The Pathway of Exogenous and Endogenous Carbohydrate Utilization in Escherichia coli: A Dual Function for the Enzymes of the Maltose Operon

The components of the maltose‐utilizing system in Escherichia coli ML, i.e. maltose permease, amylomaltase and maltodextrin phosphorylase, were re‐investigated, with the following findings: The concentrating mechanism (permease) is the rate‐limiting step in the maltose‐utilizing system. Maltose and higher maltodextrins, up to an average chain length of at least 10 glucosyl units, can be taken up at the same rate. Maltodextrins of an average chain length of 20 glucosyl units are restricted in their uptake, as reflected in the growth‐rate of the organism. Growth studies with extensively purified maltose show that this sugar, in contrast to maltotriose and higher maltodextrins, is a poor inducer of the system. Pullulanase is co‐induced by maltodextrins, along with the other enzymes. The pullulanase seems to be bound to the cell wall and has properties very similar to those of the well‐studied pullulanase of Aerobacter aerogenes. The complete system, now referred to as the maltodextrin‐utilizing system, comprises the enzymes pullulanase, maltodextrin permease, (a name replacing maltose permease) amylomaltase and maltodextrin phosphorylase. This group of enzymes was also demonstrated to be present in Streptococcus mutans and A. aerogenes. In addition, A. aerogenes possesses an inducible α‐amylase. This last enzyme permits a wider range of branched 1,4‐α‐glucans to be utilized by the latter organism. The enzymes of glycogen degradation in E. coli NCTC 5928 were investigated. This organism utilizes isoamylase to debranch the reserve polysaccharide, liberating the component chains. These maltodextrin chains are subsequently metabolized by the enzymes of the maltodextrin‐utilizing system. We suggest that E. coli uses pullulanase for the debranching of extracellular carbohydrates, while isoamylase is presumed to function intracellularly in the degradation of glycogen. Maltodextrins are provided for intracellular utilization in E. coli by either of two routes. Pullulanase and permease function in the degradation and uptake of extracellular branched α‐glucans. Isoamylase produces maltodextrins during the debranching process of intracellular glycogen. Maltodextrins arising by either route are metabolized to glucose and glucose 1‐phosphate by the concerted actions of amylomaltase and maltodextrin phosphorylase. A. aerogenes and S. mutans probably have similar enzymic machinery, the former also having an extracellular α‐amylase to assist exogenous glucan degradation. 9. A general scheme for the metabolism of branched 1,4‐α‐glucans in bacteria and plants is proposed.