Figure 2.

Generation of succinate in mitochondria. (A) Succinate is an intermediate in the citric acid cycle, and is converted by succinate dehydrogenase (also called complex II) to fumarate. When high H⁺ gradients over the inner membrane are present, or when the oxygen pressure is low, complex I, II, III, and IV will be inhibited, leading to accumulation of succinate. (B) Overview of the citric acid cycle under normal physiological conditions. (C) Cellular stress such as hypoxia may affect normal functioning of the citric acid cycle and induce a part of the cycle to run in reverse, ultimately leading to increased succinate production. See text for more details and references.