Carbohydrate Metabolism 1

Question 1

GLUT 2

Answer 1

• Low affinity transporter in hepatocytes and pancreatic cells
• Captures excess glucose primarily for storage
• When the glucose concentration drops below the K_m for the transporter, the remainder bypasses the liver and enters the peripheral circulation
• The liver will pick up excess glucose and store it preferentially after a meal when BGLs are high
• In the ß-islet cells of the pancreas, GLUT 2, along with the glycolytic enzyme, glucokinase, serves as the glucose sensor for insulin release

Question 2

GLUT 4

Answer 2

• Present in adipose tissue/muscle and responds to the glucose concentration in peripheral blood
• Rate of glucose transport in these tissues is increased by insulin, stimulating the movement of additional GLUT 4 transporters to the membrane by a mechanism involving exocytosis
• The transporter is saturated when BGLs are just a bit higher than normal
• When someone has high BGLs. the transporters will still permit only a constant rate of glucose influx because they will be saturated (zero-order kinetics)
• GLUT 4 can increase their intake of glucose by increaing the number of GLUT 4 transporters on their surface

Question 3

Diabetes Mellitus

Answer 3

• Caused by a disruption of the insulin/GLUT mechanism
• Type 1 =Insulin is absent and cannot stimulate the insulin receptor
• Type 2 =The insulin receptor becomes insensitive to insulin and fails to bring GLUT 4 transporters to the cell surface.
• Blood glucose levels rise resulting in; increased urination, increased thirst, and ketoacidosis. Long term symptoms are blindness, heart attacks, strokes, and nerve damage.

Question 4

Glycogen

Answer 4

• Muscle stores excess glucose as glycogen

Question 5

Dihydroxyacetone Phosphate (DHAP)

Answer 5

• Adipose tissue requires glucose to form this (DHAP)
• DHAP is converted to glycerol phosphate to store incoming fatty acids as triacylglycerols

Question 6

Red Blood Cells

Answer 6

• Glycolysis is the only energy yielding pathway available
• They lack mitochondria (which are required for the citric acid cycle, ETC, oxidative phosphorylation, and fatty metabolism (ß-oxidation))

Question 7

Glycolysis

Answer 7

• Converts glucose into 2 pyruvate molecules, releasing energy captured in two substrate level phosphorylations and one oxidation reaction
• If a cell has mitochondria and oxygen, the energy carriers produced in glycolysis (NADH) can feed into the aerobic respiration pathway to generate energy for the cell
• If either mitochondria or oxygen is lacking (RBCs or exercising skeletal muscle), glycolysis may occur anaerobically, although some of the available energy is lost
• In the liver, glycolysis is part of the process by which excess glucose is converted to fatty acids for storage.

Question 8

Glycolysis Rate Limiting Enzyme

Answer 8

• Phosphofructokinase-1 (PFK-1)
• Fructose 6-Phosphate = phosphorylated to Fructose 1,6-Bisphosphate using ATP
• Inhibited: ATP, citrate, and glucagon (indirectly)
• Activated: AMP, Insulin (indirectly)
• Cell turns off glycolysis when it has high ATP
• Cell turns on glycolysis when it needs energy (high AMP)

Question 9

PFK-2

(Phosphofructokinase-2)

Answer 9

• Activate: Insulin
• After activated by insulin, fructose 6-phosphate is converted to fructose 2,6 bisphosphate (Activating PFK-1)
• Inhibit: Glucagon
• Glucagon lowers fructose 2,6 bisphosphate, inhibiting PFK-1
• Found in the liver

Question 10

NAD+ ——-> NADH

Answer 10

• Reduction reaction
• Glyceraldehyde 3-Phosphate Dehyrogenase catalyzes an oxidation and addition of inorganic phosphate, Pi , to its substrate, glyceraldehyde 3-phosphate
• Equals: Production of 1,3 bisphosphateglycerate and the reductions reaction
• Aerobic Glycolysis: NADH can be oxidized by ETC