Figure 1.

Reactive oxygen species (ROS) are oxygen-derived free radicals produced by oxidative metabolic pathways (chiefly the mitochondrial respiratory chain). Small amounts of ROS are needed to ensure normal sperm cell functions such as capacitation, hyperactivation, acrosome reaction and sperm-oocyte fusion, but excess ROS production induces oxidative stress (OS). Seminal fluid and sperm cells are rich in antioxidant molecules (superoxide dismutase -SOD-, catalase -CAT- and glutathione peroxidase 4—Gpx4), which counterbalance the effect of ROS. Spermatozoa are susceptible to OS because their plasma membrane is rich in polyunsaturated fatty acids that undergo lipid peroxidation, which in turn increases ROS production and induces DNA fragmentation and sperm apoptosis. Coenzyme Q10 is an obligatory co-factor with strong antioxidant properties that counteract OS by reducing ROS production in mitochondria and protect spermatozoa membranes from lipid peroxidation.