy, Ataxia, Retinitis Pigmentosa (NARP), Maternally Inherited Leigh's Syndrome (MILS) and Leber's Hereditary Optic Neuropathy (LHON) [5] and (ii) nuclear DNA (nDNA) mutations, which are inherited as Mendelian disorders. A recent review provided an update on the contribution of nuclear genes that impair mitochondrial respiration in patients and have been characterized in yeast [6]. More than 150 distinct genetic mitochondrial dysfunction syndromes characterized by a diminished OXPHOS capacity have been described [5,7–11]. Typical clinical traits include visual/hearing defects, encephalopathies, cardiomyopathies, myopathies, diabetes, liver and renal dysfunctions [12–14]. In other cases, mitochondria participate to the cascades of pathogenic events leading to the onset of several diseases, but they are not linked to their genetic origins. Mitochondria are damaged during the reperfusion of ischemic heart, age-related diseases and all the major neurodegenerative diseases—-Parkinson's (PD), Alzheimer's (AD) and motor neuron diseases such as Amyotrophic Lateral Sclerosis (ALS).

In this scenario, ATP synthase has been shown to participate to the pathogenesis of different human diseases. The mitochondrial enzyme occupies the IMM of the organelle and forms dimers. Each monomeric unit (Figure 1), as shown in the latest dimeric mammalian enzyme by electron cryo-microscopy [15], is an assembly of 28 polypeptide chains of 17 different subunits organized into a catalytic globular domain, which is attached to an intrinsic membrane domain by a central stalk and a peripheral stalk [16]. This membranebound enzyme is a rotary machine. The membrane-bound rotor consists of eight identical c subunits (c- ring) in close association with a single a (or ATP6) subunit and is attached to the asymmetrical central stalk (subunits γ, δ and e) [17,18], which extends from the membrane domain and penetrates into the extrinsic globular catalytic domain along its central axis. As the central stalk rotates, it causes structural changes in the three catalytic sites, found mainly in each of the three β subunits, which alternate with three α subunits in the spherical extrinsic domain [16,19]. These structural changes lead to the enzyme's catalytic activity. The peripheral stalk, composed of the subunits oligomycin sensitivity conferral protein (OSCP), b, d, F6 and the membrane extrinsic region of A6L (or ATP8), links the external surface of the catalytic domain (F1) to the a subunit in the membrane domain (Fo) [20,21]. The subunits e, f, g, A6L and 6.8 proteolipid also contribute to the membrane domain of the peripheral stalk [22–24], and in the dimeric complex, some of them are involved in forming the interface between monomers [24]. Another subunit, previously known as diabetes-associated protein in insulin sensitive-tissues (DAPIT) [23], may be involved in the formation of links between dimer units in the rows of dimers [25]. In this review, we mainly focus on mutations in mitochondrial and nuclear genes encoding ATP synthase subunits and factors important for their association to human diseases. Moreover, we describe the contribution of this enzyme to the pathogenic mechanisms of cardiovascular, neurodegenerative diseases and neurodevelopmental disorders. Due to space constraints, the modulation and regulation of the ATP synthase in cancer has not been addressed in this review; however, for details, see [26,27]. attached to the asymmetrical central stalk (subunits γ, δ and e) lytic sites, found mainly in each of the three β subunits, which alternate with three α subzyme's catalytic activity. The pe –

––Parkinson's (PD), Alzheimer's (AD) and motor n

**Figure 1.** Subunit composition of the bovine ATP synthase monomer is mapped according to [15], (Protein Data Bank (PDB): 6ZQM). In the upper part, the subunits α(3) and β(3) of the catalytic domain are red and yellow, respectively; the three central stalk subunits γ, δ and ε, are blue, indigo and green. In the lower part, the membrane domain is composed of the c8-ring and the a subunit (dark grey and light blue); the supernumerary subunits e, f, g, A6L, 6.8PL and DAPIT are khaki, straw yellow, forest green, brick red, lime green and dark pink. On the right of the model, the peripheral stalk subunits OSCP, b, d and F6 are teal, light pink, orange and magenta.

#### **2. Gene Mutations of ATP Synthase and Its Assembly Factors in Human Disease**

Disorders caused by ATP synthase deficiencies can be classified depending on the mitochondrial or nuclear genetic origin (Table 1). These diseases are often severe encephaloor cardiomyopathies and manifest shortly after birth. Interestingly, they are less frequent than other OXPHOS-related diseases [28].

**Table 1.** Human pathogenic mutations occurring in ATP synthase subunits and assembly factors. The mutant subunits, or the mutant assembly factors of the ATP synthase found in human diseases are listed. Their specific nucleotide and amino acid substitutions and the related consequences on ATP synthase activity, assembly or mitochondrial morphology are summarized (nd, not defined).


Note. (\*) indicates a STOP codon.
