Module 7: Carbohydrate Metabolism

Question 1

Carbohydrates storage

Answer 1

Carbohydrates not required immediately for energy are stored as glycogen in our muscle and liver
Once glycogen stores are filled, excess carbohydrates are converted to fat for storage

Question 2

Glycogen

Answer 2

highly branched structure with a single reducing end and several non- reducing ends
alpha-1,4 bonds link the glucose subunits linearly, while the alpha- 1,6 bonds form the branch points

Question 3

Glycogen phosphorylase

Answer 3

catalyzes the phosphorolysis of glycogen with the addition of Pi to form glucose-1-phosphate, which can be converted by phosphoglucomutase to glucose-6-phosphate

Question 4

Glycogen synthase

Answer 4

catalyzes the synthesis of glycogen from glucose-1-phosphate, using UTP

Question 5

What causes Glycogen synthase to be activated?

Answer 5

activated by high concentrations of glucose-6-phosphate (signals plenty of carbohydrate available)

Question 6

What causes Glycogen phosphorylase to be activated/inhibited?

Answer 6

activated by high concentrations of AMP
(signals energy status of the cell is quite low)
inhibited by high concentrations of ATP
(signals ample energy supply)

Question 7

Glycogen synthase (phosphorylation/dephosphorylation)

Answer 7

Phosphorylation by kinase =inactive form

Removal of the phosphate group by phosphatase=active form

Question 8

Glycogen phosphorylase

phosphorylation/dephosphorylation

Answer 8

Phosphorylation by kinase =active form

Removal of the phosphate group by phosphatase=inactive form

Question 9

Which of these hormones: insulin, glucagon, epinephrine, lead to phosphorylation/dephosphorylation?

Answer 9

Insulin leads to dephosphorylation
Glucagon leads to phosphorylation
Epinephrine leads to phosphorylation

Question 10

When is insulin release and what enzyme is activated/inactivated?

Answer 10

released when blood glucose levels elevated
activate glycogen synthase (abundance of glucose that can be used to replenish the glycogen stores)
inactivate glycogen phosphorylase (glucose can be used for energy)

Question 11

When is glucagon release and what enzyme is activated/inactivated?

Answer 11

released when blood glucose levels drop
inactivates glycogen synthase
activates glycogen phosphorylase

Question 12

Glycolysis

Answer 12

occurs in cytosol
one 6-carbon glucose molecule is split into two three-carbon pyruvate molecules, in addition to 2 ATP molecules and 2 NADH molecules

Pyruvate further oxidized in the mitochondria
NADH further oxidized by ETC inside the mitochondria

Question 13

What is special about rxns 1 & 3 of glycolysis?

Answer 13

Enzymes that catalyze reactions 1 and 3 are targets for the control of flux through glycolysis –have a large change in free energy and therefore irreversible

Question 14

Step 1 of glycolysis

regulations

Answer 14

catalyzed by hexokinase

inhibited by its product, glucose-6- phosphate (signal enough product to proceed)

Question 15

Step 3 of glycolysis

regulations

Answer 15

catalyzed by phosphofructokinase
inhibited by high concentrations of ATP (signal adequate energy levels) and citrate (signals enough substrate to proceed)

stimulated by high concentrations of AMP and ADP (signal low energy)
and fructose-2,6-bisphosphate

Question 16

Fermentation

Answer 16

pathway that produces ATP with no net oxidation of carbon (no O2, no ETC to regenerate NAD+)

Question 17

Strategy 1: Formation of Lactate

Answer 17

employed by microorganisms and some eukaryotic cells
enzyme lactate dehydrogenase oxidizes NADH, reducing pyruvate to lactate, and regenerating NAD+

Glucose+2ADP+2Pi–>2Lactate +2ATP

Question 18

Strategy 2: Formation of Ethanol

Answer 18

occur in yeast
ethanol is produced rather than lactate

Glucose+2ADP+2Pi–>2Ethanol+2CO2 +2ATP

Question 19

Gluconeogenesis

Answer 19

synthesis of glucose that occurs when the body’s stores of glycogen are low
occurs in liver (and kidneys)

Precursors such as some amino acids, some citric acid cycle intermediates and lactate via pyruvate can be converted to oxaloacetate–>converted to glucose

2Pyruvate+4ATP+2GTP+2NADH–>
Glucose+4ADP+2GDP+6Pi+2NAD+

Question 20

Given the large changes in free energy that we saw in glycolysis, how can the reverse pathway of gluconeogenesis proceed?

Answer 20

Gluconeogenesis is not exactly the reverse of glycolysis
Steps 1, 3 and 10, catalyzed by glucokinase, phosphofructokinase and pyruvate kinase must be by-passed as these reactions are irreversible
By-passed by glucose-6- phosphatase, fructose bisphosphatase, PEP carboxykinase and pyruvate carboxylase

Question 21

What inhibits fructose bisphosphatase?

Answer 21

inhibited by high concentrations of AMP and fructose-2,6-bisphosphate

Question 22

When is gluconeogenesis/glycolysis stimulated?

Answer 22

when the energy state of the cell is high (high ATP, low AMP), gluconeogenesis is stimulated
when the energy state of the cell is low (lowATP, high ADP and AMP), glycolysis is stimulated

Question 23

Pentose Phophate Pathway

Answer 23

alternate pathway by which glucose is broken down to generate NADPH
also provides ribose-5-phopshate for the biosynthesis of nucleotides

3Glucose-6-phosphate+6NADP+–>
2Fructose-6-phosphate (glycolysis)+
glyceraldehyde-3-phosphate (glycolysis)+
3CO2 (waste)+6NADPH (reducing power)