respiratory complex

any of several groups of physically associated components of the respiratory chain. The concept arose during experiments to purify individual components, when it was found that membrane particles could be isolated each with a consistently characteristic composition. Four main complexes have been identified: complex I, which contains primarily NADH dehydrogenase; complex II, which contains succinate dehydrogenase; complex III, which contains cytochromes b and c ₁; and complex IV, which contains cytochrome a /a ₃ together with copper-containing proteins. Complexes I, II, and III all contain additionally a number of proteins rich in iron and sulfur. The F_o and F₁ inunits of the ATPase associated with the respiratory chain have been termed complex V (see H⁺ -transporting ATP synthase). See also complex i, ii, iii, iv.

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