To start succinate dehydrogenase is an enzyme complex which is a catalyst for reactions that occur in the mitochondria. Succinate dehydrogenase (SDH) is also referred to as succinate-coenzyme Q reductase (SQR) or respiratory Complex II but in all cases the term refers to an enzyme complex. Succinate dehydrogenase plays a role in both the Krebs cycle and electron transport where most of the energy is derived from aerobic respiration. Interestingly, if something happens where SDH enzyme malfunctions (a mutation occurs) it sends a signal out of low oxygen. At this point SDH, which plays a role in tumor suppression, ceases to suppress.