Lecture 17: Carbohydrate Metabolism

Question 1

Carbohydrate Metabolism

Answer 1

• In anabolic state, insulin signaling increases glucose localization
• Essential for ATP production from simple monosaccharides(Glc, Fru, Gal) in cells
• Excess glucose stored as glycogen when ATP
• In catabolic state, epinephrine and glucagon signal for glycogen breakdown leading to ATP production or glucose release in liver

Question 2

Glycolysis

Answer 2

• 10 step catabolic pathway in cytoplasm that uses glucose/other simple monosaccharides to make energy
• Starts with glucose(6 carbon molecule)
• Stage 1(Steps 1-5): 2 ATPs used to prepare glucose molecule for breakdown, generating 2 GAPs(3 carbon molecule)
• Stage 2(Steps 6-10): Produces 2 pyruvates(3 carbon molecule), 2 NADH, and 4 ATP(net 2 ATP). ATP produced by substrate-level phosphorylation
• Pyruvate can be oxidized in mitochondria to generate more NADH
• Pathway is exergonic

Question 3

Glycolysis: 10 Steps to Pyruvate

Answer 3

• During glycolysis, glucose is broken down to two pyruvate molecules, generating ATP and NADH
• Involves 10 enzymatic reactions
• Fructose and Galactose converted to intermediates of the pathway
  - Fructose converted to DHAP and Glyceraldehyde-3-Phosphate
  - Galactose is converted to Glucose-6-Phosphate

Question 4

Regulation of Glycolysis

Answer 4

• Step 1: Hexokinase inhibited by build up of glucose-6-phosphate
• Step 3: Phosphofructokinase 1(PFK-1) inhibited by amount of ATP or citrate, and activated by amount of AMP, ADP, or Fructose 2,6-bisphosphate
• Step 10: Pyruvate kinase is inhibited by phosphorylation and other allosteric regulators
• All three steps involve large ΔG and are irreversible

Question 5

Step 1: Hexokinase

Answer 5

• Glucose is phosphorylated using ATP
• Hexokinase adds phosphate to glucose
• Traps glucose in cell(GLUT can’t bind to Glucose-6-P), Only liver cells have enzyme that reverses this step)
• Glucose-6-P used in cell to generate energy
  
  • Can be stored as glycogen in liver/muscle cells

Question 6

Hexokinase Binding

Answer 6

• ATP and Mg bind to large lobe away from glucose binding in active site
• Substrates bind in active site via charged and polar residues
• Results in conformational change(enzyme closes around substrate)
• C6 hydroxyl attacks gamma phosphate producing Glucose-6-Phosphate

Question 7

Step 3: Phosphofructokinase(PFK-1)

Answer 7

• Uses ATP(2nd ATP requiring step of glycolysis)
• PFK-1 transfers phosphate group from ATP to C1 hydroxyl of F-6P
• PFK-1 inhibited allosterically by ATP and citrate
  - activated by AMP/ADP and Fructose 2,6 bisphosphate

Question 8

Allosteric regulation of PFK-1

Answer 8

• PFK-1 has 2 ATP binding sites(active site and allosteric site)
• ATP binding to allosteric site inhibits PFK-1
• AMP or ADP binding to allosteric site stimulates PFK-1

Question 9

Step 10: Pyruvate Kinase

Answer 9

• Pyruvate kinase inhibited by phosphorylation and other allosteric regulators
• Active in high blood glucose(insulin)
• Less active in low blood glucose(glucagon, cortisol, epinephrine)

Question 10

Regulation of Glycolysis

Answer 10

• Glycolysis: Convert monosaccharides to energy. Favourable, catabolic pathway
• What if the cell has enough energy?
  - Glycolysis will slow down
• If cell doesn’t have enough energy: Glycolysis stimulated
• Feedback inhibition, allosteric regulation and phosphorylation can regulate spontaneous reactions in glycolysis.
• When glycolysis inhibited, excess glucose can be stored as glycogen in liver and muscle

Question 11

Glycogen Synthesis

Answer 11

• Excess glucose can be stored as glycogen by glycogen synthase(anabolic pathway)
• Glycolysis slows down when ATP levels rise
• Glucose phosphorylated by hexokinase, but isomerized by phosphoglucomutase to form Glucose-1-phosphate
• UDP-glucose pyrophosphorylase uses UTP to activate Glucose-1-phosphate to make UDP glucose
• Glycogen synthase requires UDP-glucose, the activated form of glucose to create new alpha(1,4) glycosidic bonds on non-reducing ends of glycogen
• Insulin signaling activates glycogen synthase

Question 12

Glycogenolysis

Answer 12

• Epinephrine and glucagon signaling leads to phosphorylation and activation of glycogen phosphorylase and inactivation of glycogen synthase via dephosphorylation
• Glycogenolysis is a phosphorylis reaction, using P to release glucose-1-phosphate from non-reducing ends
• Glucose-1-phosphate cannot be used directly in glycolysis and must be converted to glucose-6-phosphate by phosphoglucomutase
• Glycogen phosphorylase is also allosterically inhibited by ATP and glucose-6-phosphate and stimulated by AMP