Figure 1.

Lactose and D-galactose catabolism in Hypocrea jecorina. The heterodisaccharide lactose is cleaved extracellularly into its monomers D-glucose and D-galactose. While D-glucose is assimilated via glycolysis (not shown), D-galactose can be converted by two different pathways. The galactokinase of the classical Leloir pathway (left) is specific for α-D-galactose and therefore, β-D-galactose has to be epimerised to the α-anomer before it can enter this pathway. A second pathway identified in Hypocrea jecorina starts with the reduction of both anomeric forms of D-galactose to galactitol. Two hypothetical pathways are drafted for the further degradation of galactitol.