BIOCHEM TEST PRACTICE
1.The Central Role of the Citric Acid Cycle in Metabolism.
i. Recall Which pathways are involved in the anaerobic metabolism of glucose? Which
pathways are involved in the aerobic metabolism of glucose? Anaerobic glycolysis is the
principal pathway for the anaerobic metabolism of glucose. The pentose phosphate pathway
can also be considered. Aerobic glycolysis and the citric acid cycle are responsible for the
aerobic metabolism of glucose.
ii. Recall How many ATPs can be produced from one molecule of glucose anaerobically?
Aerobically? Anaerobically, two ATPs can be produced from one glucose molecule.
Aerobically, this figure is 30 to 32, depending on in which tissue it is occurring.
iii. Recall What are the different names used to describe the pathway discussed in
this chapter? The citric acid cycle is also called the Krebs cycle, the tricarboxylic acid
cycle, and the TCA cycle.
iv. Recall What is meant by the statement that a pathway is amphibolic? Amphibolic means
that the pathway is involved in both catabolism and anabolism.
2. The Overall Pathway of the Citric Acid Cycle.
i. Recall In what part of the cell does the citric acid cycle take place? Does this differ
from the part of the cell where glycolysis occurs? The citric acid cycle takes place in the
mitochondrial matrix. Glycolysis takes place in the cytosol.
ii. Recall How does pyruvate from glycolysis get to the pyruvate dehydrogenase complex?
There is a transporter on the inner mitochondrial matrix that allows pyruvate from the cytosol
to pass into the mitochondria.
iii. Recall What electron acceptors play a role in the citric acid cycle? NAD+ and FAD are
the primary electron acceptors of the citric acid cycle.
iv. Recall What three molecules produced during the citric acid cycle are an indirect or
direct source of high-energy compounds? NADH and FADH2 are indirect sources of
energy produced in the TCA cycle. GTP is a direct source of energy.
3. How Pyruvate Is Converted to Acetyl-CoA.
i. Recall How many enzymes are involved in mammalian pyruvate dehydrogenase?
What are their functions? Five enzymes are involved in the pyruvate dehydrogenase
complex of mammals. Pyruvate dehydrogenase transfers a two-carbon unit to TPP and
releases CO2. Dihydrolipoyl transacetylase transfers the two-carbon acetyl unit to lipoic
acid and then to coenzyme A. Dihydrolipoyl dehydrogenase reoxidizes lipoic acid and
reduces NAD+ to NADH. Pyruvate dehydrogenase kinase phosphorylates PDH. PDH
phosphatase removes the phosphate.
, ii. Recall Briefly describe the dual role of lipoic acid in the pyruvate
dehydrogenase complex. Lipoic acid plays a role both in redox and in acetyl-
transfer reactions.
iii. Recall What is the advantage to the organization of the PDH complex? Five enzymes
are all in close proximity for efficient shuttling of the acetyl unit between molecules and
efficient control of the complex by phosphorylation.
iv. Recall In the PDH reaction alone, we can see cofactors that come from four different
vitamins. What are they? Thiamine pyrophosphate comes from the B vitamin thiamine.
Lipoic acid is a vitamin. NAD+ comes from the B vitamin niacin. FAD comes from the
B vitamin riboflavin.
4. The Individual Reactions of the Citric Acid Cycle
i. Recall Why is the reaction catalyzed by citrate synthase considered a condensation
reaction? A condensation reaction is one in which a new carbon–carbon bond is formed.
The reaction of acetyl-CoA and oxaloacetate to produce citrate involves formation of
such a carbon– carbon bond.
ii. Recall What does it mean when an enzyme has the name synthase? It means that the
reaction catalyzed by the enzyme produces the product that is part of the name and
does not require a direct input of energy from a high-energy phosphate. Thus, citrate
synthase catalyzes the synthesis of citrate without using ATP to do it.
iii. Biochemical Connections What is fluoroacetate? Why is it used? Fluoroacetate is a
poison that is produced naturally in some plants and is also used as a poison against
undesirable pests. It is poisonous because it is used by citrate synthase to make
fluorocitrate, which is an inhibitor of the citric acid cycle.
iv. Recall With respect to stereochemistry, what is unique about the reaction catalyzed
by aconitase? The reaction involves an achiral molecule (citrate) being converted to a
chiral one (isocitrate).
v. Recall In which steps of the aerobic processing of pyruvate is CO2 produced?
Conversion of pyruvate to acetyl-CoA, conversion of isocitrate to -ketoglutarate, and
conversion of -ketoglutarate to succinyl-CoA.
1.The Central Role of the Citric Acid Cycle in Metabolism.
i. Recall Which pathways are involved in the anaerobic metabolism of glucose? Which
pathways are involved in the aerobic metabolism of glucose? Anaerobic glycolysis is the
principal pathway for the anaerobic metabolism of glucose. The pentose phosphate pathway
can also be considered. Aerobic glycolysis and the citric acid cycle are responsible for the
aerobic metabolism of glucose.
ii. Recall How many ATPs can be produced from one molecule of glucose anaerobically?
Aerobically? Anaerobically, two ATPs can be produced from one glucose molecule.
Aerobically, this figure is 30 to 32, depending on in which tissue it is occurring.
iii. Recall What are the different names used to describe the pathway discussed in
this chapter? The citric acid cycle is also called the Krebs cycle, the tricarboxylic acid
cycle, and the TCA cycle.
iv. Recall What is meant by the statement that a pathway is amphibolic? Amphibolic means
that the pathway is involved in both catabolism and anabolism.
2. The Overall Pathway of the Citric Acid Cycle.
i. Recall In what part of the cell does the citric acid cycle take place? Does this differ
from the part of the cell where glycolysis occurs? The citric acid cycle takes place in the
mitochondrial matrix. Glycolysis takes place in the cytosol.
ii. Recall How does pyruvate from glycolysis get to the pyruvate dehydrogenase complex?
There is a transporter on the inner mitochondrial matrix that allows pyruvate from the cytosol
to pass into the mitochondria.
iii. Recall What electron acceptors play a role in the citric acid cycle? NAD+ and FAD are
the primary electron acceptors of the citric acid cycle.
iv. Recall What three molecules produced during the citric acid cycle are an indirect or
direct source of high-energy compounds? NADH and FADH2 are indirect sources of
energy produced in the TCA cycle. GTP is a direct source of energy.
3. How Pyruvate Is Converted to Acetyl-CoA.
i. Recall How many enzymes are involved in mammalian pyruvate dehydrogenase?
What are their functions? Five enzymes are involved in the pyruvate dehydrogenase
complex of mammals. Pyruvate dehydrogenase transfers a two-carbon unit to TPP and
releases CO2. Dihydrolipoyl transacetylase transfers the two-carbon acetyl unit to lipoic
acid and then to coenzyme A. Dihydrolipoyl dehydrogenase reoxidizes lipoic acid and
reduces NAD+ to NADH. Pyruvate dehydrogenase kinase phosphorylates PDH. PDH
phosphatase removes the phosphate.
, ii. Recall Briefly describe the dual role of lipoic acid in the pyruvate
dehydrogenase complex. Lipoic acid plays a role both in redox and in acetyl-
transfer reactions.
iii. Recall What is the advantage to the organization of the PDH complex? Five enzymes
are all in close proximity for efficient shuttling of the acetyl unit between molecules and
efficient control of the complex by phosphorylation.
iv. Recall In the PDH reaction alone, we can see cofactors that come from four different
vitamins. What are they? Thiamine pyrophosphate comes from the B vitamin thiamine.
Lipoic acid is a vitamin. NAD+ comes from the B vitamin niacin. FAD comes from the
B vitamin riboflavin.
4. The Individual Reactions of the Citric Acid Cycle
i. Recall Why is the reaction catalyzed by citrate synthase considered a condensation
reaction? A condensation reaction is one in which a new carbon–carbon bond is formed.
The reaction of acetyl-CoA and oxaloacetate to produce citrate involves formation of
such a carbon– carbon bond.
ii. Recall What does it mean when an enzyme has the name synthase? It means that the
reaction catalyzed by the enzyme produces the product that is part of the name and
does not require a direct input of energy from a high-energy phosphate. Thus, citrate
synthase catalyzes the synthesis of citrate without using ATP to do it.
iii. Biochemical Connections What is fluoroacetate? Why is it used? Fluoroacetate is a
poison that is produced naturally in some plants and is also used as a poison against
undesirable pests. It is poisonous because it is used by citrate synthase to make
fluorocitrate, which is an inhibitor of the citric acid cycle.
iv. Recall With respect to stereochemistry, what is unique about the reaction catalyzed
by aconitase? The reaction involves an achiral molecule (citrate) being converted to a
chiral one (isocitrate).
v. Recall In which steps of the aerobic processing of pyruvate is CO2 produced?
Conversion of pyruvate to acetyl-CoA, conversion of isocitrate to -ketoglutarate, and
conversion of -ketoglutarate to succinyl-CoA.
