Sucrase-isomaltase complex

Hunziker, W., Spiess, M., Semenza, G., and Lodish, H. F. (1986). The sucrase-isomaltase complex Primary structure, membrane orientation, and evolution of a stalked, intrinsic brush border protein. Cell 46, 227-234. 

The brush border enzymes with disaccharidase and ohgosaccharidase activity are listed in Table 48-1. The sucrase-isomaltase complex comprises most of the sucrase, isomaltase, and maltase (80%) activity of the small intestine. It hydrolyzes sucrose to its constituent monosaccharides, cleaves glucose from a-limit dextrins with 1,6 bonds, and hydrolyzes maltose. The activity of the complex is fourfold to fivefold greater in the jejunum than in the ileum. Changes in diet have a marked effect on the expression of the complex starvation leads to a rapid decline in activity, which is rapidly restored on refeeding. AH small intestinal saccha-ridases may decrease with infection or inflammation of the small bowel to the extent that carbohydrate malabsorption... 

Sucrase-isomaltase complex An enzyme complex comprised of two enzyme units. Both units have high a-l,4-glucosidase activity and will hydrolyze maltose and maltotriose to glucose. The sucrase unit will also hydrolyze sucrose to fructose and glucose, whereas the isomaltase unit will hydrolyze a-1,6 bonds found in isomaltose and the limit dextrins of starch. 

Individuals with genetic deficiencies of the sucrase-isomaltase complex show symptoms of sucrose intolerance but are able to digest normal amounts of starch in a meal, without problems. The maltase activity in the glucoamylase complex, and residual activity in the sucrase-isomaltase complex (which is normally present in excess of need) is apparently sufficient to digest normal amounts of dietary starch. 

Which of the bonds in the structure above are hydrolyzed by the sucrase-isomaltase complex Which by glucoamylase ... 

Bonds (1) and (3) would first be hydrolyzed by glucoamylase. Bond (2) would require isomaltase. Bonds (4) and (5) could then be hydrolyzed by the sucrase-isomaltase complex, or by the glucoamylase complex, all of which can convert maltotriose and maltose to glucose. 

Fig. 27.5. The major portion of the sucrase-isomaltase complex, containing the catalytic sites, protrudes from the absorptive cells into the lumen of the intestine. Other domains of the protein form a connecting segment (stalk), and an anchoring segment that extends through the membrane into the cell. The complex is synthesized as a single polypeptide chain that is split into its two enzyme subunits extracellularly. Each subunit is a domain with a catalytic site (sucrase-maltase) and isomaltase-maltase sites. In spite of their maltase activity, these catalytic sites are often called just sucrase and isomaltase.

The specificity of conduritol B epoxide is such that -glucosidases from widely differing sources have been found to react with loss of enzymic activity from various Aspergillus species, > yeast, snail Helix pomatia) sweet almonds, and mammals. The only exceptions have been the / -glucosidases from garbanzo plants Cicer arietum L.) and from Alocasia macrorrhiza The only other enzymes that have been found to be covalently inhibited are a-glucosidase from yeast Saccharo-myces cerevisiae) and the sucrase-isomaltase complex from rabbit small intestine. ... 

Suberic acid, 506 Substrate analogs, 8, 9 Subtilisin, 22, 27, 75, 206, 207 Subtilisin BPN, 215 Succinimide esters, 96 Sucrase-isomaltase complex, 377,379 Suicide reagents, 9, 10 Sulfonyl azides, 78 a-Sulfonyl nitrenes, 78 Sulfoxides, 135... 

Amino-acid sequences around the essential carboxylic acid group in the active sites of the sucrose a-D-glucohydrolase oligo-l,6-glucosidase (sucrase-isomaltase) complex of rabbit small intestines have been determined by selective labelling with [ H]conduritol-B epoxide. ... 

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