Lecture 18

Question 1

T or F, glycerol is released during triacylglycerol hydrolysis in adipose tissue

Answer 1

True, and can enter glycolysis as DHAP

Question 2

T or F, galactose and fructose are good energy sources yet lack dedicated pathways for processing

Answer 2

True

Question 3

Which enzymes are involved in the metabolism of fructose into the glycolytic pathway?

Answer 3

• • fructokinase
• • fructokinase phosphate aldolase
• • triose kinase
• • triose phosphate isomerase

Question 4

Briefly describe the fructose1-phosphate pathway.

Answer 4

1. phosphorylation of fructose to yield F 1-P
2. splitting of F 1-P into glyceraldehyde and DHAP via aldol cleavage
3. phosphorylation of glyceraldehyde to GAP

GAP –> glycolysis

Question 5

Which enzyme is involved in the formation of galactose to glycolytic intermediates?

Answer 5

galactokinase

– there’s a release of ADP and H+

Question 6

True or false

Increasing the pH of the environment will increase galactose metabolism

Answer 6

true. H+ is product of one of the steps so removing it would shift it towards products

Question 7

Which of the following does not occur as a result of low lactase levels?
A) build up of lactic acid
B) buildup of methane and hydrogen gas
C) formation of ethanol
D) osmotic change that results in drawing water into the intestine

Answer 7

C

Question 8

Why are some people “lactose intolerant”

Answer 8

– deficiency in lactase enzyme prevents conversion of lactose to galactose and glucose

– lactose build-up allows microorganisms in the colon to ferment it to lactic acid, with methane and hydrogen gas as byproducts. Result is flatulence

– in addition, lactate produced by microorganisms draws water into intestine, causing diarrhea

– if that weren’t enough, severe gas and diarrhea hinder uptake of fats, proteins, and other nutrients

Question 9

T or F, in animal metabolism, glucose is derived mainly from dietary polysaccharides, starch from plants and glycogen from meat as well as the body’s own glycogen stores

Answer 9

True

Question 10

true or false

alpha 14 linkages give glycogen a helical structure whereas b 1 4 linkages of cellulose give it a stacked configuration

Answer 10

true

Question 11

True or false

2/3 of glucose exists in its beta pyranose formation

Answer 11

true; 66% where as 33% of it exists in alpha pyranose and less than 1% exists in open-chain form

Question 12

When does branching occur in glycogen?

Answer 12

– it occurs every 10 - 12 residues

Question 13

True or false

non reducing ends are a result of branching in glycogen that allow for quick degradation when needed

Answer 13

True

Question 14

T or F, when energy demands increase suddenly, glycogen is a readily mobilized source of glucose

Answer 14

true

Question 15

T or F, controlled utilization of glycogen is what keeps blood-sugar levels constant between meals

Answer 15

True; glycogen supplies glucose to brain, which uses glucose as primary fuel

Question 16

T or F, branching producing a compact structure for cellular storage and also creates the presence of many non-reducing ends allows for quick degradation when energy is needed

Answer 16

True

Question 17

What are the 4 enzymatic activities allow glycogen to be processed into useable glucose 6-phosphate (feeds into glycolytic pathway):

Answer 17

– 1 activity cleaves glycogen to form glucose 1-phosphate

– 2 activities remodel glycogen so it can be cleaved again

– 1 activity coverts glucose 1-P to glucose 6-P

Question 18

Which of the following is NOT a typical step in glycogen breakdown?
A) phosphorylase of glycogen to produce a glucose 1 phosphate
B) hydrolysis of glycogen to form glucose via glycogen phosphorylase
C) cleavage of alpha 1 6 bonds by alpha 1 6 glucosidase
D) transfer of 3 glucose subunits to non-reducing ends via transferase

Answer 18

B

Question 19

True or false

hexokinase is responsible for phosphorylating the final glucose liberated from a branch point

Answer 19

True

Question 20

True or false

glycogen phosphorylase is not able to function within 2 residues of a branch

Answer 20

False, 4 –> cleavage of alpha 1,4 linkages by glycogen phosphorylase does not occur within 4 residues of the branch site

Question 21

True or false

transferase is responsible for transferring one glucose unit at a time from the branch to a non reducing end

Answer 21

false, 3 units at a time

Question 22

T or F, phosphorolytic cleavage of glycogen is energetically advantageous because the released sugar is already phosphorylated

Answer 22

True; no need to burn ATP

Question 23

What enzyme is responsible for the removal of the final glucose unit from the branch

Answer 23

– alpha 1-6 glucosidase

– as final glucose unit is removed from branch, free glucose is then phosphorylated by hexokinase to be used in glucose

– now the glycogen chain can be completely broken into monomers by phosphorylase

Question 24

Which of the following is true of alpha 1 6 glucosidase?
A) it adds a phosphoryl group to the glucose residue
B) it is capable of hydrolyzing a 6 long glucose residue chain
C) It removes a glucose unit and phosphorylates it.
D) none of the above

Answer 24

D; it removes final glucose unit from branch and none of these describe that

Question 25

True or false

phosphoglucomutase contains a modified phenylalanine to mutate G1P to G6P

Answer 25

false. modified serine residude; transfers phosphoryl group C-1 to C-6

Question 26

True or false

glycogen breakdown occurs only in muscles and the liver

Answer 26

True

Question 27

true or false

G6Pase is not present in the liver

Answer 27

false, it is not present in muscles

Question 28

What is the general structure of glycogen phosphorylase?

Answer 28

a multisubunit enzyme that has catalytic sites and a site that contains a serine residue.

– there is an a and b form and each form can assume 2 activity states R (relaxed) and T (tense)

Question 29

Which states are glycogen phosphoylase A and B more likely to exist in at equilibrium?

Answer 29

– a is found mostly in the active, R state while b is the opposite, it exists mostly in the active T state

Question 30

T or F, a and b interconvert by virtue of serine phosphorylation ( a is the phosphorylated form, b is not)

Answer 30

True

Question 31

T or F, phosphorylase is mostly in b form in muscle

Answer 31

true; it is regulated allosterically by the cell’s energy charge

Question 32

How is phoshporylase b regulated allosterically?

Answer 32

– by cell’s energy change

– AMP binds nucleotide binding site in a manner that favors R state

– ATP binds nucleotide binding site in a manner that favors T state

G 6-P favors T state, example of feedback inhibition

Question 33

T or F; muscle phosphorylase a is not subject to the same type of allosteric regulation as muscle phosphorylase b

Answer 33

True

Question 34

Which of the following is true about GP A and B?
A) GP A contains a phosphoserine residue
B) GP B has a covered active site in the phosphorylated state
C) conversion of B to A involved the use of 1 ATP molecule
D)
Phosphoserine residues stabilize the tense state

Answer 34

A

Question 35

What is the difference in phosphorylase b in resting and active muscle?

Answer 35

– resting = phosphorylase b is in predominant form; AMP is low, ATP is high and thus the enzyme is inhibited (T state)

– active = AMP rises, ATP decreases, and phosphorylase b is activated to R state

Question 36

T or F, hormone signaling (adrenaline) can induce phosphorylation of b, converting some of it to even more active a form

Answer 36

True

Question 37

Which of following is TRUE concerning the energy level of the body when it comes to GP activity in the non-liver isoform
A) in a state where ATP levels is high, one would expect the stabilization of the R state
B) one would expect high levels of G6P to stabilize the T state
C) glucose would tend to stabilize the T state of the enzyme
D) extraneous exercise would tend to stabilize the T state of GP

Answer 37

B; because remember feedback inhibition

Question 38

true or false

High ratio of AMP:ATP would result in the stabilization of the R state of GP B

Answer 38

true

Question 39

true or false

hydrolysis of ATP to the allosteric site results in switch from the R to T state

Answer 39

– false. ATP hydrolysis results in changing of isoform state due to changing serine residue nature.

– Binding of ATP to allosteric sites not hydrolysis, switches between R/T

Question 40

T or F, in the liver, phosphorylase is primarily active in a form

Answer 40

Ture; 98% identical to muscle form

Question 41

T or F, liver phosphorylase is not subject to allosteric control by the cell’s energy charge

Answer 41

True; liver phosphorylase is regulated by glucose

Question 42

Describe the regulation of phosphorylase by glucose in the liver.

Answer 42

– binding of glucose to liver phosphorylase causes a R to T transition and thereby inhibits the enzyme

– Why? Don’t need to breakdown glycogen if glucose levels are high

Question 43

T or F, the 2% difference between muscle and liver phosphorylase matters

Answer 43

True; liver and muscle phosphorylase are 98% identical at the aa level yet are subject to very different modes of allosteric regulation