About half of the enzymes known today require the presence of a cofactor, coenzyme, or prosthetic group, such a compound plays a direct role in catalysis by the enzyme in which it occurs (in the following, the term cofactor also comprises the connotation of a prosthetic group but not that of a coenzyme, as the latter is considered here to be a helper or cosubstrate in the reaction). In the past, identification of these compounds has been a matter of combined action of nutritional and biochemical research, this approach ended about 30 years ago as it was thought that all existing coenzymes, cofactors, and vitamins had been discovered. It was, therefore, a surprise when it subsequently appeared that the list should be extended because of results provided by research on the enzymology of methane dissimilation by methanotrophic bacteria and that on methane formation by methanogenic bacteria, generating several new cofactors [1]. However, most of them seemed to have a specialized role in Archaea, although it appears now that this view should not be taken too strictly (e.g., factor420, a deazaflavin with very low redox potential that on its discovery was considered ideally suited to have a role only in the pathway of methanogenesis, has now been found to function as a cofactor in several enzymes occurring in aerobic Eubacteria [2]). Soon after its discovery in methanol dehydrogenase, pyrroloquinoline quinone, 2,7,9-tricarboxy-1H-pyrrolo[2,3-f ]quinoline-4,5-dione (PQQ) (Figure 1), appeared to be exceptional in that sense that it was found in several different bacterial dehydrogenases (Table 1). Subsequently, many other enzymes were detected and were at

Figure 1 Structures of PQQ (1), TTQ (2), TPQ (3), and LTQ (4).