Carbohydrates - Skildum Flashcards Preview

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Flashcards in Carbohydrates - Skildum Deck (17)
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1
Q

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?

A

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.

2
Q

What are the main dietary carbohydrates?

A
  • fructose => monosaccharide
  • lactose (Gal + Glu) => disaccharide
  • sucrose (Fru + Glu) => disaccharide
  • amylose (a1,4 bonds) => polysaccharide
  • amylopectin (a1,6 and a1,4 branches) => polysaccharide
3
Q

Whhich dietary carbohydrate’s structure most resembles glycogen?

A

Amylopectin (longer chains before the branches)

4
Q

What does amylase do?

A
  • Amylase is an endoglycosidase.
    • It cuts a-1,4 bonds in polysaccharides.
    • Amylase activity is highest in the duodenum (secreted from pancreas).
5
Q

What are the four products of Starch digested by Salivary + Pancreatic Amylase?

A
  • Maltose
  • Isomaltose
  • Trisaccharides
  • Dextrins
6
Q

What are the four disaccharidases of the brush border (microvilli)?

A
  • Disaccharidases of the brush border:
      1. Glucoamylase
      1. Sucrase / Isolmaltase complex
      1. Trehalase
      1. b-glycosidase complex (degrades lactose)
7
Q

What does Glucoamylase do?

A
  • 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.
8
Q

What does the Sucrase / Isolmaltase complex do?

A
  • 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.
9
Q

What does Trehalase do?

A
  • 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.
10
Q

What does the Beta-glycosidase complex do?

A
  • 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.
11
Q

What is the consequence of a loss of lysosomal Beta-glycosidase?

A
  • 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
12
Q

How do monosaccharides get absorbed in the gut?

A
  • 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.
13
Q

How does bacteria help with the digestion of carbohydrates?

A
  • 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.
14
Q

What happens in the absence of lactase?

A
  • 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)
15
Q

What is the metabolic fate of carbohydrates?

A
  • 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
16
Q

What is fructose? How is it produced?

A
  • Fructose is a naturally occurring monosaccharide found in honey.
  • It is also produced by sucrase acting on sucrose to produce glucose and fructose.
  • High fructose corn syrup is a mixture of 55% fructose, 45% glucose.
17
Q

Does high fructose corn syrup cause obesity?

A

No?

Fructose does not have a caloric value any higher than other stuff we eat.