Answer key
FCH 530 Homework 8
1. What is the rate-limiting step of glycolysis? Which enzyme is regulated and by
what? The conversion of fructose-6-phosphate (F6P) to fructose-1,6bisphosphate (F1,6P) by phosphofructokinase is the rate limiting step in
glycolysis. Phosphpfructokinase is enhanced allosterically by AMP
(activator) and inhibited by ATP and citrate.
2. What are the energy producing steps of glycolysis? Which enzymes are
involved? Step 6, conversion of glyceraldehyde-3-phosphate (GAP) to 1,3bisphosphoglycerate (1,3-BPG) via glyceraldehydes-3-phosphate
dehydrogenase (GAPDH) produces NADH. Step 7, conversion of 1,3-BPG to
3-phosphoglycerate (3PG) via phosphoglycerate kinase (PGK) produces
ATP. Step 10, conversion of phosphoenolpyruvate (PEP) to pyruvate via
pyruvate kinase (PK) produces ATP.
3. What are the energy consuming steps? Which enzymes are involved? Step 1,
conversion of glucose to glucose-6-phosphate via hexokinase uses ATP. Step
3, conversion of fructose-6-phosphate to fructose-1,6-bisphosphate via
phosphofructokinase uses ATP.
4. What is the total amount of energy in joules generated by 1 mol of glucose that is
processed via glycolysis? Every mol of glucose processed via glycolysis will
yield 2 mol ATP and 2 mol NADH. The amount of energy in ATP  ADP +
Pi is -30.5 kJ/mol (Table 16-3). The amount of energy stored in NADH can
be calculated by the nernst equation: G = -nFE =-2*(96,494 Joules *V-1
mol-1)*(0.315) = -60.8 kJ/mol. Thus the total energy in joules would be
2*30.5 + 2*60.8 = 182.6 kJ/mol
5. Write out a detailed mechanism for the conversion of GAP to 1,3-BPG by
GAPDH. Show chemical structures of substrates and amino acids in the active
site of the enzyme and all electron flow.
Answer key
6. What are the two main types of fermentation pathways? What are the key
differences between these pathways? The two main types are lactate
fermentation and alcohol fermentation. Key differences in the pathways is
that for lactate fermentation, we have direct oxidation of NADH coupled to
the reduction of pyruvate by lactate dehydrogenase to produce lactate (single
enzyme reaction). For alcohol production, two enzymes and steps are needed
to go from pyruvate to ethanol. First pyruvate is decarboxylated by
pyruvate decarboxylase to acetaldehyde. Next acetaldehyde is reduced to
ethanol by NADH to regenerate NAD+.
7. Write out a detailed mechanism for the pyruvate decarboxylase enzyme. Show
chemical structures of substrates and amino acids in the active site of the enzyme
and all electron flow.
Answer key
8. Write out the Entner-Doudoroff pathway for glucose utilization. Show chemical
structures of substrates and write out the enzyme names and any cofactors or
coenzymes involved. Include the intermediates and enzymes for the alcoholic
fementation steps. See page 625 in text. For alcohol production see slide 21
from lecture 21.
9. What are the enzymes that regulate glycolysis? What are the activators? What
are the inhibitors? Hexokinase-inhibited by glucose-6-phosphate.
Phosphofructokinase-1, inhibited by ATP, citrate, NADH, activated by Pi,
AMP, fructose-6-phosphate, fructose-2,6-bisphosphate. Pyruvate kinase,
inhibited by ATP, acetyl-CoA, cAMP, activated by fructose-1,6-phosphate.
10. What are the differences and similarities of PFK1 and PFK2? How do they help
regulate glycolysis? PFK1 catalyzes the production of fructose-1,6bisphosphate from fructose-6-phosphate. PFK2 converts fructose-6phosphate to fructose-2,6-bisphosphate. PFK1 regulates glycolysis by
producing a feedforward activator (fructose-1,6-phosphate) of pyruvate
kinase. PFK1 itself is inhibited by ATP and citrate and activated by AMP
and fructose-2,6-phosphate. PFK2 produces fructose-2,6-phosphate from
fructose-6-phosphate and can override the energy charge regulation of the
cell (i.e. when glucose concentrations are high but you need glycolysis for an
anabolic role). PFK2 is activated by citrate and inhibited by camp and
3PGA.
11. What are the enzyme involved in glycogen formation and degradation? What are
their functions? How are they regulated?
Glycogen formation:
Answer key
1. UDP-glucose pyrophosphorylase-formation of UDP-glucose from glucose1-phosphate and UTP.
2. glycogen synthase-transfers glucosyl unit from UDP-glucose to glycogen’s
nonreducing ends to form an -(1,4)-glycosidic bond. Regulated by
phosphorylation of serine residue in the enzyme. Phosphorylated is
inactivated. Unphosphorylated is active. Adrenline (epinephrine) results
in the inactivation of glycogen synthase.
3. glycogen branching enzymes-transfers minimum 11 residue glycosyl units
to a new branch point on the glycogen main chain at least 4 residues away
from other branch points.
Glycogen degradation:
Requires 3 enzymes:
1. Glycogen phosphorylase (phosphorylase) catalyzes glycogen phosphorylysis
(bond cleavage by the substitution of a phosphate group) and yields glucose1-phosphate (G1P). inhibited by ATP, G6P, glucose, activated by AMP.
Unphosphorylated is inactive. Phosphorylated is active. Adrenline
(epinephrine) results in the activation of glycogen phosphorylase.
2. Glycogen debranching enzyme removes glycogen’s branches, allowing
glycogen phosphorylase to complete it’s reactions. It also hydrolyzes -(1,6)linked glucosyl units to yield glucose. 92% of glycogen’s glucse residues are
converted to G1P and 8% to glucose.
3. Phosphoglucomutase converts G1P to G6P-can either go through glycolysis
(muscle cells) or converted to glucose (liver).
12. What are the main products produced by the pentose phosphate pathway? Why
are these important? Main products are NAPH and ribose-5-phosphate.
NADPH utilizes the free energy of metaboite oxidation for biosynthesis and
ribose-5-phosphate is essential for nucleotide biosynthesis.