The answer is cytochromes.
Cytochromes are iron-containing proteins responsible for ATP production via electron transport chain. The electron transport chain consists of Complex I, coenzyme Q, Complex II, Complex III, cytochrome c, Complex IV. Electrons from NADPH from the Krebs cycle are transported to Complex I and they are passed to coenzyme Q. Electrons from Complex II are passed to coenzyme Q as well. From coenzyme Q, electrons are passed to Complex III, then to cytochrome c, then to Complex IV.
