Coenzyme Q (CoQ) is a lipid essential for oxidative phosphorylation in the mitochondrial inner membrane. It transfers electrons from complexes I and II to complex III in the electron transport chain, contributing to the membrane potential that permits ATP synthesis. It also receives electrons from many other mitochondrial dehydrogenases, being, for example, essential for de novo pyrimidine biosynthesis. Due to its redox properties, it is a potent membrane antioxidant as well. Primary CoQ deficiencies are rare and heterogeneous clinical conditions caused by biallelic pathogenic variations in any of the nuclear-coded genes responsible for CoQ biosynthesis. Secondary CoQ deficiencies are caused by defects not directly related to its endogenous biosynthesis. Patients can benefit from early diagnosis since they can be treated with CoQ supplementation and, possibly in the future, for primary CoQ deficiencies, with other molecules that could work as bypass treatments. Next-generation sequencing (NGS) has had an important impact on primary CoQ deficiency genetic diagnosis. Still, CoQ steady-state measurements are still critical not only in the clinical setting but also for research. Moreover, functional assays are essential to experimentally confirm new potential pathogenic variants identified by NGS.

The scope of this Collection is to bring together a series of methods for CoQ measurements and other related functional studies.