Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2008
Publication Date: 4/1/2008
Citation: Quezada-Calvillo, R., Simm, L., Ao, Z., Hamaker, B.R., Quaroni, A., Brayer, G.D., Sterchi, E.E., Robayo-Torres, C.C., Rose, D.R., Nichols, B.L. 2008. Luminal starch substrate "Brake" on maltase-glucoamylase activity is located within the glucoamylase subunit. Journal of Nutrition. 138:685-692. Interpretive Summary: Starch is the major animal food source of energy from plant tissues. In the plant, starch exists as semicrystalline granules of glucose polymers. The digestion of starch granules requires two steps: solubilization by secreted amylase enzymes and glucose liberation by intestinal-bound enzymes. Here we show in mice that there are two sets of membrane-bound enzymes. Maltase-glucoamylase is a high activity, low amount enzyme that is essential for rapid glucose production from snacks, which is inhibited by larger intakes and proves that the inhibited activity is in the unattached end. The alternate enzyme, sucrase-isomaltase, is a low activity and high amount enzyme that is uninhibited and constrains glucose production by large meals.
Technical Abstract: The detailed mechanistic aspects for the final starch digestion process leading to effective alpha-glucogenesis by the 2 mucosal alpha-glucosidases, human sucrase-isomaltase complex (SI) and human maltase-glucoamylase (MGAM), are poorly understood. This is due to the structural complexity and vast variety of starches and their intermediate digestion products, the poorly understood enzyme-substrate interactions occurring during the digestive process, and the limited knowledge of the structure-function properties of SI and MGAM. Here we analyzed the basic catalytic properties of the N-terminal subunit of MGAM (ntMGAM) on the hydrolysis of glucan substrates and compared it with those of human native MGAM isolated by immunochemical methods. In relation to native MGAM, ntMGAM displayed slower activity against maltose to maltopentose (G5) series glucose oligomers, as well as maltodextrins and alpha-limit dextrins, and failed to show the strong substrate inhibitory "brake" effect caused by maltotriose, maltotetrose, and G5 on the native enzyme. In addition, the inhibitory constant for acarbose was 2 orders of magnitude higher for ntMGAM than for native MGAM, suggesting lower affinity and/or fewer binding configurations of the active site in the recombinant enzyme. The results strongly suggested that the C-terminal subunit of MGAM has a greater catalytic efficiency due to a higher affinity for glucan substrates and larger number of binding configurations to its active site. Our results show for the first time, to our knowledge, that the C-terminal subunit of MGAM is responsible for the MGAM peptide's "glucoamylase" activity and is the location of the substrate inhibitory brake. In contrast, the membrane-bound ntMGAM subunit contains the poorly inhibitable "maltase" activity of the internally duplicated enzyme.