Electron donors can also form charge transfer complexes with electron acceptors. Quinones are electrophilic Michael acceptors stabilised by conjugation. Electron acceptors participate in electron-transfer reactions. Thioglycoligases catalyze the condensation of activated glycosides and various thiol containing acceptors. The electron transport chain comprises an enzymatic series of electron donors and acceptors. In general, ligands are viewed as electron donors and the metals as electron acceptors. Class II oxidases are Quinol oxidases and can use a variety of terminal electron acceptors. Phenothiazines are electron donors, forming charge-transfer salts with many acceptors. Prokaryotes can utilize a variety of electron acceptors. Many denitrifying bacteria can also use ferric iron () and some organic electron acceptors. This chain of electron acceptors is known as an electron transport chain. MFC BOD sensors underestimate BOD values in the presence of these electron acceptors. Only methanogenesis and fermentation can occur in the absence of electron acceptors other than carbon. Many bacteria use inorganic compounds as hydrogen acceptors to regenerate the NAD. Common electron acceptors used by this subclass are NAD, FAD, and NADP.