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Flashcards in Chapter 14 Deck (46)
1

What is glycogen?

branched polymer of glucose

storage form of glucose in cytoplasm

synthesis and degradation largely in liver and skeltal muscle

--cardiac muscle and kidney store in smaller quantities 

2

How can glycogen be stored?

single granules: skeletal muscle

--stored as energy for muscle contraction

fast twitch: glucose converted primarily to lactate

slow twitch: glucose is completely oxidized

clusters of granules: liver

--stored to be mobilized during hypoglycemia

3

What is the structure of the granule?

central protein core (glycogenin) with polyglucose chains radiaing outward

--only linear chains: highest density of glucose near core

--branched: highest density of glucose at periphery; allow more rapid release of glucose

4

How is glycogen synthesized?

core protein glycogenin

--begins with glucose 6 phosphate

converted to glucose 1 phosphate and activated UDP-glucose for addition to glycogen chain by glycogen synthase

--glycogen synthase is rate limiting enzyme 

5

What is the function of glycogen synthase? What activates and inhibits it?

forms alpha 1,4 glycosidic bonds in linear glucose chains of granule

--activated by: insulin in the liver and skeletal muscle

--inhibited by glucagon and epinephrine in the liver and epinephrine in skeletal muscle

6

What is the function of branching enzyme (alpha 1,4:alpha 1,6 transferase)?

introduces alpha 1,6 linked branches

--hydrolyzes one of the alpha 1,4 bonds to release a block of oligoglucose, which is moved and added in different location

--forms an alpha 1,6 bond to create a branch

7

What is the rate limiting enzyme of glycogenolysis?

glucagon phosphorylase

--breaks alpha 1,4 glycosidic bonds using Pi

--glucose 1 phosphate formed is converted to glucose 6 phosphate by same mutase used in glucose synthase

--cannot break alpha 1,6 bonds; stops when it nears outermost branch points 

8

What activates and inhibits glycogen phosphorylase?

Liver:

activated: epinephrine, glucagon

inhibited by: insulin

Muscle: 

activated by: epinephrine, AMP, Ca2+ (through calmodulin)

inhibited by: inulin and ATP

9

What is the function of debranching enzyme (glucosyl alpha 1,4: alpha 1,4 transferase and alpha 1,6 glucosidase)?

deconstructs branches in glycogen

--breaks an alpha 1,4 bond adjacent to branch point and moves it to small oligoglucose chain released to the exposed end of the other chain

--forms new alpha 1,4 bond

--hydrolyzes alpha 1,6 bond, releasing the single reisdue at the branch point as free glucose (only free glucose produced directly in glycogenolysis)

10

What is von Gierke disease?

glucose 6 phosphatase deficiency

fasting hypoglycemia, lactic acidosis, hepatomegaly

--glycogen deposits in the liver (G6P stimulates glycogen sythesis and glycogenolysis in inhibited)

--hyperuricemia predisposing to gout; decreased Pi causes increased AMP, which is degraded to uric acid--lactate slows uric acid excretion in the kidney

--hyperlipidemia with skin xanthomas; elevation of triglycerides (VLDL)

--fatty liver

--ingestion of galactose or fructose and adminstration of glucagon and epinephrine causes no increase in blood glucose

11

What is myophosphorylase deficiency?

McArdle deficiency

muscle glycogen phosphorylase deficiency

unable to properly break down glycogen to G6P in muscles

symptoms: 

--exercise intolerance during initial phase of high intensity exercise

--muscle cramping

--possible myoglobinuria

--recovery or second wind after 10-15 minutes of exercise

ameliorated by drinking sucrose rich soft drinks--provides dietary glucose for muscles to use 

12

What is hepatic glycogen phosphorylase deficiency?

Hers disease

relatively mild becuase gluconeogensis compensates for lack of glycogenolysis

symptoms: hypoglycemia, hyperlipidemia, hyperketosis are mild

hepatomegaly, growth retardation in early childhood

13

What is lysosomal alpha 1,4 glucosidase deficiency?

Pompe disease

deficient enzyme normally reisdes in lysosome

responsible for digseting glycogen-like material accumulating in endosomes (inclusion bodies)

tissues most severely affected arethose that normally have glycogen stores 

infantile onset: massive cardiomegaly--cause of death by 2 years of age

symptoms: slowly progressing muscle weakness involving arms and legs, difficulty breathing, enlarged liver

14

What is Andersen disease?

15

What is Cori disease?

16

What is gluconeogenesis?

promoted by glucagon and epinephrine and inhibited by insulin

--gluconeogenesis when glycogen reserves drop after 12 hours

after 24 hours, it is sole source of glucose

17

What are important substrates for gluconeogenesis?

glycerol 3 phosphate (from triacylglycerol in adipose)

lactate (from anaerobic glycolysis)

gluconeogenic amino acids (protein from muscle)

18

What are the ketogenic and glucogenic amino acids?

19

What are the steps of gluconeogenesis?

20

What can and cannot be converted to glucose in the liver?

can be converted:

*dietary fructose and galactose in liver

*odd number cabon fatty acids yield small amount of propionyl CoA that is gluconeogenic

cannot be converted:

*acetyl CoA to glucose

*FA converted to acetl CoA so not major source of glucose

21

What enzyme coonverts lactate to pyruvate in gluconeogenesis?

lactate dehydrogenase oxidizes lactate --> pyruvate

22

What is the function of alanine in gluconeogenesis?

important gluconeogenic amino acid

converted to pyruvate via alanine aminotransferase (ALT or GPT)

23

What converts glycerol 3 phopsphate to dihydroxyacetone phosphate (DHAP)?

glycerol 3 phosphate dehydrogenase: oxidizes

24

What is the function of pyruvate carboxylase?

mitochondrial enzyme requiring biotin

--activated by acetyl CoA from beta oxidation

--product oxaloacetate (TCA intermediate) can't leave mitochondria

--reduced to malate that can leave via malate chuttle

--in cytoplasm: malate reoxidized to OAA

25

What is the function of phosphoenolpyrvuate carboxykinase (PEPCK)?

in cytoplasm

induced by glycagon and coritsol

converts OAA to phosphoenolpyrvuate (PEP)

requires GTP

PEP then converted F1,6bisphosphate 

26

What is the function of fructose 1,6 bisphosphate?

in cytoplasm

key control point in gluconeogenesis

hydrolyzes phosphate from F1,6 bisphosphate

phosphatases oppose kinases

activated by: ATP

inhibited by: AMP, fructose 2,6 bisphosphate

F26bisphosphate: produced by PFK2 controls gluconeogensis and glycolysis (in liver)

--PFK2 is activated by insulin and inhibited by glucagon

--glucagon will lower F26BP and stimulate gluconeogenesis

--insulin will increase F26BP and inhibit gluconeogenesis

27

What is the function of glucose 6 phosphatase?

lumen of ER

G6P is transported into ER

free glucose transported back into cytoplasm where it leaves cell

only in liver

absense in skeletal muscle: why muscle glycogen can't serve as source of blood glucose

28

What are the gluconeogenic amino acids

alanine=major

18 of 20 (all except leucine and lysine) are gluconeogenesis

most are converted to TCA intermediates then to malate

29

How is gluconeogenesis and beta oxidation related?

glucose produced by hepatic gluconeogenesis goesn't represent energy source for liver

gluconeogenesis requires expenditure of ATP that is provided by beta-oxidation of FA

--during hypoglycemia, adipose tissue releases fatty acids by breaking down triglyceride 

30

What occurs with biotin deficiency?

symptoms: alopecia, scaly dermatitis, waxy pallow, acidosis (mild)

causes: raw egg whites (avidin)

long term home TPN

31

How is acetyl CoA from FA utilized?

can't be converted to glucose but can be converted to ketone bodies as alternative fuel for cells, including brain

--chronic hypoglycemia: accompanied by increase in ketone bodies

32

How does acetyl CoA regulate mitochondrial enzymes that use pyruvate?

pyruvate carboxylase & pyruvate dehydrogenase

--between meals, when FA oxidized in liver for energy, accumulating acetyl CoA activates pyruvate carboxylase and gluconeogensis and inhibits PDH,thus preventing conversion of lactate and alanine to acetyl CoA

--in well fed, absorptive state (insulin), accumulating acetyl CoA is shuttle into cytoplasm for FA synthesis; OAA in necessary for transport and acetyl CoA can stimulate its formation from pyruvate 

33

What is the Cori cycle?

during fasting, lactate from RBC (and exercising skeletal muscle( is converted to glucose in the liver that can be returned to the RBC or muscle

34

What is the alanine cycle?

msucles releases alanine delivering a gluconeogenic substrate (pyruvate) and an amino group for urea synthesis 

35

Why are alcoholics suscpetible to hypoglycemia?

alcohol is metabolized to acetate (acetyl Coa) 

high amounts fo cytoplasmic NADH formed by alcohol dehydrogenase and acetaldehyde dehydrogenase interfere with gluconeogensis

high NADH favors formation of:

--lactate from pyruvate

--malate from OAA in cytoplasm

--glycerol 3 phosphate from DHAP

diverts important gluconeogenic substrates from entering the pathway

accumulation of cytoplasmic NADH and glycerol 3P-->lipid accumulation in alcoholic liver disease

--free FA released form adipose in part enter the liver where beta oxidation is very slow (high NADH) 

in presence of high glycerol 3 phosphate, fatty acids are inappropriately sotred in liver as triglyceride

 

 

36

What occurs with extreme exercise and alcohol consumption?

excessive alcohol consumption --> hypoglycermia after running marathon

--exercising muscle, lactic acid builds up in muscle due to anaerobic glycolysis ,causing muscle cramping and pain

lactate spills into blood and converted to glucose in liver (Cori cycle)

to carry out gluconeogensis NAD is required by lactate dehydrogenase to oxidize lactate to pyruvate

but much of NAD is being used for ethanol metabolism and is unavailable for lactate oxidation

result=metabolic acidosis and hypoglycemia

37

What is the hexose monophosphate shunt? (pentose phosphate pathway)

cytoplasm of all cells

functions=NADPH production and source of ribose 5P for nucleotide synthesis

38

What is the rate limiting enzyme of the HMS?

Glucose 5 phosphate dehydrogenase (G6PDH)

--induced by insulin and NADP

--inhibited by NADPH

39

What are the importance of the frcutose 6P and glyceraldehyde 3P intermediates in the HMS?

can feed back into glycolysis

can make pentoses without going through G6PDH reaction via transketolase (thiamine requiring enzyme)

--transketolase is only thiamine enzyme in RBC

40

What are the functions of NADPH?

biosynthesis

maintenance of a supply of reduced glutathione to protect against reactive oxygen species (ROS)

bactericidal activity in polymorphonuclear leukocytes (PMN)

41

What is favism?

ingestion of Fava beans may cause severe hemolysis in G6PDH individuals 

symptoms: pallor, hemoglobinuria, jaundice, severe anemia 24-48 hours after ingestion of beans

common in: Greece, Italy, Spain, Portugal, Turkey

42

What is chronic granulomatous disease (CGD)?

deficiency of NADPH oxidase in PMN

susceptible to infection by catalase positive organsms (Staph. aureus, Klebsiella, Escherichia coli, Candida, Aspergillus)

negative nitroblue tetrazolium test: positive diagnosis

43

What is a benefit of G6PDH deficiency?

many parasites (plasmodium) are deficient in antioxidant mechanisms--making them susceptible to oxygen radicals

--G6PDH deficiency: ability of erythrocytes to detoxify oxygen radicals is impaired

accumulation of radicals in erythrocytes in G6PDH deficiency gives protection against malaria

44

What is G6PDH deficiency?

hemolytic anemia, symptoms resemble CGD (insufficient activity of G6PDH in PMN to generate NADPH from the NADPH oxidase bactericidal system)

allelic heterogeneity

X linked recessive

acute episodic or rarely chornic hemolysis

female carrier: increased resistance to malaria

most likely cause of hemolytic episode is overwhelming infection, often pneumonia (viral and bacterial) or infectious hepatitis

mutation may decrease the activity od G6PDH sufficiently to cause chronic nonspherocytic hemolytic anemia

 

45

What is the function of ROS in RBC?

RBC contain a large amount of oxygen

prone to spntaneously generate ROS that dmage protein and lipid in cell

in presence of ROS, hemoglobin may precipitate (heinz bodies) and membrane lipids may undergo peroxidation, weakening the membrane and causing hemolysis

as peroxides form, they are rapidly destroyed by the glutathione peroxidase/glutatione reductase system in the red bloo cell

--NADPH required by glutathione reductase is supplied by teh HMP shutn in the erythrocyte 

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