In Krebs cycle, dehydrogenation reaction occurs four times; among which there are three points in the cycle where ${\text{NAD}}^{+}$ is reduced to ${\text{NADH}}^{+}{\text{H}}^{+}$ and one point where ${\text{FAD}}^{+}$ is reduced to ${\text{FADH}}_2$ when succinate is converted to fumarate.