Formation and Hydrolysis of Maltohexaose by an Extracellular Exo-maltohexaohydrolase

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An extracellular exo-maltohexaohydrolase [EC 3.2.1.98] from a Klebsiella pneumoniae (Aerobacter aerogenes) mutant produced about 40% maltohexaose (G6) from short-chain amylose (DP=23). Mostly G6 was produced from maltooligosaccharides larger than G6 by an exomechanism action. It also hydrolyzed G6 and shorter maltooligosaccharides to give smaller maltooligosaccharides. Its position specificity of action on G3 through G8 was studied with maltodextrins specifically labeled at the reducing-end glucose unit with 14C. The highest frequency of cleavage was at the second bond from the reducing end in G3 through G6. For G7 and G8, the sixth bond from the nonreducing end of the substrate was cleaved with absolute specificity by the exo-mechanism action. Kinetic parameters of the exo-maltohexaohydrolase on various substrates were also studied. The Michaelis constant (Km)for short-chain amylose was the smallest among the various substrates examined. G6 was also formed from G4 by a transfer action of the enzyme, with an action pattern dependent on the substrate concentration.
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