Carbohydrates - Skildum Flashcards
Carbs account for about 50% of the calories in the Western diet. Glucose is the main carb circulating in our blood and used for fuel by our cells. What fraction of the total calories in a Western diet are from glucose?
0%
Glucose is the most important circulating carbohydrate. Our diet contains almost no glucose.
***We eat other carbohydrates (polysaccharides, disaccharides, fructose), and they are converted to glucose by our body.
What are the main dietary carbohydrates?
- fructose => monosaccharide
- lactose (Gal + Glu) => disaccharide
- sucrose (Fru + Glu) => disaccharide
- amylose (a1,4 bonds) => polysaccharide
- amylopectin (a1,6 and a1,4 branches) => polysaccharide
Whhich dietary carbohydrate’s structure most resembles glycogen?
Amylopectin (longer chains before the branches)
What does amylase do?
- Amylase is an endoglycosidase.
- It cuts a-1,4 bonds in polysaccharides.
- Amylase activity is highest in the duodenum (secreted from pancreas).
What are the four products of Starch digested by Salivary + Pancreatic Amylase?
- Maltose
- Isomaltose
- Trisaccharides
- Dextrins
What are the four disaccharidases of the brush border (microvilli)?
- Disaccharidases of the brush border:
- Glucoamylase
- Sucrase / Isolmaltase complex
- Trehalase
- b-glycosidase complex (degrades lactose)
What does Glucoamylase do?
- Glucoamylase / maltase is an exoglycosidase.
- It cleaves a-1,4 bonds of maltose to form two molecules of glucose.
- (***can clip off one glucose at a time***)
- It cuts glucose off the non-reducing ends of starch.
- Glucoamylase activity is highest in the ileum.
- It cleaves a-1,4 bonds of maltose to form two molecules of glucose.
What does the Sucrase / Isolmaltase complex do?
- The Sucrase – Isomaltase complex has two extracellular domains with different substrate specificities.
- Sucrase cuts sucrose into glucose and fructose
- Isomaltase cuts the a-1,6 bond in isomaltose. (cut branching/branch point glucoses)
- Sucrase-isomaltase activity is highest in the jejunem.
What does Trehalase do?
- Trehalase has only one catalytic site, and one substrate: Trehalose.
- Trehalose: Two glucose units bonded through the number 1 carbons.
- Found in insects, algae, mushrooms and other fungi.
What does the Beta-glycosidase complex do?
- Beta-glycosidase complex is a glycophosphatidylinositol (GPI) glycan anchored protein with two catalytic domains.
- Glucosyl ceramide domain:
- Cuts glucose and galactose from glucosylceramide and galatosylceramide
- Lactase domain:
- Splits the 1,4 bond in lactose to make galactose and glucose.
- Beta-glycosidase activity is highest in the jejunem.
- Glucosyl ceramide domain:
What is the consequence of a loss of lysosomal Beta-glycosidase?
- Gaucher’s Disease:
- genetic disorder in which glucocerebroside (a sphingolipid, also known as glucosylceramide) accumulates in cells (WBCs and macrophages) and certain organs (spleen, liver, kidneys, lungs, brain)
- characterized by:
- bruising
- fatigue
- anemia
- low blood platelet count
- enlargement of the liver and spleen
How do monosaccharides get absorbed in the gut?
- When carbohydrate concentration in the lumen exceeds that of the blood, they can enter the cell through facilitated diffusion.
- Na+/glucose cotransporters (symporter)
- Facilitated glucose transporters -Na+/K+ ATPase (sets up Na+ gradients for symporter)
- When the concentration in the lumen is lower than in the blood, the cell must expend energy get the monosaccharides inside.
How does bacteria help with the digestion of carbohydrates?
- Amylose and amylopectin can also be a substrate for fermentation by gut bacteria.
- Short chain fatty acids (acetate, propionate, and butyrate) produced by bacteria are used as fuel by colonocytes.
What happens in the absence of lactase?
- Without lactase → bacteria ferment lactose to lactic acid.
- Water enters the lumen of the gut to balance difference in proton concentration
- causes diarrhea ==> Lactose intolerance (congenital or acquired)
What is the metabolic fate of carbohydrates?
- Glycolysis: All tissues
- Glycogenogenesis: All tissues
- Fatty acid synthesis: Liver
- Cholesterol synthesis: Liver
- Amino acid synthesis: Mainly liver, but most tissues
- Glycosylation of proteins, lipids: All tissues
