*Review* **Organization of the Respiratory Supercomplexes in Cells with Defective Complex III: Structural Features and Metabolic Consequences**

**Michela Rugolo , Claudia Zanna and Anna Maria Ghelli \***

Department of Pharmacy and Biotechnology, University of Bologna, 40126 Bologna, Italy; michela.rugolo@unibo.it (M.R.); claudia.zanna@unibo.it (C.Z.)

**\*** Correspondence: annamaria.ghelli@unibo.it

**Abstract:** The mitochondrial respiratory chain encompasses four oligomeric enzymatic complexes (complex I, II, III and IV) which, together with the redox carrier ubiquinone and cytochrome *c*, catalyze electron transport coupled to proton extrusion from the inner membrane. The protonmotive force is utilized by complex V for ATP synthesis in the process of oxidative phosphorylation. Respiratory complexes are known to coexist in the membrane as single functional entities and as supramolecular aggregates or supercomplexes (SCs). Understanding the assembly features of SCs has relevant biomedical implications because defects in a single protein can derange the overall SC organization and compromise the energetic function, causing severe mitochondrial disorders. Here we describe in detail the main types of SCs, all characterized by the presence of complex III. We show that the genetic alterations that hinder the assembly of Complex III, not just the activity, cause a rearrangement of the architecture of the SC that can help to preserve a minimal energetic function. Finally, the major metabolic disturbances associated with severe SCs perturbation due to defective complex III are discussed along with interventions that may circumvent these deficiencies.

**Keywords:** respiratory complexes; respiratory supercomplexes; oxidative stress; mitochondrial DNA; *MTCYB* mutations; cytochrome *b*; complex III; mitochondrial diseases
