Regulation of Carbon Metabolism in Filamentous Fungi

Abstract

Fungi are primarily soil organisms and as agents of decay produce extracellular enzymes degrading many natural polymers, especially polysaccharides (Section 2). The majority of fungi also utilize a wide range of growth substrates including poly-and oligosaccharides, hexoses, pentoses, many organic acids and alcohols, but cannot metabolize C1 compounds such as methanol (Cooney and Levine, 1972). Although most filamentous fungi are obligate aerobes, they exhibit high affinities for oxygen and thus grow at very low oxygen concentrations (Bull and Bushell, 1976). It is therefore difficult to demonstrate true anaerobic growth, which is found only in a few fungi such as Blastocladiella ramosa (Held et al, 1969), Mucor species (Bartnicki-Garcia and Nickerson, 1962), and Fusarium oxysporum (Gunner and Alexander, 1964). The capacity for fermentation in a limiting concentration of oxygen is, however, more common. For example, a lactic acid fermentation is found in aquatic lower fungi, an alcoholic fermentation in F. lini, and fermentations yielding lactic acid, ethanol, and CO2 in Rhizopus species (see Cochrane, 1976). Anaerobic growth or fermentation may demand specific nutritional requirements such as thiamine and nicotinic acid in Figure 1. Relationships between primary and secondary metabolism in filamentous fungi. The origins of major classes of secondary metabolites produced by filamentous fungi are shown in relation to the central pathways of intermediary metabolism. Apart from the 3-lactam antibiotics, no attempt is made to show the many fungal products that arise by further reactions between secondary metabolites of diverse origins. The biosynthesis of certain organic acids is discussed in some detail (Section 3.4) and illustrated in Figs. 5–7.