BIOC*3560 Part C: Carbohydrate Metabolism

Define metabolism

the sum of all chemical transformations that occur in an organism for it to stay alive

The "sum of all chemical transformations that occur in an organism for it to stay alive" is termed:
a) catabolism
b) energism
c) anabolism
d) metabolism
e) none of the above

d) metabolism

Define anabolism

synthesis of biomolecules; the synthesis of complex molecules in living organisms from simpler ones together with the storage of energy

Which of the following is the term for 'synthesis of biomolecules'?
a) anabolism
b) catabolism
c) metabolism
d) degradation
e) none of the above

a) anabolism

Define catabolism

degradation of biomolecules; the breakdown of complex molecules in living organisms to form simpler ones, together with the release of energy

Which of the following is the term for "degradation of biomolecules"?
a) anabolism
b) catabolism
c) metabolism
d) synthesis
e) none of the above

b) catabolism

List the three general types of pathways

1. converging catabolism
2. cyclic pathways
3. diverging anabolism

In any given metabolic pathway, what is the rate of most of the enzymes limited by?
a) substrate-limited
b) product-limited
c) enzyme-limited
d) inhibitor-limited
e) activator-limited

a) substrate-limited

T/F: "There can only be one rate limiting step per metabolic pathway."

False, there can be multiple rate-limiting steps

Which step in a metabolic pathway will determine the overall speed of the pathway?
a) always the first step
b) always the last step
c) always either the first or last step
d) whichever step is rate limiting
e) no single step can determine the speed of a whole pathway

d) whichever step is rate limiting

Rate limiting steps are often______________ under cellular conditions.
a) exergonic & reversible
b) exergonic & irreversible
c) endergonic & reversible
d) endergonic & irreversible
e) endothermic & reversible

b) exergonic & irreversible

Which steps in metabolic pathways are usually targets of metabolic regulation?

Those catalyzing exergonic, rate-limiting reactions

Why are pathways regulated only at rate-limiting steps, and not all steps?

This allows the cell to regulate the overall rate of the metabolic pathway without expending too many resources

List the 10 ways enzymes can be regulated

*1.* Extracellular signals
*2.* Transcription of specific gene(s)
*3.* mRNA degradation
*4.* mRNA translation on ribosome
*5.* Protein degradation (ubiquitin, proteasome)
*6.* Enzyme sequestered in organelle (e.g., ER)
*7.* Enzyme binds substrate
*8.* Enzyme binds ligand (allosteric effector)
*9.* Enzyme undergoes phosphorylation/ dephosphorylation
*10.* Enzyme combines w/ regulatory protein

Being strongly _____________ makes reactions essentially irreversible.

exergonic

a) How many steps in glycolysis are irreversible?
b) Which steps are they?

a) three
b) 1, 3 & 10

List the three irreversible steps of glycolysis and the enzymes that catalyze them

Step 1: hexokinase
Step 3: phosphofructokinase-1
Step 10: pyruvate kinase

Which reaction does hexokinase catalyze?

The reaction that allows entry of glucose into glycolysis; step 1 of glycolysis:

glucose + ATP ➡ glucose 6-phosphate + ADP

Which of the following is the hexokinase isozyme found in muscle?
a) hexokinase a
b) hexokinase b
c) hexokinase I
d) hexokinase IV

c) hexokinase I

Which of the following is the hexokinase isozyme found in the liver?
a) hexokinase a
b) hexokinase b
c) hexokinase I
d) hexokinase IV

d) hexokinase IV

Glucokinase is another name for:
a) hexokinase a
b) hexokinase b
c) hexokinase I
d) hexokinase IV
e) none of the above

d) hexokinase IV

T/F: "The the two hexokinase isozymes -- hexokinase I & hexokinase IV -- are transcribed from the same gene, but receive different post-translational modifications."

False, they are from two different genes

T/F: "Hexokinase I normally has maximal activity."

True
^Recall that hexokinase I is the isozyme found in muscles

T/F: "Hexokinase IV normally has maximal activity."

False
^Hexokinase *I* normally has maximal activity

T/F: "Hexokinase I is inhibited by high [glucose 6-phosphate]."

True
Hexokinase I is inhibited by high [Glucose 6-P]
Hexokinase IV is inhibited by high [Fructose 6-P]

T/F: "Hexokinase IV is inhibited by high [glucose 6-phosphate]."

False
Hexokinase I is inhibited by high [Glucose 6-P]
Hexokinase IV is inhibited by high [Fructose 6-P]

T/F: "Hexokinase IV is inhibited by high [fructose 6-phosphate]."

True
Hexokinase I is inhibited by high [Glucose 6-P]
Hexokinase IV is inhibited by high [Fructose 6-P]

T/F: "Hexokinase I is inhibited by high [fructose 6-phosphate]."

False
Hexokinase I is inhibited by high [Glucose 6-P]
Hexokinase IV is inhibited by high [Fructose 6-P]

T/F: "Hexokinase IV has a lower affinity for glucose than hexokinase I."

True

T/F: "Hexokinase I has a lower affinity for glucose than hexokinase IV."

False

Corrected statement:
Hexokinase IV has a lower affinity for glucose than hexokinase I

Inhibition of hexokinase IV by fructose 6-P is effected through _____________________

glucokinase regulatory protein

T/F: "Hexokinase I has a relatively large K0.5."

False

T/F: "Hexokinase IV has a relatively large K0.5."

True

T/F: "Hexokinase I has a relatively poor K0.5."

False

T/F: "Hexokinase IV has a relatively poor K0.5."

True

Which hexokinase isozyme increases its rate when blood glucose is higher than optimal (~5 mM)

hexokinase IV (glucokinase; liver isozyme) *does*
hexokinase I (muscle isozyme) does *not*

Which hexokinase isozyme has a higher Km?

Hexokinase IV (glucokinase)--> liver

*When* & *how* does glucokinase regulatory protein inhibit hexokinase IV?

When [fructose 6-P] is high, *glucokinase regulatory protein* sequesters hexokinase IV in the nucleus

Once glucokinase regulatory protein has sequestered hexokinase IV in the nucleus, what causes hexokinase to be localized back into the cytosol? Why/ how does this happen?

High [glucose] causes cytosolic localization of hexokinase IV, because it weakens the enzyme-regulator interaction.

Which reaction does phosphofructokinase-1 catalyze?

The third reaction of glycolysis:

fructose 6-phosphate + ATP ➡ fructose 1, 6-bisphosphate + ADP

Which of the following is an inhibitor of phosphofructokinase-1?
a) AMP
b) ADP
c) ATP
d) fructose 2,6-bisphosphate
e) none of the above

c) ATP

Which of the following is NOT an activator of phosphofructokinase-1?
a) AMP
b) ADP
c) ATP
d) fructose 2,6-bisphosphate
e) none of the above

c) ATP

T/F: "Every glucose 6-P molecule in a cell has the same fate."

False, glucose 6-P has several possible fates in the cell

Phosphorylation by _____________________ commits fructose 6-P to glycolysis.

phosphofructokinase-1

Phosphorylation by PFK-1 commits fructose 6-phosphate(in equilibrium with ____________________) to glycolysis.

glucose 6-phosphate

What effect does ATP have on phospofructokinase-1? How does it accomplish this?

It lowers PFK-1's affinity for fructose 6-phosphate, by binding to an allosteric site on PFK-1

What effect do AMP & ADP have on phosphofructokinase-1? How do they accomplish this?

They promote PFK-1 activity by relieving inhibition caused by ATP

What effect does citrate have on phosphofructokinase-1? How does it accomplish this?

Citrate contributes to the inhibition of PFK-1 activity, by increasing the inhibition caused by ATP

What effect does fructose 2, 6-bisphosphate have on phosphofructokinase?

It is a strong activator

T/F: "High [ATP] greatly reduces the affinity of PFK-1 for fructose 6-P."

True

T/F: "High [AMP] greatly reduces the affinity of PFK-1 for fructose 6-P."

False

T/F: "High [fructose 2,6-bisphosphate] greatly reduces the affinity of PFK-1 for fructose 6-P."

False
^fructose 2,6-bisphosphate is a strong activator of PFK-1

T/F: "When [ATP] is low, higher fructose 6-phosphate affinity allows PFK-1 to become more active."

True

T/F: "When [ADP] is low, higher fructose 6-phosphate affinity allows PFK-1 to become more active."

False

T/F: "When [fructose 2,6-bisphosphate] is low, higher fructose 6-phosphate affinity allows PFK-1 to become more active."

False, low [fructose 2,6-bisphosphate] would not increase PFK-1's affinity for F6P, as F2,6-BP is a strong activator of PFK-1. However, removing a strong inhibitor, such as ATP, would increase PFK-1 activity.

Which reaction does pyruvate kinase catalyze?

The tenth step of glycolysis:

phosphoenolpyruvate + ADP ➡ pyruvate + ATP

In the 10th reaction of glycolysis, pyruvate kinase transfers ________ from phosphoenolpyruvate to _______.

Pi to ADP

High _______________ allosterically inhibits pyruvate kinase, decreasing its affinity for phosphoenolpyruvate (PEP)

[ATP]

T/F: "High [ATP] allosterically inhibits pyruvate kinase, decreasing its affinity for phosphoenolpyruvate (PEP)."

True

T/F: "High [ATP] allosterically activates pyruvate kinase, increasing its affinity for phosphoenolpyruvate (PEP)."

False.

Corrected statement:
High [ATP] allosterically *inhibits* pyruvate kinase, *decreasing* its affinity for phosphoenolpyruvate (PEP).

List the four inhibitors of pyruvate kinase.

- ATP
- acetyl-CoA
- long chain fatty acids
- alanine

Name an allosteric activator of pyruvate kinase (PK)

fructose 1,6-bisphosphate

Which of the following does NOT inhibit pyruvate kinase (PK)?
a) acetyl CoA
b) long chain fatty acids
c) ATP
d) fructose 1,6-bisphosphate
e) alanine

d) fructose 1,6-bisphosphate

__________________ (intermediate of glycolysis) can be interconverted with alanine via transamination.

pyruvate

Which hormone can activate PKA in the liver?

glucagon

T/F: "The liver isoform of pyruvate kinase can be inactivated by glucagon."

True

T/F: "The muscle isoform of pyruvate kinase can be inactivated by glucagon."

False, only the liver isoform is affected by glucagon

When the liver isoform of pyruvate kinase is phosphorylated, it becomes:
a) activated
b) inactivated
c) is unaffected by phosphorylation

b) inactivated

The activated liver isoform of pyruvate kinase is:
a) phosphorylated
b) unphosphorylated

b) unphosphorylated

PKA catalyzes the reaction that:
a) activates the liver isoform of pyruvate kinase
b) deactivates the liver isoform of pyruvate kinase
c) activates the muscle isoform of pyruvate kinase
d) deactivates the muscle isoform of pyruvate kinase

b) deactivates the liver isoform of pyruvate kinase

Approximately how many hours/days worth of glucose is stored in the body?

1 day (as glycogen)

What is gluconeogenesis?

The synthesis of glucose from *non-glucose* precursors

List the three main precursors for gluconeogenesis.

- lactic acid
- glycerol
- glucogenic amino acids

How is step 10 of glycolysis bypassed? [step 1 of gluconeogenesis]

In the forward direction (i.e., in glycolysis) phosphoenolpyruvate is converted to pyruvate.

In the reverse direction (i.e., in glucogenogenesis), pyruvate needs to be converted to phosphoenolpyruvate.

In Mitochondria:
- *pyruvate* enters the mitochondria
- in the mitochondria, pryuvate is converted to *oxaloacetate* (by pyruvate carboxylase)
- oxaloacetate cannot leave the mitochondria, so it is converted into *malate* (by mitochondrial malate dehydrogenase)
- the malate then leaves the mitochondria for the cytosol

In cytosol:
- in the cytosol, malate is converted back to *oxaloacetate* (by cytosolic malate dehydrogenase)
- oxaloacetate is converted to *phosphoenolpyruvate* (by phosphoenolpyruvate carboxykinase) <-- requires *GTP*

Why is a malate intermediate used in the 1st step of gluconeogenesis, instead of just exporting oxaloacetate directly from the mitochondrial matrix -> cytosol?

It has to do with NADH.

Recall that (liver) mitochondria will be breaking down fatty acids (beta-oxidation), which produces NADH.

Also recall that gluconeogenesis consumes NADH (glyceraldehyde 3-phosphate dehydrogenase).

Therefore the NADH needs to get out of the matrix, into the cytosol. However NADH cannot be directly exported from the mitochondria.

The solution is for NADH to be consumed in the mitochondrial malate dehydrogenase reaction, and then produced on the cytosolic side by cytosolic malate dehydrogenase.

When lactate is the feedstock for gluconeogenesis, which enzyme produces cytosolic NADH?

lactate dehydrogenase

Which reaction is calatyzed by *fructose 1,6-bisphosphatase*?

Step 8 of gluconeogenesis:

fructose 1,6-bisphosphate + H2O ➡ fructose 6-phosphate + Pi

Fructose 1,6-bisphosphatase converts _______________ to _________________.

fructose 1,6-bisphosphate ➡ fructose 6-phosphate

Which step of gluconeogensis corresponds to the following steps of glycolysis?
a) 1
b) 3
c) 10

a) 10
b) 8
c) 1

State whether each of the following enzymes is part of glycolysis, gluconeogensis, or both.

a) phosphoenolpyruvate carboxykinase
b) hexokinase IV
c) phosphofructokinase-1
d) cytosolic malate dehydrogenase
e) enolase
f) glyceraldehyde 3-phosphate dehydrogenase
g) fructose 1,6-bisphosphatase
h) pyruvate kinase
i) hexokinase I
j) glucose 6-phosphatase
k) triose phosphate isomerase
l) mitochondrial malate dehydrogenase
m) phosphoglycerate mutase
n) phosphoglycerate kinase
o) pyruvate carboxylase
p) phosphohexose isomerase
q) aldolase

a) phosphoenolpyruvate carboxykinase- *gluconeogenesis*
b) hexokinase IV - *glycolysis*
c) phosphofructokinase-1 - *glycolysis*
d) cytosolic malate dehydrogenase - *gluconeogenesis*
e) enolase - *both*
f) glyceraldehyde 3-phosphate dehydrogenase - *both*
g) fructose 1,6-bisphosphatase - *gluconeogenesis*
h) pyruvate kinase - *glycolysis*
i) hexokinase I - *glycolysis*
j) glucose 6-phosphatase - *gluconeogenesis*
k) triose phosphate isomerase - *both*
l) mitochondrial malate dehydrogenase - *gluconeogenesis*
m) phosphoglycerate mutase - *both*
n) phosphoglycerate kinase - *both*
o) pyruvate carboxylase - *gluconeogenesis*
p) phosphohexose isomerase - *both*
q) aldolase - *both*

In step 8 of gluconeogensis, F1,6-BP ➡ F 6-P. This reaction is:
a) a phosphatase reaction
b) endergonic
c) a kinase reaction
d) exergonic
e) 2 of the above

e) 2 of the above

(a & d) --> phosphatase reaction & exergonic

Which reaction is catalyzed by glucose 6-phosphatase?

It catalyzes the dephosphorylation of G6P; the 10th step of gluconeogenesis:

glucose 6-phosphate + H2O ➡ glucose + Pi

Which of the following enzymes is expressed in few tissues (such as the liver, kidney, and small intestine --> a.k.a. gluconeogenic tissues)?
a) pyruvate carboxylase
b) phosphofructokinase-1
c) glucose 6-phosphatase
d) phosphoenolpyruvate carboxykinase

c) glucose 6-phosphatase

Why is it important that the regulation of glycolysis and gluconeogenesis be coordinated?

Because glycolysis and gluconeogenesis are opposing cellular processes, so having them run in parallel would simply waste energy

Name two allosteric modulators that increase the activity of PFK-1 [glycolysis, step3] but decrease the activity of FBPase-1 [gluconeogenesis, step8].

*F2,6-BP* & AMP

_________________________ allosterically regulates phosphofructokinase-1 and fructose 1,6-bisphosphatase-1 in a reciprocal manner.
a) fructose 2,6-bisphosphate
b) ATP
c) citrate
d) none of the above

a) fructose 2,6-bisphosphate

(also AMP, to a lesser degree)

Fructose 2,6-bisphosphate greatly increases _________________ activity.
a) pyruvate carboxylase
b) fructose 1,6-bisphosphatase
c) phosphofructokinase-1
d) all of the above
e) none of the above

c) phosphofructokinase-1

In the *absence* of F2,6-BP, phosphofructokinase-1 has _______________ affinity for fructose 6-P.
a) high
b) low
c) unaffected by F2,6-BP

b) low

In the *presence* of F2,6-BP, phosphofructokinase-1 has _______________ affinity for fructose 6-P.
a) high
b) low
c) unaffected by F2,6-BP

a) high

In the presence of F2,6-BP, the affinity of phophofructokinase-1 for fructose 6-P....
a) increases >10x
b) increases >100x
c) decreases >10x
d) decreases >100x
e) decreases <1000x

b) increases >100x

What effect does the presence of F2,6-BP have on fructose 1,6-bisphosphatase-1 activity?
a) drastically increases it
b) drastically decreases it
c) slightly increases it
d) slightly decreases it
e) has no effect

b) drastically decreases it

In the *absence* of F2,6-BP, fructose 1,6-BPase-1 has _______________ affinity for fructose 1,6-BP.
a) high
b) low
c) no change

a) high

In the *presence* of F2,6-BP, the affinity of fructose 1,6-bisphosphatase-1 for fructose 1,6-bisphosphate...
a) increases >10x
b) increases >100x
c) decreases >10x
d) decreases >100x
e) decreases <1000x

c) decreases >10x

Which two enzymes control *fructose 2,6-bisphosphate* concentration?

phosphofructokinase-2 (PFK-2)
&
fructose 2,6-bisphosphatase (FBPase-2)

Increased [F26BP]...
a) stimulates glycolysis
b) inhibits gluconeogenesis
c) inhibits glycolysis
d) stimulates gluconeogenesis
e) 2 of the above

e) 2 of the above
(a & b)

Which enzyme activates FBPase-2 (fructose 2,6-bisphosphatase)?

PKA (a.k.a. cAMP dependent protein kinase)

Which enzyme activates PFK-2?

phosphoprotein phosphatase

Which of the following is activated by insulin?
a) phosphoprotein phosphatase
b) PKA
c) both
d) neither

a) phosphoprotein phosphatase

Which of the following is activated by glucagon?
a) phosphoprotein phosphatase
b) PKA
c) both
d) neither

b) PKA

note: in diagram cAMP dependent protein kinase = PKA

When blood sugar is low, which of the following becomes active? Why?
a) phosphofructokinase-2 (PFK-2)
b) fructose 2,6-bisphosphatase (FBPase-2)
c) both
d) neither

b) fructose 2,6-bisphosphatase (FBPase-2)

b/c when blood sugar is low, glucagon is released, and glucagon stimulates PKA, which activates FBPase-2 via phosphorylation, while simultaneously inactivating phosphoprotein phosphatase