The gene of malQ encoding 4-α-glucanotransferase (amylomaltase) is located in the malPQ operon of Escherichia coli K12. In an effort to understand the function of MalQ enzyme in maltodextrin and glycogen metabolism of E. coli, the reactions of the enzyme with several types of carbohydrate were carried out under the optimal conditions. The enzyme catalyzed the hydrolysis of the α-1,4 glucosidic linkage of linear maltodextrins, releasing the reducing-end glucose of dextrins, and it also transferred glycosyl residues onto the non-reducing end of an acceptor via a disproportionation reaction. The smallest substrate that MalQ recognized of this reaction mode was maltose. Glucose was not substrate but a great acceptor for this enzyme. In the reaction with amylose substrate, the enzyme performed intramolecular transglycosylation to produce the cyclic form having degree of polymerization (DP) from DP20 to DP33 with DP24 as the main product. These data maybe help to understand MalQ mechanism in vivo.