Carbohydrate metabolism

Question 1

Fermentation

Answer 1

Occurs under anaerobic conditions. Regenerates glycolysis substrates rather than proceeding to citric acid cycle.

Question 2

Fermentation

Key enzyme

Answer 2

Lactate dehydrogenase converts pyruvate to lactate and converts NADH to NAD+.

Question 3

Pentose phosphate pathway (PPP)

Answer 3

Conversion of glucose 6-phosphate to NADPH and ribose-5-phosphate (5-carbon sugar used for nucleic acid synthesis).

Question 4

Pentose phosphate pathway (PPP)

Location

Answer 4

Occurs in the cytoplasm of the cell.
In terms of organs, occurs mainly in the liver and other lipid-synthesizing tissues.

Question 5

Pentose phosphate pathway (PPP)

Regulation

Answer 5

Activated by insulin and inhibited by NADPH.

Question 6

Pentose phosphate pathway (PPP)

Phases

Answer 6

• Oxidative phase: irreversible reaction that starts with G6P and produces NADPH and ribulose-5-phosphate.
• Non-oxidative phase: reversible reactions that produce ribose-5-phosphate for nucleic acid synthesis.

Question 7

Pentose phosphate pathway (PPP)

Key enzymes

Answer 7

Rate-limiting enzyme is glucose-6-phosphate dehydrogenase (G6PD) which produces NADPH.

Question 8

Pentose phosphate pathway (PPP)

Link to glycolysis

Answer 8

Because the non-oxidative phase is reversible, products from this phase can be shared in both directions with glycolysis.
In other words, glycolysis can feed into the PPP, and PPP can feed into glycolysis.
These products include fructose 6-phosphate and glyceraldehyde 3-phosphate.

Question 9

Pentose phosphate pathway (PPP)

Glucose 6-phosphate dehydrogenase (G6PD)

Answer 9

Catalyzes the conversion of glucose 6-phosphate to 6-phosphogluconate with production of NADPH.
The rate-limiting step of the pentose phosphate pathway.

Question 10

Glycogenolysis

Location

Answer 10

Glycogen breakdown occurs in the liver or muscle tissues where glycogen is stored.

Question 11

Glycogenolysis

Regulation

Answer 11

Glycogenolysis is stimulated by catecholamines in peripheral tissues or by glucagon in the liver.

Question 12

Glycogenolysis

Key enzymes

Answer 12

Glycogen phosphorylase and debranching enzyme.

Question 13

Glycogenolysis

Glycogen phosphorylase

Answer 13

The rate limiting reaction. Breaks α-1,4 glycosidic bonds between glucose molecules by adding inorganic phosphate (Pi) in a phosphorolysis reaction.

Question 14

Glycogenolysis

Debranching enzyme

Answer 14

Breaks α-1,6 glycosidic branches in glycogen through hydrolysis.

Question 15

Glycogenolysis

Link to other pathways

Answer 15

Glycogenolysis results in glucose-6-phosphate which feeds into glycolysis and fermentation.

Question 16

Glycogenesis

Location

Answer 16

Glycogen production occurs in liver and muscle cells. Glycogen is then stored in the liver and skeletal muscle cells.

Question 17

Glycogenesis

Regulation

Answer 17

Stimulated by insulin.

Question 18

Glycogenesis

Key enzymes

Answer 18

Remember glycogen synthase and branching enzyme.

Question 19

Glycogenesis

Glycogen synthase

Answer 19

Synthesizes α-1,4 glycosidic bonds between glucose molecules.

Question 20

Glycogenesis

Branching enzyme

Answer 20

Creates α-1,6 glycosidic branches in glycogen.