Menaquinones in bacteria - occurrence, function and biosynthesis.

Menaquinones (MKs), known mainly as respiratory quinones in bacteria, are found in the majority of bacterial species and show great variety as regards to the structure, i.e. the length of the side-chain, its saturation, ring methylation and other modifications. MK function in bacteria is attributed mainly to its function in anaerobic respiration, but the number of potential electron donors and acceptors is very numerous. However, their function is far beyond contribution in respiration. For example, they take part in bacterial photosynthesis, mediate extracellular electron transport or participate in cellular signalling. Moreover, respiratory MK pool of bacteria, is an electron acceptor from DsbA-DsbB two-component system engaged in the formation of disulfide bridges in various target proteins. The classical and a new MK biosynthetic pathway in bacteria, so-called futalosine pathway, have been recently thoroughly characterized and nearly all the enzymatic steps recognized, showing in some cases unusual catalytic mechanisms. Finally, examples of the analytical methods of MKs determination are presented, including practical hints.