Figure 3.

General scheme for the autocatalytic synthesis of a coenzyme. The coenzyme (carrier molecule) and its loaded form are denoted by Q and Q*, respectively. A, X and F are external compounds provided to the cycle. B is an intermediate in the cycle. G and Y are by-products of the cycle. B can be considered an autocatalyst if X is provided as an external compound. However, if X is an intermediate of the whole network (that is, an internal compound) then providing B does not necessarily launch the cycle because biosynthesis of X might require the presence of coenzyme Q*.