Download BCHM 463 Supplemental Problems for Friday, April 2, 2004 1. Write

BCHM 463
Supplemental Problems for Friday, April 2, 2004
1. Write the net reaction for glycolysis.
Glucose + 2 ADP + 2 NAD+ + 2 Pi
2 Pyruvate + 2 ATP + 2 NADH + 4 H+ + 2 H2O
2. During glycolysis, how many ADP molecules are converted to ATP. Explain this
answer with regard to your answer to #1.
4 ADP molecules are converted into ATP. There is a net gain of only 2 ATP
molecules because 2 are consumed during the first stage of glycolysis.
3. What are the three metabolically irreversible steps of glycolysis? What general type
of reaction is catalyzed by these enzymes? Why are these reactions irreversible?
Rxn 1: hexokinase
Rxn 3: phosphofructo kinase
Rxn 10: pyruvate kinase
All three reactions involve group transfer reactions.
The reactions are irreversible because they have a large negative ∆G. Note: ∆G
should not be confused with ∆G°´. See Table 14.1 for more information.
4. The following is an intermediate formed along the glycolysis pathway. This is a
substrate or product of what enzymes? Provide the net reaction for each catalytic
process that produces or consumes this intermediate.
OPO3-2
HO
HO
phosphoglucose
isomerase
O
OH
O
-2O PO
3
OH
HO
OH OH
phosphofructose
kinase
OPO3-2
O
-2O PO
3
OH
OH
HO
OH
PGI
G6P
F6P + ATP
F6P
PFK
FBP + ADP + H+
Mg2+
5. Name the compounds that serve as phosphoryl group donors in glycolysis and also
name the enzymes that catalyze these phosphoryl transfer steps.
1,3-BPG
ATP
Glucose
hexokinase
phosphoglycerate
kinase
G6P
1,3-BPG
F6P
phosphofructose
kinase
ATP
PEP
Pi
GAP
ADP
FBP
GAPDH
1,3-BPG
pyruvate
kinase
PEP
Pyruvate
ADP
ATP
3PG
BCHM 463
Supplemental Problems for Friday, April 2, 2004
6. Phosphoglycerate mutase catalyzes the conversion of 3-phosphoglycerate to 2phosphoglycerate. 2,3-Bisphosphoglycerate (2,3-BPG) is formed as an intermediate.
Describe the catalytic process that forms and consumes this intermediate. What
cofactors, if any, are necessary for these transformations? Does 2,3-BPG serve any
physiological function?
3PG
2,3-BPG
2PG
Each step is a group transfer step. First a phosphoryl group is transferred from the
enzyme to the substrate to generate the bisphospho intermediate. This is followed by
transfer of the second phosphoryl group back to the enzyme.
Although not mentioned in the book, this enzyme is Mg2+ dependent in some
organisms and also occasionally requires 2,3-BPG as a cofactor to reactivate the
enzyme if it becomes dephosphorylated.
Recall that 2,3-BPG is an allosteric modulator of hemoglobin in erythrocytes
although the 2,3-BPG produced by escape from phosphoglycerate mutase is neglible.
This of course means that there is some other source of 2,3-BPG in erythrocytes.
7. In Greek mythology, the Three Fates, or Moirae, were the goddesses who controlled
the destiny of everyone from the time they were born to the time they died. Pyruvate
also has three fates: fill in the boxes below to indicate the fate of pyruvate produced
during glycolysis.
Glucose
Glycolysis
Pyruvate
anaerobic
conditions
2 EtOH + 2CO2
aerobic
conditions
CO2
2 AcetylCoA
citric acid
cycle
4 CO2 + 4 H2O
anaerobic
conditions
2 Lactate
BCHM 463
Supplemental Problems for Friday, April 2, 2004
8. When lactate is converted to pyruvate, which of the following, if any, are true?
a. NAD+ is converted to NADH
b. ADP is converted to ATP
c. NADH is converted to NAD+
d. ATP is converted to ADP
e. FAD is converted to FADH
A is the only answer that is true. Remember that the reaction catalyzed by lactate
dehyrogenase is freely reversible.
9. Which enzymes in glycolysis utilize covalent catalysis during conversion of substrate
into product? Write the enzyme names and the net reaction catalyzed by each.
GAPDH
1,3-BPG + NADH + H+
GAP + Pi + NAD+
O
Enz
OPO3-2
S
OH
acylenzyme
intermediate
10. Glycolysis is regulated by controlling which enzymes? Identify the mechanisms of
control for each. (e.g. product inhibition or feedback inhibition)
Rxn 1: hexokinase: product inhibition by Glc6P
Rxn 3: phosphofructo kinase: ATP is an allosteric inhibitor; AMP, ADP, F2,6P, and
other compounds overcome this inhibition and therefore serve as activators
Rxn 10: pyruvate kinase: product inhibition by ATP (allosteric); FBP acts as an
allosteric activator
11. Xylose has the same structure as glucose except that it has a hydrogen atom at C-5 in
place of a hydroxymethyl group. The rate of ATP hydrolysis catalyzed by
hexokinase is markedly enhanced by the addition of xylose. Why?
Although xylose does NOT serve as a substrate for hexokinase, it does bind and
induce the conformational change found with glucose binding. The xylose binding
event positions a water molecule to serve as a nucleophile to hydrolyze ATP.
.