Glycogen Metabolism

Question 1

Glycogen Structure

Answer 1

• glycogen is a branched chain homopolysaccharide made up of a-D-glucose
• primary glycosidic bond is an a(1,4) linkage
• after 8-10 glucoses, there is a branch formed by an a(1,6) linkage
• single glycogen molecule may be very large

Question 2

Glycogenin protein core

Answer 2

1st building block of glycogen

Question 3

Glycogen Metabolism

Answer 3

• constant blood glucose supply is essential for life
• glucose is the preferred source of energy for brain or in cells w/ lacking mitochondria or in excercising muscle where it is the substrate for anaerobic glycolysis

Question 3

branching of glycogen

Answer 3

greater amount of glucose that can be stored in a small space

Question 4

What sources are blood glucose obtained from?

Answer 4

• Diet- may be sporadic, not always reliable
• Gluconeogenesis- slow in responding to falling glucose levels
• Glycogen breakdown- glucose stored in a rapidly mobilizable form

Question 5

What do depletion of glycogen stores result in?

Answer 5

results in glucose synthesis by gluconeogenesis using a.a from body’s proteins as substrates

Question 6

What is the function of liver glycogen?

Answer 6

functions to maintain blood glucose levels, primarily during the initial stages of fasting

Question 6

Where is glycogen stored?

Answer 6

stored in liver and skeletal muscle

Question 7

What is the function of muscle glycogen?

Answer 7

functions as a fuel reserve for ATP synthesis during muscle contraction

Question 8

Glycogen degradation requires 2 enzymes

Answer 8

1. Glycogen phosphorylase
2. Debranching enzyme

Question 9

Glycogen phosphorylase

Answer 9

• cleaves a(1-4) linkages
• produces glucose 1-phosphate

Question 10

Debranching enzyme part A

Answer 10

A) First glycosyl (4,4) transferase removes the outer 3 of 4 glucose molecules from the branch to the straight chain forming a new a(1-4) bond

Question 11

Debranching enzyme part B

Answer 11

Amylo (1,6) glucosidase removes the remaining glucose

Question 12

Products of glycogen degradation

Answer 12

Glucose 1-P and glucose

Question 12

Glycogen synthesis

Answer 12

substrate of glycogen synthesis is UDP-glucose (activated form of glucose)

Question 13

Glycogen synthesis requires 2 enzymes

Answer 13

1. Glycogen Synthase
2. Branching enzyme forms the branches

Question 14

What is Glycogen synthase

Answer 14

catalyzes transfer of glucose from UDP-glucose to growing chain

Question 14

What is Branching enzyme

Answer 14

introduces the a(1,6) linkages

Question 15

Reciprocal regulation of glycogen synthesis and degradation

Answer 15

• synthesis stimulated by high levels of glucose and ATP (well-fed state)
• breakdown stimulated by low glucose and energy levels (fasted state)

Question 15

Regulation of glycogen synthase and phosphorylase at 2 levels

Answer 15

1. Allosteric
2. Hormonal

Question 16

1. Allosteric

Answer 16

Glycogen synthase is activated by elevated levels of glucose 6-P ATP while elevated G6P, ATP and glucose inhibit glycogen phosphorylase

Question 16

2. Hormonal

Answer 16

• Elevated insulin levels increase glycogen synthesis
• Elevated glucagon increase degradation