Starch is a predominant carbohydrate in all major foods as a digestible macronutrient, which contributes a substantial amount of calories in the human diet. Most starches are composed of 2 types of macromolecules, amylose and amylopectin, with approximate weight amount of 15% to 30% and 85% to 70%, respectively (1). The exceptions are of waxy and high-amylose varieties having 0% to 5% amylose and 50% to 90% amylose, respectively. Amylose, a polymer of α-D-glucopyranosyl units mainly linked by the α-1,4 bonds, is defined as a linear molecular chain with minor branching. Amylopectin is defined a branched polymer of α-D-glucopyranosyl units linked by both α-1,4 and α-1,6 linkages. In starch granules, there are semicrystalline regions alternating with amorphous regions as ring-like structures. The semicrystalline structure is formed and developed from double helices of linear branched chains of cluster-like organized amylopectin molecules. Raw starch granules from different botanical sources display various granule size molecular and crystalline structures.
Without processing such as cooking, raw starch is not easily digested in the human body if its crystalline structure is not destroyed, which generally displays slowly digestible property. Gelatinized starch or raw starch is digested to glucose in the small intestine and undigested or “resistant starch” is used by microorganisms in the large intestine, producing short-chain fatty acids. Six enzymatic degradation steps involve the digestion of starch molecules to glucose in the human body. The salivary and pancreatic α-amylases (α-1,4 endoglucosidases) hydrolyze starch to soluble glucose oligomers with linear and branched structures. They are converted to glucose in the human small intestine by the combined action of mucosal maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI). Both MGAM and SI with exoglucosidic activity could hydrolyze α-1,4 and α-1,6 linkages from nonreducing ends of linear chains of glucose oligomers and polymers to release free glucose as the final step in small intestinal digestion. SI displays more hydrolytic activity on branched α-1,6 linkages than MGAM. MGAM substrate specificity somewhat overlaps with that of SI (2–4).
Raw starch digestion with both salivary and pancreatic α-amylases, as well as bacterial α-amylases and fungi glucoamylases, are widely recognized; however, the nature of raw starch digestion by MGAM and SI is less well known, even though there is a hypothesis that human mucosal maltase activities is an alternate pathway for starch digestion when salivary and pancreatic α-amylase activities are inhibited or reduced because of malnutrition and immaturity of the gastrointestinal tract. We have reported that recombinant human N-terminal MGAM (rMGAM-N) can digest raw starch in a unique way: the degradation of native starch granules showed a surface furrowed pattern in random, radial, or tree-like arrangements that differed substantially from the erosion patterns of α-amylase and fungi amyloglucosidase (5).
We have demonstrated again that the human mucosal MGAM and SI can hydrolyze raw starch. The digestion displayed similar patterns to those found in the previous study with rMGAM-N, which were observed with scanning electron microscopy (Fig. 1) and onsite observation of atomic force microscopy. Furthermore, the digestion profiles were proven with wild-type and MGAM knockout (null) mice mucosal solution using different kinds of raw starches. The unique digestion pattern of raw starch by MGAM and SI in human and mouse, which is different from α-amylases and fungi glucoamylases, may be a universal pattern with applications to other mammalian species.
In terms of glucose production, α-amylases activity amplifies glucose production because of the production of available substrates with nonreducing ends for MGAM and SI. Pretreatment of starch granules with α-amylase resulted in an expected amplification on glucose production. The resistant property of raw starch (mainly from its crystalline structure) is dominated by being resistant to α-amylases, followed by MGAM and SI. This would apply to retrogradated starch molecules in foods after processing (eg, cooking), in which semicrystalline structures are normally formed during storage. Therefore, α-amylases could be considered to be one of supplements for patients with digestion disorder of starchy foods. It may be applied in patients with congenital sucrase-isomaltase deficiency when starchy foods are consumed. Further efficacy studies are needed, however, to prove this hypothesis for patients with congenital sucrase-isomaltase deficiency.
1. Jane J, Chen YY, Lee LF, et al. Effects of amylopectin branch chain length and amylose content on the gelatinization and pasting properties of starch. Cereal Chem
2. Quezada-Calvillo R, Robayo-Torres CC, Opekum AR, et al. Contribution of mucosal maltase-glucoamylase activities to mouse small intestinal starch alpha-glucogenesis. J Nutr
3. Nichols BL, Quezada-Calvillo R, Robayo-Torres CC, et al. Mucosal maltase-glucoamylase plays a crucial role in starch digestion and prandial glucose homeostasis of mice. J Nutr
4. Sim L, Quezada-Calvillo R, Sterchi EE, et al. Human intestinal maltase-glucoamylase: crystal structure of the N-terminal catalytic subunit and basis of inhibition and substrate specificity. J Mol Biol
5. Ao Z, Quezada-Calvillo R, Sim L, et al. Evidence of native starch degradation with human small intestinal maltase-glucoamylase (recombinant). FEBS Lett