Bl. Nichols et al., HUMAN SMALL-INTESTINAL MALTASE-GLUCOAMYLASE CDNA CLONING - HOMOLOGY TO SUCRASE-ISOMALTASE, The Journal of biological chemistry, 273(5), 1998, pp. 3076-3081
It has been hypothesized that human mucosal glucoamylase (EC 3.2.1.20
and 3.2.1.3) activity serves as an alternate pathway for starch digest
ion when luminal cu-amylase activity is reduced because of immaturity
or malnutrition and that maltase-glucoamylase plays a unique role in t
he digestion of malted dietary oligosaccharides used in food manufactu
ring, As a first step toward the testing of this hypothesis, we have c
loned human small intestinal maltase-glucoamylase cDNA to permit study
of the individual catalytic and binding sites for maltose and starch
enzyme hydrolase activities in subsequent expression experiments, Huma
n maltase-glucoamylase was purified by immunoisolation and partially s
equenced, Maltase-glucoamylase cDNA was amplified from human intestina
l RNA using degenerate and gene-specific primers with the reverse tran
scription-polymerase chain reaction. The 6,513-base pair cDNA contains
an open reading frame that encodes a 1,857-amino acid protein (molecu
lar mass 209,702 Da). Maltase-glucoamylase has two catalytic sites ide
ntical to those of sucrase-isomaltase, but the proteins are only 59% h
omologous, Both are members of glycosyl hydrolase family 31, which has
a variety of substrate specificities, Our findings suggest that diver
gences in the carbohydrate binding sequences must determine the substr
ate specificities for the four different enzyme activities that share
a conserved catalytic site.