Biosynthesis

Question 1

what is glycogen

Answer 1

polysaccharide storage form of glucose
- first line of defense against declining blood glucose concentration

Question 2

glycogenesis

Answer 2

glucose converted into glycogen
- activated in the liver and muscle after a meal

Question 3

glycogenolysis

Answer 3

releasing glucose to maintain blood glucose concentration
- degraded b/w meals
- not sufficent for 12h fast

hepatic glycogenesis and glycogenolysis important for maintaing blood glucose

Question 4

gluconeogenesis

Answer 4

de novo synthesis of glucose (in hepatic pathway)
- uses amino acids from muscle protein as precursor of glucose
- increases more slowly, becomes primary source of glucose concentration about 8h into postabsorptive state

Question 5

what are they two types of glycosidic linkages contained in glycogen

Answer 5

alpha 1 > 4 linked glucose residues with alpha 1 > 6 branches spaced about every 4-6 residues along the alpha 1 > 4 chain

close up of the structure of glycogen

Question 6

glucokinase

Answer 6

an enzyme specific for glucose and converts to glucose 6 phosphate

Question 7

pathway of glycogenesis from glucose involves 4 steps

Answer 7

1. glc6P into glc1P by phosphoglucomutase
2. activate glc1P to UDP by enzyme UDP glc pyrophosphorylase
3. transfer UDPglc to alpah 1>4 by glycogen synthase
4. alpha 1 >4 chain exceeds 8 residues glycogen branching enzyme (transglycosylase) transfer into alpha 1 >6

Question 8

what is the regulatory enzyme for glycogenesis

Answer 8

glycogen synthase rather than UDP glucose pyrophosphorylase

Question 9

glycogenolysis

glycogen phosphorylase

Answer 9

removes abundant external alpha 1>4 linked glucose residues in glycogen
- releases glucose from glycogen in form of glc1P

Question 10

glycogenolysis

phosphoglucomutase

Answer 10

glc1P is isomerized by phosphoglucomutase to glc6P
- bypasses requirement of ATP

Question 11

glycogenolysis

phosphorylase

Answer 11

rate limiting step in glycogenolysis is catalyzed by phosphorylase
- glc6P released glucose from liver by glucose 6 phosphatase exits via GLUT2

Question 12

what three hormones counterregulate glycogenolysis and glycogenesis

Answer 12

• insulin
• glucagon
• cortisol

Question 13

what are the three hormonal activators of glycogenolysis

Answer 13

• glucagon
• epinephrine
• cortisol

Question 14

glucagon

Answer 14

• secreted by alpha cells
• activate hepatic glycogenolysis for maintenance of normal glood glucose
• increases b/w meals
• dec during a meal
• chronically inc during fasting

Question 15

hepatic glycogenolysis activated in response to acute and chronic stress. what are they?

Answer 15

• physiologic (inc blood glucose during exercise)
• pathologic (blood loss)
• psychologic (acute/chronic threats)

Question 16

acute stress activates glycogenolysis through what hormone

Answer 16

epinephrine
- released by adrenal medulla
- during prolonged or stressful exercise

augments effects of glucagon during hypoglycemia = rapid heartbeat, sweating, anxiety

Question 17

how is cortisol released to induce glycogenolysis

Answer 17

increased blood concentrations of adrenocortical steroid homrone cortisol
- can be chronically elevated under continuously stressful conditions

Question 18

mechanism of action of glucagon

Answer 18

G proteins are found on the inside of the plasma membrane
- they bind GTP & GDP

Question 19

mechanism of action of glucagon

Answer 19

Gprotein=conformational change, dissocation of a-subunit that binds to adenylate cyclase = converts cytoplasmic ATP to cAMP (second messenger)

Question 20

mechanism of action of glucagon

Answer 20

cAMP binds to PKA = dissociation of inhibitory subunits relieving inhibition of PKA

Question 21

mechanism of action of glucagon

Answer 21

hormonal activation of glycogen phosphoylase involves phosphorylation of many molecules by PKA and activates many molecules of glycogen phosphorylase

Question 22

what opposes the action of glucagon and stimulates gluconeogenesis

Answer 22

insulin
- secreted in response to rise in blood glucose after meal
- turns off glycogen phosphoylase and activating glycogen synthase promoting glucose storage

Question 24

why cant muscle glycogen replenish blood glucose

Answer 24

it lacks both glucagon receptor and glc6Pase

Question 25

what activates muscle glycogenolysis

Answer 25

activates in response to epinephrine through cAMP dependent beta adrenergic receptor

Question 27

what primary sources are carbon skeleton provided from

gluconeogenesis requires both sources of energy for biosynthesis and source of carbon for formation of backbone of glucose molecule.

Answer 27

• lactate produced in tissues such as the red cell and muscle
• AA derived from muscle protein
• glycerol released from triglycerides during lipolysis in adipose tissue

Question 28

Summary

Answer 28