BIOCHEM ACS Exam Study Guide – Comprehensive
American Chemical Society Biochemistry Exam Preparation
Notes, Practice Questions, Metabolism Pathways, Enzyme
Kinetics, Molecular Biology Concepts & High-Yield Review
for ACS Biochemistry Final
Question 1: Which amino acid contains a thiol group in its side chain that can form disulfide
bonds under oxidizing conditions?
A. Methionine
B. Cysteine
C. Serine
D. Threonine
CORRECT ANSWER: B. Cysteine
Rationale: Cysteine contains a sulfhydryl (-SH) group in its side chain that can undergo oxidation
to form disulfide bonds (-S-S-) with another cysteine residue. This covalent linkage is critical for
stabilizing tertiary and quaternary protein structures. Methionine contains a thioether linkage
that cannot form disulfide bonds, while serine and threonine contain hydroxyl groups.
Question 2: In the Michaelis-Menten equation, what does Km represent?
A. The maximum velocity of the reaction
B. The substrate concentration at which the reaction velocity is half of Vmax
C. The turnover number of the enzyme
D. The dissociation constant for the enzyme-product complex
CORRECT ANSWER: B. The substrate concentration at which the reaction velocity is half of
Vmax
Rationale: Km, the Michaelis constant, is defined as the substrate concentration at which the
reaction rate equals half of Vmax. It provides a measure of the enzyme's affinity for its
substrate; a lower Km indicates higher affinity. Vmax represents maximum velocity, kcat is the
turnover number, and Km is not directly the dissociation constant for the enzyme-product
complex.
Question 3: Which of the following metabolic pathways occurs exclusively in the
mitochondria of eukaryotic cells?
A. Glycolysis
B. Pentose phosphate pathway
C. Citric acid cycle
D. Fatty acid synthesis
CORRECT ANSWER: C. Citric acid cycle
Rationale: The citric acid cycle (Krebs cycle) occurs in the mitochondrial matrix of eukaryotic
cells. Glycolysis and the pentose phosphate pathway occur in the cytosol, while fatty acid
synthesis primarily occurs in the cytosol (with acetyl-CoA transported from mitochondria). The
mitochondrial localization of the citric acid cycle is essential for coupling with oxidative
phosphorylation.
Question 4: What is the primary structural difference between DNA and RNA?
A. DNA contains uracil; RNA contains thymine
B. DNA contains deoxyribose; RNA contains ribose
,C. DNA is single-stranded; RNA is double-stranded
D. DNA has phosphodiester bonds; RNA does not
CORRECT ANSWER: B. DNA contains deoxyribose; RNA contains ribose
Rationale: The sugar component distinguishes DNA from RNA: DNA contains 2'-deoxyribose
(lacking a hydroxyl group at the 2' carbon), whereas RNA contains ribose (with a 2'-OH group).
DNA contains thymine while RNA contains uracil. DNA is typically double-stranded while RNA is
typically single-stranded, though exceptions exist. Both contain phosphodiester bonds in their
backbone.
Question 5: Which enzyme catalyzes the committed step in glycolysis?
A. Hexokinase
B. Phosphofructokinase-1
C. Pyruvate kinase
D. Aldolase
CORRECT ANSWER: B. Phosphofructokinase-1
Rationale: Phosphofructokinase-1 (PFK-1) catalyzes the phosphorylation of fructose-6-
phosphate to fructose-1,6-bisphosphate, which is the committed and rate-limiting step of
glycolysis. This step is highly regulated by allosteric effectors including ATP, ADP, AMP, and
citrate. Hexokinase catalyzes the first step but glucose-6-phosphate can enter other pathways;
pyruvate kinase catalyzes a later regulated step.
Question 6: The isoelectric point (pI) of an amino acid is the pH at which:
A. The amino acid is fully protonated
B. The amino acid is fully deprotonated
C. The amino acid has no net electrical charge
D. The amino acid exhibits maximum solubility
CORRECT ANSWER: C. The amino acid has no net electrical charge
Rationale: The isoelectric point (pI) is the pH at which an amino acid or protein carries no net
electrical charge because the number of positive and negative charges are equal. At this pH, the
molecule does not migrate in an electric field and often exhibits minimum solubility. The pI is
calculated as the average of the pKa values bracketing the zwitterionic form.
Question 7: Which of the following is a competitive inhibitor of succinate dehydrogenase?
A. Cyanide
B. Malonate
C. Oligomycin
D. Rotenone
CORRECT ANSWER: B. Malonate
Rationale: Malonate is a classic competitive inhibitor of succinate dehydrogenase because it
structurally resembles succinate and binds to the active site without undergoing catalysis. This
inhibition can be overcome by increasing succinate concentration. Cyanide inhibits cytochrome
c oxidase, oligomycin inhibits ATP synthase, and rotenone inhibits Complex I of the electron
transport chain.
Question 8: In the Watson-Crick model of DNA, adenine pairs with thymine via how many
hydrogen bonds?
,A. One
B. Two
C. Three
D. Four
CORRECT ANSWER: B. Two
Rationale: In B-form DNA, adenine forms two hydrogen bonds with thymine (N6 of adenine to
O4 of thymine, and N1 of adenine to N3 of thymine). Guanine forms three hydrogen bonds with
cytosine, contributing to the higher thermal stability of GC-rich DNA regions. These specific
base-pairing interactions are fundamental to DNA replication and transcription fidelity.
Question 9: Which coenzyme is derived from vitamin B3 (niacin) and functions as an electron
carrier in oxidation-reduction reactions?
A. FAD
B. NAD+
C. Coenzyme A
D. Thiamine pyrophosphate
CORRECT ANSWER: B. NAD+
Rationale: NAD+ (nicotinamide adenine dinucleotide) is derived from niacin (vitamin B3) and
functions as a soluble electron carrier, accepting two electrons and one proton to form NADH
during catabolic reactions. FAD is derived from riboflavin (B2), CoA from pantothenic acid (B5),
and TPP from thiamine (B1). NAD+/NADH participates in numerous dehydrogenase reactions
throughout metabolism.
Question 10: What type of enzyme catalyzes the transfer of a phosphate group from ATP to a
substrate?
A. Hydrolase
B. Isomerase
C. Kinase
D. Lyase
CORRECT ANSWER: C. Kinase
Rationale: Kinases are transferases that catalyze the transfer of a phosphate group (typically
from ATP) to a specific substrate molecule, a process called phosphorylation. This post-
translational modification regulates enzyme activity, signal transduction, and metabolic flux.
Hydrolases catalyze bond cleavage with water, isomerases rearrange molecular structures, and
lyases cleave bonds without hydrolysis or oxidation.
Question 11: Which of the following amino acids is classified as essential for humans?
A. Alanine
B. Glycine
C. Leucine
D. Serine
CORRECT ANSWER: C. Leucine
Rationale: Leucine is one of nine essential amino acids that cannot be synthesized de novo by
humans and must be obtained from the diet. Essential amino acids include histidine, isoleucine,
, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Alanine, glycine,
and serine are nonessential because they can be synthesized from metabolic intermediates.
Question 12: The Bohr effect describes how hemoglobin's oxygen-binding affinity is inversely
related to:
A. Temperature and 2,3-BPG concentration
B. Carbon dioxide concentration and hydrogen ion concentration
C. Nitric oxide concentration and pH
D. Chloride ion concentration and osmolarity
CORRECT ANSWER: B. Carbon dioxide concentration and hydrogen ion concentration
Rationale: The Bohr effect refers to the decrease in hemoglobin's oxygen affinity with
increasing CO2 and H+ concentration (lower pH). This facilitates oxygen unloading in
metabolically active tissues where CO2 production and acidity are elevated. Protons and CO2
stabilize the T-state of hemoglobin through allosteric interactions. Temperature and 2,3-BPG
also affect oxygen affinity but are distinct from the classic Bohr effect definition.
Question 13: Which metabolic intermediate serves as a branch point between glycolysis and
the pentose phosphate pathway?
A. Fructose-6-phosphate
B. Glucose-6-phosphate
C. Glyceraldehyde-3-phosphate
D. Pyruvate
CORRECT ANSWER: B. Glucose-6-phosphate
Rationale: Glucose-6-phosphate is a critical metabolic branch point that can enter glycolysis (via
phosphoglucose isomerase), the pentose phosphate pathway (via glucose-6-phosphate
dehydrogenase), or glycogen synthesis (via phosphoglucomutase). The flux through each
pathway is regulated by cellular needs for ATP, NADPH, and ribose-5-phosphate. This node
allows metabolic flexibility in response to redox and biosynthetic demands.
Question 14: In competitive inhibition, how does the inhibitor affect Km and Vmax?
A. Km increases; Vmax increases
B. Km decreases; Vmax unchanged
C. Km increases; Vmax unchanged
D. Km unchanged; Vmax decreases
CORRECT ANSWER: C. Km increases; Vmax unchanged
Rationale: Competitive inhibitors bind reversibly to the enzyme's active site, competing with
substrate. This increases the apparent Km (more substrate is needed to reach half Vmax) but
does not affect Vmax because sufficient substrate can outcompete the inhibitor. In
noncompetitive inhibition, Vmax decreases while Km remains unchanged. Mixed inhibition
affects both parameters.
Question 15: Which of the following is NOT a function of cholesterol in mammalian cells?
A. Modulating membrane fluidity
B. Precursor for steroid hormone synthesis
C. Direct participation in electron transport chain
D. Precursor for bile acid synthesis
American Chemical Society Biochemistry Exam Preparation
Notes, Practice Questions, Metabolism Pathways, Enzyme
Kinetics, Molecular Biology Concepts & High-Yield Review
for ACS Biochemistry Final
Question 1: Which amino acid contains a thiol group in its side chain that can form disulfide
bonds under oxidizing conditions?
A. Methionine
B. Cysteine
C. Serine
D. Threonine
CORRECT ANSWER: B. Cysteine
Rationale: Cysteine contains a sulfhydryl (-SH) group in its side chain that can undergo oxidation
to form disulfide bonds (-S-S-) with another cysteine residue. This covalent linkage is critical for
stabilizing tertiary and quaternary protein structures. Methionine contains a thioether linkage
that cannot form disulfide bonds, while serine and threonine contain hydroxyl groups.
Question 2: In the Michaelis-Menten equation, what does Km represent?
A. The maximum velocity of the reaction
B. The substrate concentration at which the reaction velocity is half of Vmax
C. The turnover number of the enzyme
D. The dissociation constant for the enzyme-product complex
CORRECT ANSWER: B. The substrate concentration at which the reaction velocity is half of
Vmax
Rationale: Km, the Michaelis constant, is defined as the substrate concentration at which the
reaction rate equals half of Vmax. It provides a measure of the enzyme's affinity for its
substrate; a lower Km indicates higher affinity. Vmax represents maximum velocity, kcat is the
turnover number, and Km is not directly the dissociation constant for the enzyme-product
complex.
Question 3: Which of the following metabolic pathways occurs exclusively in the
mitochondria of eukaryotic cells?
A. Glycolysis
B. Pentose phosphate pathway
C. Citric acid cycle
D. Fatty acid synthesis
CORRECT ANSWER: C. Citric acid cycle
Rationale: The citric acid cycle (Krebs cycle) occurs in the mitochondrial matrix of eukaryotic
cells. Glycolysis and the pentose phosphate pathway occur in the cytosol, while fatty acid
synthesis primarily occurs in the cytosol (with acetyl-CoA transported from mitochondria). The
mitochondrial localization of the citric acid cycle is essential for coupling with oxidative
phosphorylation.
Question 4: What is the primary structural difference between DNA and RNA?
A. DNA contains uracil; RNA contains thymine
B. DNA contains deoxyribose; RNA contains ribose
,C. DNA is single-stranded; RNA is double-stranded
D. DNA has phosphodiester bonds; RNA does not
CORRECT ANSWER: B. DNA contains deoxyribose; RNA contains ribose
Rationale: The sugar component distinguishes DNA from RNA: DNA contains 2'-deoxyribose
(lacking a hydroxyl group at the 2' carbon), whereas RNA contains ribose (with a 2'-OH group).
DNA contains thymine while RNA contains uracil. DNA is typically double-stranded while RNA is
typically single-stranded, though exceptions exist. Both contain phosphodiester bonds in their
backbone.
Question 5: Which enzyme catalyzes the committed step in glycolysis?
A. Hexokinase
B. Phosphofructokinase-1
C. Pyruvate kinase
D. Aldolase
CORRECT ANSWER: B. Phosphofructokinase-1
Rationale: Phosphofructokinase-1 (PFK-1) catalyzes the phosphorylation of fructose-6-
phosphate to fructose-1,6-bisphosphate, which is the committed and rate-limiting step of
glycolysis. This step is highly regulated by allosteric effectors including ATP, ADP, AMP, and
citrate. Hexokinase catalyzes the first step but glucose-6-phosphate can enter other pathways;
pyruvate kinase catalyzes a later regulated step.
Question 6: The isoelectric point (pI) of an amino acid is the pH at which:
A. The amino acid is fully protonated
B. The amino acid is fully deprotonated
C. The amino acid has no net electrical charge
D. The amino acid exhibits maximum solubility
CORRECT ANSWER: C. The amino acid has no net electrical charge
Rationale: The isoelectric point (pI) is the pH at which an amino acid or protein carries no net
electrical charge because the number of positive and negative charges are equal. At this pH, the
molecule does not migrate in an electric field and often exhibits minimum solubility. The pI is
calculated as the average of the pKa values bracketing the zwitterionic form.
Question 7: Which of the following is a competitive inhibitor of succinate dehydrogenase?
A. Cyanide
B. Malonate
C. Oligomycin
D. Rotenone
CORRECT ANSWER: B. Malonate
Rationale: Malonate is a classic competitive inhibitor of succinate dehydrogenase because it
structurally resembles succinate and binds to the active site without undergoing catalysis. This
inhibition can be overcome by increasing succinate concentration. Cyanide inhibits cytochrome
c oxidase, oligomycin inhibits ATP synthase, and rotenone inhibits Complex I of the electron
transport chain.
Question 8: In the Watson-Crick model of DNA, adenine pairs with thymine via how many
hydrogen bonds?
,A. One
B. Two
C. Three
D. Four
CORRECT ANSWER: B. Two
Rationale: In B-form DNA, adenine forms two hydrogen bonds with thymine (N6 of adenine to
O4 of thymine, and N1 of adenine to N3 of thymine). Guanine forms three hydrogen bonds with
cytosine, contributing to the higher thermal stability of GC-rich DNA regions. These specific
base-pairing interactions are fundamental to DNA replication and transcription fidelity.
Question 9: Which coenzyme is derived from vitamin B3 (niacin) and functions as an electron
carrier in oxidation-reduction reactions?
A. FAD
B. NAD+
C. Coenzyme A
D. Thiamine pyrophosphate
CORRECT ANSWER: B. NAD+
Rationale: NAD+ (nicotinamide adenine dinucleotide) is derived from niacin (vitamin B3) and
functions as a soluble electron carrier, accepting two electrons and one proton to form NADH
during catabolic reactions. FAD is derived from riboflavin (B2), CoA from pantothenic acid (B5),
and TPP from thiamine (B1). NAD+/NADH participates in numerous dehydrogenase reactions
throughout metabolism.
Question 10: What type of enzyme catalyzes the transfer of a phosphate group from ATP to a
substrate?
A. Hydrolase
B. Isomerase
C. Kinase
D. Lyase
CORRECT ANSWER: C. Kinase
Rationale: Kinases are transferases that catalyze the transfer of a phosphate group (typically
from ATP) to a specific substrate molecule, a process called phosphorylation. This post-
translational modification regulates enzyme activity, signal transduction, and metabolic flux.
Hydrolases catalyze bond cleavage with water, isomerases rearrange molecular structures, and
lyases cleave bonds without hydrolysis or oxidation.
Question 11: Which of the following amino acids is classified as essential for humans?
A. Alanine
B. Glycine
C. Leucine
D. Serine
CORRECT ANSWER: C. Leucine
Rationale: Leucine is one of nine essential amino acids that cannot be synthesized de novo by
humans and must be obtained from the diet. Essential amino acids include histidine, isoleucine,
, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Alanine, glycine,
and serine are nonessential because they can be synthesized from metabolic intermediates.
Question 12: The Bohr effect describes how hemoglobin's oxygen-binding affinity is inversely
related to:
A. Temperature and 2,3-BPG concentration
B. Carbon dioxide concentration and hydrogen ion concentration
C. Nitric oxide concentration and pH
D. Chloride ion concentration and osmolarity
CORRECT ANSWER: B. Carbon dioxide concentration and hydrogen ion concentration
Rationale: The Bohr effect refers to the decrease in hemoglobin's oxygen affinity with
increasing CO2 and H+ concentration (lower pH). This facilitates oxygen unloading in
metabolically active tissues where CO2 production and acidity are elevated. Protons and CO2
stabilize the T-state of hemoglobin through allosteric interactions. Temperature and 2,3-BPG
also affect oxygen affinity but are distinct from the classic Bohr effect definition.
Question 13: Which metabolic intermediate serves as a branch point between glycolysis and
the pentose phosphate pathway?
A. Fructose-6-phosphate
B. Glucose-6-phosphate
C. Glyceraldehyde-3-phosphate
D. Pyruvate
CORRECT ANSWER: B. Glucose-6-phosphate
Rationale: Glucose-6-phosphate is a critical metabolic branch point that can enter glycolysis (via
phosphoglucose isomerase), the pentose phosphate pathway (via glucose-6-phosphate
dehydrogenase), or glycogen synthesis (via phosphoglucomutase). The flux through each
pathway is regulated by cellular needs for ATP, NADPH, and ribose-5-phosphate. This node
allows metabolic flexibility in response to redox and biosynthetic demands.
Question 14: In competitive inhibition, how does the inhibitor affect Km and Vmax?
A. Km increases; Vmax increases
B. Km decreases; Vmax unchanged
C. Km increases; Vmax unchanged
D. Km unchanged; Vmax decreases
CORRECT ANSWER: C. Km increases; Vmax unchanged
Rationale: Competitive inhibitors bind reversibly to the enzyme's active site, competing with
substrate. This increases the apparent Km (more substrate is needed to reach half Vmax) but
does not affect Vmax because sufficient substrate can outcompete the inhibitor. In
noncompetitive inhibition, Vmax decreases while Km remains unchanged. Mixed inhibition
affects both parameters.
Question 15: Which of the following is NOT a function of cholesterol in mammalian cells?
A. Modulating membrane fluidity
B. Precursor for steroid hormone synthesis
C. Direct participation in electron transport chain
D. Precursor for bile acid synthesis
