Ch. 14: Carbohydrate Metabolism

Question 1

NADPH

Answer 1

A strong reductant, available in biosynthetic and detoxification pathways, used to reduce reactive oxygen species

Question 2

What is the main product of the pentose phosphate pathway (PPP)?

Answer 2

NADPH

Question 3

What are the main functions of the pentose phosphate pathway?

Answer 3

1. Provide NADPH
2. Maintain homeostasis by providing carbons for nucleotide and amino acid biosynthesis

Question 4

What happens with the PPP if ATP is needed?

Answer 4

Glucose-6-P is reserved for glycolysis

Question 5

What molecule does the PPP start with?

Answer 5

Glucose-6-P

Question 6

What the intermediates of the oxidative phase of PPP?

Answer 6

1. 6-Phosphogluconolacetone
2. 6-phosphogluconate
3. Ribulose-5-P

Question 7

What is the rate limiting step of the oxidative phase of the PPP?

Answer 7

Glucose-6-P to 6-phosphogluconolacetone

Question 8

What are the 3 enzymes involved in the oxidative phase of the PPP?

Answer 8

1. Glucose-6-phosphate dehydrogenase
2. Lactonase
3. 6-phosphogluconate dehydrogenase

Question 9

How is the buildup of harmful oxygen radicals in cells prevented?

Answer 9

By the detoxification reactions that require reduced glutathione and the enzyme glutathione peroxidase

Question 10

Why is NADPH required as a coenzyme?

Answer 10

It is required as a coenzyme in the glutathione reductase reaction to convert one molecule of oxidized glutathione (GSSG) to two molecules of reduced glutathione (GHS), which is a highly reactive electron donor in coupled redox reactions.

Question 11

Carbon shuffle

Answer 11

When transketolase or transaldolase is put into another molecule, both are molecules that partake in the PPP

Question 12

What do transketolase and transaldolase do in the PPP?

Answer 12

Catalyze the reactions of the non-oxidative phase of PPP, both link glycolysis to the PPP

Question 13

Allosteric Regulation of NADPH

Answer 13

In high levels of NADP+, it will act as an activator for the PPP and the formation of NADPH; in high levels of NADPH, it will act as an inhibitor and stop PPP from occurring, and favor glycolysis and ATP formation instead

Question 14

What are the four main carbon sources for glucose synthesis?

Answer 14

1. Lactate
2. Amino acids
3. Glycerol
4. CO2 Fixation (in plants)

Question 15

Glyconeogenesis

Answer 15

Production of glucose from noncarbohydrate compounds; occurs mostly in the cytoplasm

Question 16

When does glyconeogensis occur?

Answer 16

It is most active in animals when glycogen stores in the liver and muscles have been depleted

Question 17

Gluconeogenic Enzymes

Answer 17

1. Pyruvate carboxylase; uses the cofactor biotin
2. Phosphoenolpyruvate carbokinase

Question 18

Biotinylated lysine group

Answer 18

This group in pyruvate carboxylase functions as a swinging arm in the reaction to carry a carbonyl group from the biotin carboxylation domain to the caryboxyltransferase domain

Question 19

Anaerobic Metabolism

Answer 19

During vigorous exercise, lactate produced in muscle cells is transported to the liver, where it is converted to pyruvate by the enzyme lactate dehydrogenase

Question 20

The Cori Cycle

Answer 20

Link between lactate and glucose metabolism between the muscle and the liver; Under extreme conditions where glucose levels are depleted

Question 21

Why can the Cori cycle be problematic?

Answer 21

It uses 4 ATP

Question 22

What is glycogen degradation and synthesis regulated by?

Answer 22

1. Glycogen phosphorylase
2. Glycogen synthase
3. Glycogen debranching and branching enzymes

Question 23

Glycogen degradation

Answer 23

Releasing units of glucose into the bloodstream when glucose levels are low

Question 24

Activity of the glycogen phosphorylase dimer

Answer 24

Stimulated by phosphorylation, which shifts equilibrium from the inactive T-state conformation to the active R-state conformation

Question 25

Glycogen debranching enzyme

Answer 25

Transfers 3 glucose units to the nearest nonreducing end; cleaves alpha (1-6) glycosidic bond to release free glucose; no debranching unless the chain is long enough

Question 26

Glycogen synthesis

Answer 26

UMP combines with GIP to form UDP glucose which is needed for the growing glycogen chain

Question 27

What is required for glycogen synthesis?

Answer 27

The synthesis of uridine diphosphate glucose (UDP glucose)

Question 28

Glycogen synthase

Answer 28

ATP is hydrolyzed to reform UTP which is needed to make UDP glucose that is in turn required for adding glucose molecules onto non-reducing ends of sugars

Question 29

Glycogen branching enzyme

Answer 29

Transfers 7 glucose residues from one end of the glycogen chain to a nearby chain

Question 30

Glycogenin

Answer 30

Anchor protein for the glycogen core particle; catalyzes the glycosyltransferase and synthesis reactions needed to generate glycogen; extends chain to 7 glucose residues

Question 31

Hormonal Regulation of Glycogen Metabolism

Answer 31

Phosphylation can inactivate glycogen synthase, thereby degrading glycogen and transporting glucose out of the cell

Question 32

When does hormonal regulation of glycogen metabolism occur?

Answer 32

Under low blood glucose conditions (e.g. fasting)

Question 33

What is used when blood glucose levels are high?

Answer 33

Insulin is used to synthesize glycogen

Question 34

What happens when blood glucose levels are low?

Answer 34

Glycogen is released from the liver and brought to the bloodstream, where it is broken down into glucose and used to bring the glucose levels back to normal.

Question 35

How many NADPH molecules are generated from the oxidative step of the PPP?

Answer 35

2 NADPH molecules

Question 36

If 6 molecules of glucose-6-P were used for the PPP, how many NADPH molecules would be produced?

Answer 36

12 NADPH molecules