FORM & FUNCTION (Glycogen)

Question 1

glucose must be maintained at

Answer 1

-5mM
*hormonal control: insulin and glucagon

Question 2

excess glucose

Answer 2

-stored as glycogen
1. Liver (up to 10% weight)
2. Muscle (up to 1-2% weight)
*muscle pool is 4-6x of liver

Question 3

glycogen

Answer 3

-60% water (heavy)
-limit to how much the body can store
-polyssaccharide
-can contain 1,000 to 12,000 glucose molecules

Question 4

glycogen synthase

Answer 4

-creates alpha-1,4 linkages

Question 5

branching enzyme (transglycosylase)

Answer 5

-creates alpha-1,6 linkages
>removes linear linkage=increases solubility

Question 6

mutations in glycogen synthase or branching enzymes

Answer 6

-lead to glycogen storage disease

Question 7

glycogenesis

Answer 7

1. Glu to Glu-6-P (by hexokinase or glucokinase)
2. Glu-6-P: glycolysis or phophoglucomutase forms Glu-1-P
3. UTP converts it to UDP-glucose=activated glucose=high energy
4. Glycogen synthase form glycogen
   *glucose by itself is low in energy requires activation to form glycogen (costs energy (UTP) to make glycogen)

Question 8

glycogen branching

Answer 8

-when 8-14 alpha-1,4 linked glucose are created = re-organized into 1,6-linkage = increases solubility

Question 9

insulin

Answer 9

-regulates/controls glycogenesis
-activates glycogen synthase THROUGH protein phosphatase 1

Question 10

insulin (steps)

Answer 10

1. Usually glycogen synthase is inactive (phosphorylated form)
2. Insulin activates protein phosphatase 1 to dephosphorylate glycogen synthase
3. Now activated (dephosphorylated form)

Question 11

glycogen synthesis regulation

Answer 11

1. High blood glucose
2. Insulin release
3. Glycogen synthase activation
4. Excess glucose stored as glycogen (contributes to decrease in blood glucose)

Question 12

glycogen reserves (major role)

Answer 12

1. Maintain blood glucose (liver)
   -provide energy supply for RBC and brain (fat cannot cross BBB)
2. Metabolic energy (muscle)
   *rapidly movilized and broken down (faster than fat)
   *can be used aerobically or anaerobically to generate ATP (via glycolysis)

Question 13

glycogenolysis mechanism (liver)

Answer 13

1. Glucagon or catecholamines
2. Bind to receptor in liver PM, get secondary messenger cAMP
3. Protein kinase A is activated and activates (add P) phosphorylase kinase
4. Inhibits glycogen synthase (removes P)
5. Activates glycogen phophorylase (adds P)
6. Breaks down 1,4 linkage (get glucose 1-phosphate molecule)

Question 14

glucagon

Answer 14

-released from pancreas due to low glucose signals

Question 15

catecholamines

Answer 15

-released form adrenal gland as a response to stress or exercise
Ex. NE or E

Question 16

phosphorylase kinase

Answer 16

-activated by protein kinase A
-inhibits glycogen synthase
-activates glycogen phophorylase (adds P)

Question 17

glycogen phosphorylase

Answer 17

-breaks down 1-4 linkage (forms glucose 1-phosphate)

Question 18

alpha 1,6 linkage breakdown

Answer 18

-debranching enzyme: alpha-1,6 glucosidase
1. Transfer a block of 3 residues to the non-reducing end
2. Cleaves single remaining alpha-1,6 linkage to directly yield glucose (goes to blood or glycolysis for ATP)

Question 19

liver glycogen breakdown overview

Answer 19

-mostly alpha 1,4-linkage breakdown via glycogen phosphorylase (90%)
>glucose 6-P can be turned to glucose via Glucose 6-phophatase
-if go to glucose, can enter blood via GLUT2 (bi-directional transport)
-10% alpha-1,6 linkage breakdown via debranching enzyme (glucose to blood of glucose to glucose 6-P)

Question 20

role of liver glycogen

Answer 20

1. Glucose release into blood (predominant function)
2. Fuel for glycolysis

Question 21

maintaining blood glucose

Answer 21

-90% of glycogen goes to Glucose 6-P (phosphorylation traps it inside the cell)
-transported to ER where glucose-6-phosphatase is located to dephosphorylate glucose (‘untraps’ it)
*bulid up of concentration gradient controls glucose flux out of liver

Question 22

glycogenolysis mechanism (muscle)

Answer 22

1. Catecholamines released form adrenal gland in response to stress or exercise (no glucagon receptor)
2. cAMP to protein kinase A
3. Inhibits glycogen synthase
4. Activate glycogen phosphorylase
   *muslce contraction releases calcium, which directly activates phophorylase kinase without cAMP

Question 23

muscles lack

Answer 23

-glucose 6-phosphatase and glucose 6-P is always trapped inside glycolysis

Question 24

role of muscle glycogen

Answer 24

-muscle GLUT4 transporter does not transport glucose back to blood
*used directly for energy
-can NOT contribute to blood glucose levels
-hexokinase ‘converts’ glucose (from the blood or glycogen breakdown) to glucose 6-P