L15: CHO digestion and uptake/CHO metabolism I

Question 1

Explain how starch is broken down. What are the enzymes responsible?

Answer 1

• Starch breakdown results in glucose, maltose, maltotriose and alpha-limit dextrin - Alpha-amylase cleaves off glucose ends of starch. It is found in our saliva and pancreas - Maltase cleaves maltose into two molecules of glucose - Oligosaccharidase cleaves maltotriose into three glucose molecules - Alpha-limit dextrin is cleaved into glucose molecules by alpha-glucosidase and isomaltase

Question 1

Explain how lactose, sucrose and maltose are broken down. What are the enzymes responsible?

Answer 1

• Lactase breaks lactose into glucose and galactose - Sucrase/isomaltase complex yields fructose and glucose from sucrose - Maltase yields glucose from maltose

Question 1

What are the hexokinase and glucokinase reactions?

Answer 1

• Glucose + ATP =(ez: hexokinase/glucose) G6P + ADP

Question 2

What is the purpose of activating sugars? What does this mean? What is the process for activating a sugar?

Answer 2

• Activating sugars refers to setting aside of sugars through phosphorylation and addition of nucleotide phosphate groups. This “charging” process dedicates a sugar to a particular metabolic pathway, preventing it from being used in multiple pathways. In addition, it adds energy to the molecule for a later process, preventing energy from being required later on. - Example of activation: Glucose is phosphorylated, then linked to nucleotide to form an NTP-sugar, then in most cases it is cleaved by a phosphorylase to an NDP-sugar.

Question 2

Explain how glucose, galactose and fructose are absorbed into the body.

Answer 2

• Glucose: enters via the SGLT1 secondary active symporter (with sodium) found on the luminal side of the intestinal epithelia. It passively exits the epithelial cells through the GLUT2 transporter to the basal side and into the serum. - Galactose: enters via the SGLT1 secondary active symporter (with sodium) found on the luminal side of the intestinal epithelia and passively exits via the GLUT2 transporter to the basal side and into the serum. - Fructose: enters via GLUT5 passively on the luminal side of the intestinal epithelia and passively exits on the basal side via the same GLUT5 transporter into the serum. GLUT2 can move it to.

Question 2

Where are the following glucose transporters found? What are their functions?

Answer 2

• SGLT1: found in small intestine, responsible for actively transporting glucose from lumen into intestinal epithelia. - GLUT1: found in all tissues, responsible for basal glucose uptake - GLUT2: found in liver, intestine and beta-cells of pancreas. In liver: removes glucose from blood. In intestine: releases glucose from epithelia into blood. In pancreas: regulates secretion of insulin. Also able to move fructose. - GLUT3: found in all tissues, responsible for basal glucose uptake - GLUT4: found in muscle and adipose tissue. Increases with endurance training, induced by insulin. - GLUT5: found in small intestine, responsible for uptake of fructose into epithelial cells and movement into blood serum.

Question 3

What tissue mostly deals with degradation of fructose? How? What is the reaction?

Answer 3

• Liver handles most of fructose degradation. Muscle handles minimal amounts. - Fructokinase (in liver, not muscle): fructose + ATP =(ez: fructokinase) F1P + ADP

Question 5

Describe ways in which glucose can be modified.

Answer 5

Question 5

What reaction does galactokinase catalyze?

Answer 5

• Galactose + ATP =(ez: galactokinase) Galactose-1-P

Question 6

Can we absorb di/tri/polysaccharides into our serum?

Answer 6

• No, only monosaccharides

Question 7

What are the monosaccharides derived from our diet that we have to consider absorbing into our gut?

Answer 7

• Glucose, fructose and galactose.

Question 7

What is raffinose?

Answer 7

• Raffinose is a sugar that is abundant in leguminous seeds (beans, lentils). It cannot be digested by humans. Its breakdown by intestinal bacteria yields gaseous byproducts.

Question 8

In what pathways does glucose feature?

Answer 8

• 1.) glycogen synthesis / glycogenolysis - 2.) gluconeogenesis / glycolysis - 3.) pentose phosphate pathway - 4.) modification pathways (proteoglycans and glycosides)

Question 10

What is the purpose of breaking polysaccharides into monosaccharides?

Answer 10

• Only monosaccharides are absorbed across intestinal epithelia.

Question 11

What would a sucrase/isomaltase deficiency cause?

Answer 11

• Intolerance to sucrose causing protracted diarrhea

Question 12

Where are the only places in the body where glucose will be found in its dephosphorylated form? Phosphorylated form? Is this the same for fructose?

Answer 12

• Non-phosphorylated glucose: intestinal lumen, intestinal epithelium and circulation. - Phosphorylated glucose: in all cells, except the intestinal epithelial cells. - Same applies for fructose.

Question 14

What is mutarotation? What is the importance of this for glucose?

Answer 14

• Mutarotation is the ability of a molecule to to react with itself to change from linear to circular form. - Sugars have reducing power, are able to donate electrons, when in linear form. - For glucose, it is able to spontaneously bind to proteins without enzyme needed. Molecules with sugars bound to them are known as glycation products. For example, it binds to Hb forming HbA1C, which can be used clinically as a determinant for glucose metabolism issues. In addition, glycation products can be produced under metabolic stress known as advanced glycation end products (AGEs). These are major determinants of inflammatory processes.

Question 15

Which sugar macromolecules are humans able to digest, which are they note? Why?

Answer 15

• Humans are able to digest starch (consisting of amylopectin and amylose) and glycogen, which are alpha-linked sugar molecules. Starch is made by plants, glycogen is made by animals. These are digested by amylases. - We are not able to digest cellulose (a structural support macromolecule) that are made by plants and fungi. These are glucose monomers that are beta-linked and we don’t have enzymes to digest them. These are our indigestible CHOs and are our source of dietary fiber.

Question 17

Provide examples of disaccharides found in our diet.

Answer 17

• Maltose: glucose-alpha1,4-glucose - Lactose: galactose-beta1,4-glucose - Sucrose: glucose-alpha1,beta2-fructose

Question 18

Which glycosidic bonds do our digestive enzymes break down?

Answer 18

• Alpha-glycosidic bonds in starch and glycogen are degraded, but not beta-glycosidic bonds found in cellulose.

Question 20

What are the functions of carbohydrates? Provide examples.

Answer 20

• Energy source (glucose) - Energy storage (glycogen) - Structural support (proteoglycans) - Backbone of nucleic acids (ribose, deoxyribose) - Modification of proteins and metabolites (GlcNAc, uronic acids)

Question 21

What is lactose intolerance?

Answer 21

• The inability to digest dietary lactose as a result of a deficiency in the lactase enzyme (many causes). Lactose is not hydrolyzed in the gut and is therefore not absorbed. It is osmotically active and draws water into the lumen of the intestine, causing diarrhea. In addition, lactose is fermented by enterobacteria and causes gas and gas-related pain.

Question 23

What is the reducing end of a sugar?

Answer 23

• Reducing ends of sugars within a di(poly)saccharide that have reducing power when in their linear form. They need to be controlled to prevent them from reacting. They donate electrons.

Question 24

How is glucose trapped into a cell?

Answer 24

• Phosphorylation via hexokinase (all cells except liver) and glucokinase (in liver) at the C6 position. Galactokinase traps galactose inside cells by phosphorylating at C1 position. - Depending on the pathway it will be used in, it will be phosphorylated at a particular carbon.