Submitted to: Journal of Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2007
Publication Date: 1/8/2008
Citation: Sim, L., Quezada-Calvillo, R., Sterchi, E.E., Nichols, B.L., Rose, D.R. 2008. Human intestinal maltase-glucoamylase: Crystal structure of the N-terminal catalytic subunit and basis of inhibition and substrate specificity. Journal of Molecular Biology. 375:782-792. Interpretive Summary: There are a series of mucosal bound enzyme activities located on the surface of the small intestine which digest food carbohydrates into free glucose for absorption. Four of these activities digest a two-glucose unit called maltose. Two of the maltases are also associated with the digestion of sugar (sucrase-isomaltase) but two are not associated with other activities (maltase-glucoamylase). The functions of the four maltase activities are conversion of starch fragments (maltosides) to free glucose. Here we report the x-ray crystallographic structure of the membrane bound maltase from maltase-glucoamylase and show the binding of an inhibitor of starch digestion (acarbose, sometimes used for treatment of adult onset diabetes) to the active site. This is the first mucosal bound enzyme activity to be crystallized and a first step toward understanding why some food starches are resistant to digestion to glucose.
Technical Abstract: Human maltase-glucoamylase (MGAM) is one of the two enzymes responsible for catalyzing the last glucose-releasing step in starch digestion. MGAM is anchored to the small-intestinal brush-border epithelial cells and contains two homologous glycosyl hydrolase family 31 catalytic subunits: an N-terminal subunit (NtMGAM) found near the membrane-bound end and a C-terminal luminal subunit (CtMGAM). In this study, we report the crystal structure of the human NtMGAM subunit in its apo form (to 2.0 A) and in complex with acarbose (to 1.9 A). Structural analysis of the NtMGAM-acarbose complex reveals that acarbose is bound to the NtMGAM active site primarily through side-chain interactions with its acarvosine unit, and almost no interactions are made with its glycone rings. These observations, along with results from kinetic studies, suggest that the NtMGAM active site contains two primary sugar subsites and that NtMGAM and CtMGAM differ in their substrate specificities despite their structural relationship. Additional sequence analysis of the CtMGAM subunit suggests several features that could explain the higher affinity of the CtMGAM subunit for longer maltose oligosaccharides. The results provide a structural basis for the complementary roles of these glycosyl hydrolase family 31 subunits in the bioprocessing of complex starch structures into glucose.