Received May 6, 2022; Revised June 9, 2022; Accepted June 10, 2022
The Complex II family encompasses membrane bound succinate:quinones reductases and quinol:fumarate reductases that catalyze interconversion of succinate and fumarate coupled with reduction and oxidation of quinone. These enzymes are found in all biological genres and share a modular structure where a highly conserved soluble domain is bound to a membrane-spanning domain that is represented by distinct variations. The current classification of the complex II family members is based on the number of subunits and co-factors in the membrane anchor (types A-F). This classification also provides insights into possible evolutionary paths and suggests that some of the complex II enzymes (types A-C) co-evolved as the whole assembly. Origin of complex II types D and F may have arisen from independent events of de novo association of the conserved soluble domain with a new anchor. Here we analyze a recent structure of Mycobacterium smegmatis Sdh2, a complex II enzyme with two transmembrane subunits and two heme b molecules. This analysis supports an earlier hypothesis suggesting that mitochondrial complex II (type C) with a single heme b may have evolved as an assembled unit from an ancestor similar to M. smegmatis Sdh2.
KEY WORDS: succinate:ubiquinone reductase, quinol:fumarate reductase, complex II, quinone binding site, cytochrome b