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= Sucrase-Isomaltase = Sucrase-isomaltase (SI) is an intestinal brush border disaccharidase that can serve as a model for the study of the regulation of intestinal gene expression as well as intestinal differentiation and development. Sucrase-isomaltase can be found in the rough endoplasmic reticulum where it is assembled as a homologous pro-enzyme dimer, it is passed through the Golgi apparatus to the surface of the villi of the small intestine where it is broken down into its mature subunits of sucrase and isomaltase by pancreatic proteases. Sucrase-isomaltase (SI) is an enterocyte-specific gene which exhibits a complex pattern of expression during intestinal development and in the adult intestinal mucosa. It has been linked to a disorder Sucrase-Isomaltase Mechanism.png called Congenital Sucrase-Isomaltase Deficiency (CSID) is a disease caused by the inability to digest certain sugars.

Structure
The sucrase-isomaltase complex is an intrinsic glycoprotein of the small intestinal brush border membrane and plays a key role in the final degradation of glycogen and starch. The N terminal of the SI encoded protein exhibits isomaltase activity whereas the C terminal of the moelcule exhibits sucrase α-glucosidase activity.

Mechanism


When put together they form sucrase-isomaltase. The enterocyte-specific transcription of the SI gene is controlled by a 183-bp evolutionarily conserved promoter located immediately upstream of the transcriptional start site. This promoter contains at least three nuclear protein-binding sites; SIF1, SIF2, and SIF3, each of which acts as a positive cis-acting element for SI transcription.

Formula for sucrose: sucrose + H2O → β-D-fructofuranose + D-glucopyranose

Formula for isomaltose: isomaltose + H2O → α-D-glucopyranose + D-glucopyranose

Evolution of Enzyme
The gene structure for sucrase-isomaltase formed from evolution due to a tandem duplication, which s a mutation or recombination event which produces identical adjacent segments on a chromosome, of a past ancestral gene. These genes then diverged because they bonde to different substrates. Also α-glucosidase in humans is encoded from another ancestral gene of maltase-glucoamylase (MGAM) due to a duplications of the previous tandem duplication to produce the Sucrase-isomaltase gene.

Sucrase-isomaltase is one of the best-characterized marker enzymes of the absorptive villose cells in the adult small intestine. Its structure, biosynthesis, and intracellular processing have been investigated in great detail both in animal model systems and in humans.

Disease Relevance
Sucrase-isomaltase has been linked to a disorder called Congenital Sucrase-Isomaltase Deficiency (CSID) is a disease caused by the inability to digest certain sugar such as sucrose (found in fruits,table sugars) and maltose (grain sugars) Congenital sucrase-isomaltase deficiency usually becomes apparent after an infant is weaned and starts to consume fruits, juices, and grains. In adults, symptoms persist, but may appear to be less severe than those experienced by children. Genetic Sucrase-Isomaltase Deficiency is not a disease that a patient can “outgrow”. In some adults, symptoms may be limited to an increase in bowel frequency, abdominal distention and flatulence, although episodic watery diarrhea upon ingestion of high levels of sucrose may occur.3 In some patients, diarrhea may alternate with constipation, leading to a misdiagnosis of Irritable Bowel Syndrome (IBS). As with pediatric patients, the clinical presentation in adults is variable. With the introduction of dietary sucrose, some patients may experience several severe effects from this disease, while others may experience only mild symptoms. All of these sugars are disaccharides, they are broken down by digestion but cannot be done so if there is a deficiency in sucrase-isomaltose. The symptoms that arise from this disease contain diarrhea, cramps and bloating and also a person may be unable to gain weight or grow. Diarrhea is one of the most common because in a normal person, sucrose bind to its active site where it breaks down into glucose and fructose producing a normal stool. However, in CSID, sucrose has no active site to bind to because the small intestine contains water, bacteria and other particles that cause increased motility of the small intestine leading to frequency urination and diarrhea. The disease contain an autosomal recessive inheritance pattern which means that both parents are carriers (do not have the gene themselves) and have the possibility of donating a gene with a mutation to their offspring. There is also evidence that CSID may be linked by heterozygous carriers leading for a new term of genetic sucrase-isomaltase deficiency to be usd for heterozygous individuals and CSID to be used for homozygous recessive individuals.