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MCAT Bio/Biochem study materials
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MCAT Bio/Biochem practice questions

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Question 1Biomolecules and the chemistry of living systems

Which bond is primarily responsible for linking amino acids into a polypeptide chain?

  1. A phosphodiester bond between the alpha-carboxyl group and the side chain
  2. A hydrogen bond between the alpha-carboxyl group and the alpha-amino group
  3. An amide bond between the alpha-carboxyl group and the alpha-amino group Correct answer.
  4. A glycosidic bond between the alpha-amino group and the side chain

Explanation

The peptide bond is an amide linkage formed by condensation between the alpha-carboxyl carbon of one amino acid and the alpha-amino nitrogen of the next, releasing water. It is a covalent bond, not a hydrogen bond, and involves the backbone rather than side chains.

Question 2Biomolecules and the chemistry of living systems

A molecule is a ketose containing six carbons. Which structure best describes it?

  1. A six-carbon sugar with an amino group replacing a hydroxyl
  2. A six-carbon sugar with a ketone at C2 and multiple hydroxyls Correct answer.
  3. A six-carbon sugar with a carboxyl group at C6
  4. A six-carbon sugar with an aldehyde at C1 and five hydroxyls

Explanation

Ketoses carry a ketone carbonyl, and in a six-carbon ketohexose such as fructose the carbonyl sits at C2 with hydroxyls on the remaining carbons. An aldehyde at C1 defines an aldose instead.

Question 3Biomolecules and the chemistry of living systems

Which interaction most directly drives the hydrophobic collapse of a globular protein in aqueous solution?

  1. Van der Waals contacts between polar and nonpolar side chains
  2. Ionic bonds between charged side chains on the protein surface
  3. The increase in entropy of water released from nonpolar surfaces Correct answer.
  4. Hydrogen bonding between backbone amides and water

Explanation

Burying nonpolar side chains releases ordered water into bulk solvent, raising the entropy of the system and lowering free energy. The hydrophobic effect is therefore entropy-driven rather than dominated by attraction between the nonpolar groups themselves.

Question 4Biomolecules and the chemistry of living systems

In a double-stranded DNA sample, adenine makes up 30 percent of the bases. What percentage of the bases is guanine?

  1. 40 percent
  2. 30 percent
  3. 70 percent
  4. 20 percent Correct answer.

Explanation

Chargaff's rules require A = T and G = C. With A at 30 percent, T is also 30 percent, leaving 40 percent split equally between G and C, so G is 20 percent.

Question 5Biomolecules and the chemistry of living systems

Which feature distinguishes the secondary structure of a protein from its tertiary structure?

  1. Secondary structure describes subunit assembly, whereas tertiary structure describes one chain
  2. Secondary structure involves covalent cross-links, whereas tertiary structure does not
  3. Secondary structure is local repeating backbone geometry, whereas tertiary structure is the overall three-dimensional fold Correct answer.
  4. Secondary structure requires chaperones, whereas tertiary structure forms spontaneously

Explanation

Secondary structure refers to recurring local patterns such as alpha helices and beta sheets stabilized by backbone hydrogen bonds. Tertiary structure is the global arrangement of all atoms in a single folded chain.

Question 6Biomolecules and the chemistry of living systems

A competitive inhibitor is added to an enzyme-catalyzed reaction. Which kinetic change is expected?

  1. Increased apparent Michaelis constant with unchanged maximum velocity Correct answer.
  2. Decreased apparent Michaelis constant with unchanged maximum velocity
  3. Decreased maximum velocity with decreased Michaelis constant
  4. Decreased maximum velocity with unchanged Michaelis constant

Explanation

A competitive inhibitor binds the free enzyme and can be outcompeted by high substrate, so maximum velocity is unchanged while the apparent Michaelis constant rises. It raises the substrate concentration needed to reach half-maximal rate.

Question 7Biomolecules and the chemistry of living systems

Which statement correctly describes the effect of a catalyst on a biochemical reaction?

  1. It raises the activation energy and shifts the equilibrium toward products
  2. It changes the standard free energy change of the reaction
  3. It lowers the activation energy and makes the reaction more exergonic
  4. It lowers the activation energy without changing the equilibrium constant Correct answer.

Explanation

Catalysts provide an alternative pathway with a lower activation energy but do not alter the free energy difference between reactants and products. Equilibrium position and standard free energy change therefore remain unchanged.

Question 8Biomolecules and the chemistry of living systems

Which property of water most directly explains its high specific heat capacity?

  1. Extensive hydrogen bonding among water molecules Correct answer.
  2. Its low molecular mass relative to other solvents
  3. Its low dielectric constant compared with organic solvents
  4. Its ability to dissolve nonpolar gases

Explanation

Breaking and reforming the extensive hydrogen-bond network requires substantial energy, so water absorbs large amounts of heat with modest temperature change. This network, not molecular mass, underlies the high specific heat.

Question 9Biomolecules and the chemistry of living systems

A fatty acid is described as unsaturated. What structural feature does this imply?

  1. One or more carbon-carbon double bonds in the hydrocarbon chain Correct answer.
  2. A fully saturated hydrocarbon chain with only single bonds
  3. An ester linkage to glycerol at both ends
  4. A carboxyl group esterified to a phosphate group

Explanation

Unsaturation refers to the presence of one or more carbon-carbon double bonds, which introduce kinks that reduce packing and lower melting temperature. Saturated chains contain only single bonds.

Question 10Biomolecules and the chemistry of living systems

Which amino acid side chain would most likely be found buried in the interior of a soluble globular protein at physiological pH?

  1. Aspartate
  2. Arginine
  3. Leucine Correct answer.
  4. Lysine

Explanation

Leucine has a nonpolar aliphatic side chain that is favorably buried away from water. Charged side chains such as lysine, arginine, and aspartate prefer the hydrated surface.

Question 11Biomolecules and the chemistry of living systems

In a reaction at equilibrium, which relationship holds between the free energy change and the reaction quotient?

  1. The free energy change is positive because the reaction has stopped
  2. The free energy change is always negative regardless of concentrations
  3. The free energy change equals zero because the reaction quotient equals the equilibrium constant Correct answer.
  4. The free energy change equals the standard free energy change at all times

Explanation

At equilibrium the reaction quotient equals the equilibrium constant, which makes the logarithmic term vanish and the actual free energy change zero. The standard free energy change may be nonzero, but it is not the quantity that is zero at equilibrium.

Question 12Biomolecules and the chemistry of living systems

Which feature of the DNA double helix is stabilized primarily by hydrophobic interactions rather than hydrogen bonds?

  1. The hydrogen bonding between guanine and cytosine
  2. The stacking of adjacent base pairs in the interior of the helix Correct answer.
  3. The pairing of adenine with thymine
  4. The covalent linkage of the sugar-phosphate backbone

Explanation

Base stacking buries the aromatic rings away from water and contributes substantial hydrophobic and van der Waals stabilization to helix formation. Base pairing itself relies on hydrogen bonds, and the backbone is covalent.

Question 13Biomolecules and the chemistry of living systems

A protein sample is treated with a reagent that reduces disulfide bonds. What level of structure is most directly disrupted?

  1. The hydrogen bonding pattern of alpha helices
  2. The tertiary structure stabilized by covalent cross-links Correct answer.
  3. The primary sequence of amino acids
  4. The hydrogen bonding between paired nucleotide strands

Explanation

Disulfide bonds are covalent cross-links between cysteine residues that stabilize the folded three-dimensional shape, so reducing them primarily disrupts tertiary structure. The primary sequence and backbone hydrogen bonding are not directly broken by reduction.

Question 14Biomolecules and the chemistry of living systems

Which observation would indicate that an enzyme follows Michaelis-Menten kinetics with a large Michaelis constant?

  1. The enzyme reaches maximal velocity at a very low substrate concentration
  2. The enzyme shows no dependence of rate on substrate concentration
  3. The enzyme reaches half-maximal velocity only at a high substrate concentration Correct answer.
  4. The enzyme's rate decreases as substrate concentration increases

Explanation

The Michaelis constant equals the substrate concentration at half-maximal velocity, so a large value means a high concentration is needed to half-saturate the enzyme. This reflects relatively weak substrate binding.

Question 15Biomolecules and the chemistry of living systems

Which structural difference distinguishes RNA from DNA?

  1. RNA contains deoxyribose and thymine in place of uracil
  2. RNA contains ribose and thymine but lacks uracil
  3. RNA contains ribose with a 2-prime hydroxyl and uracil in place of thymine Correct answer.
  4. RNA contains deoxyribose with a 2-prime hydroxyl and uracil

Explanation

RNA uses ribose, which has a hydroxyl at the 2-prime position, and pairs uracil with adenine instead of thymine. DNA uses deoxyribose and thymine.

Question 16Biomolecules and the chemistry of living systems

A solution contains a weak acid with a pKa of 5. At pH 6, which statement best describes the mixture?

  1. The protonated form predominates by roughly tenfold
  2. The two forms are present in equal amounts
  3. The acid is fully protonated regardless of pH
  4. The deprotonated form predominates by roughly tenfold Correct answer.

Explanation

When pH exceeds pKa by one unit, the Henderson-Hasselbalch relationship gives a base-to-acid ratio of about ten, so the deprotonated form predominates. Equal amounts occur only when pH equals pKa.

Question 17Biomolecules and the chemistry of living systems

Which type of bond connects the phosphate group to the sugar in a nucleotide?

  1. A peptide bond
  2. A glycosidic bond
  3. A phosphodiester bond
  4. A phosphoester bond Correct answer.

Explanation

Within a single nucleotide, the phosphate is joined to the sugar by a phosphoester linkage. Phosphodiester bonds instead connect successive nucleotides in a polymer.

Question 18Biomolecules and the chemistry of living systems

Which characteristic of a phospholipid bilayer arises directly from the amphipathic nature of its components?

  1. The covalent cross-linking of adjacent lipid tails
  2. The spontaneous formation of a sealed compartment with polar head groups facing water Correct answer.
  3. The complete exclusion of water from both surfaces of the bilayer
  4. The uniform distribution of cholesterol in the hydrophobic core

Explanation

Amphipathic lipids bury their hydrophobic tails away from water while polar head groups face the aqueous environment, which favors sealed bilayer structures. This self-assembly is driven by the hydrophobic effect.

Question 19Biomolecules and the chemistry of living systems

An enzyme-catalyzed reaction has a standard free energy change that is positive. What can be concluded about the reaction under standard conditions?

  1. It cannot proceed in either direction under any conditions
  2. It must be coupled to ATP hydrolysis to occur at all
  3. It is nonspontaneous as written under standard conditions Correct answer.
  4. Its equilibrium constant is greater than one

Explanation

A positive standard free energy change means the reaction is nonspontaneous under standard conditions, corresponding to an equilibrium constant below one. It can still proceed if cellular concentrations make the actual free energy change negative.

Question 20Biomolecules and the chemistry of living systems

Which amino acid is unique in having its side chain covalently linked to the backbone nitrogen, restricting its conformational flexibility?

  1. Serine
  2. Glycine
  3. Proline Correct answer.
  4. Cysteine

Explanation

Proline's side chain cyclizes onto the backbone nitrogen, forming a rigid ring that limits rotation about the peptide bond. This property makes proline a common helix breaker.

Question 21Biomolecules and the chemistry of living systems

A researcher measures reaction velocity at several substrate concentrations and finds that velocity increases linearly without saturating even at very high substrate levels. Which explanation is most consistent with this result?

  1. The substrate concentration is far below the Michaelis constant throughout the range Correct answer.
  2. The enzyme has a very small Michaelis constant and is saturated throughout
  3. The enzyme concentration exceeds the substrate concentration at all points
  4. The enzyme is subject to strong product inhibition

Explanation

When substrate is well below the Michaelis constant, the Michaelis-Menten equation approximates a linear dependence on substrate, so no plateau is observed. Saturation appears only as substrate approaches and exceeds the Michaelis constant.

Question 22Biomolecules and the chemistry of living systems

Two enzymes catalyze the same reaction but have different Michaelis constants, with enzyme A having the smaller value. Assuming similar maximum velocities, which enzyme is more efficient at low substrate concentration?

  1. Neither, because efficiency depends only on activation energy
  2. Enzyme B, because a larger Michaelis constant indicates tighter binding
  3. Enzyme A, because it reaches a given fraction of maximal velocity at lower substrate Correct answer.
  4. Both are equally efficient because maximum velocity is the same

Explanation

A smaller Michaelis constant indicates tighter substrate binding, so the enzyme approaches half-maximal velocity at a lower substrate concentration. At low substrate, this corresponds to greater fractional saturation and higher efficiency.

Question 23Biomolecules and the chemistry of living systems

A mutation replaces a surface lysine with glutamate in a protein that normally functions at pH 7. Which change in net charge is expected?

  1. The protein becomes more negatively charged by two units Correct answer.
  2. The protein becomes more positively charged by one unit
  3. The protein becomes more positively charged by two units
  4. The net charge is unchanged because both residues are polar

Explanation

At pH 7, lysine carries a positive charge and glutamate carries a negative charge, so replacing one with the other shifts the net charge by two units toward negative. Charge, not polarity alone, determines the electrostatic effect.

Question 24Biomolecules and the chemistry of living systems

In a beta sheet, which arrangement of hydrogen bonds is observed?

  1. Hydrogen bonds between backbone amides spaced four residues apart in a helix
  2. Hydrogen bonds between the phosphate groups of the backbone
  3. Hydrogen bonds between side chains within a single strand
  4. Hydrogen bonds between backbone amides of adjacent strands Correct answer.

Explanation

Beta sheets are stabilized by hydrogen bonds between backbone amide groups of neighboring strands, which may be parallel or antiparallel. Side chains project alternately above and below the sheet and are not the source of the interstrand bonding.

Question 25Biomolecules and the chemistry of living systems

A buffer is prepared from a weak acid whose pKa is 4.8. Which pH range provides the greatest buffering capacity?

  1. A range centered near pH 6.8
  2. A range centered near pH 4.8 Correct answer.
  3. A range centered near pH 3.8
  4. A range centered near pH 7.4

Explanation

Buffering capacity is greatest when pH is close to the pKa, where the acid and conjugate base are present in comparable amounts. The effective range typically spans about one pH unit on either side of the pKa.

Question 26Biomolecules and the chemistry of living systems

Which interaction is primarily responsible for the secondary structure of a protein?

  1. Hydrophobic clustering of nonpolar side chains
  2. Ionic bonding between oppositely charged side chains
  3. Hydrogen bonding between backbone amide and carbonyl groups Correct answer.
  4. Covalent disulfide linkages between cysteine residues

Explanation

Secondary structure (alpha helices and beta sheets) arises from regular hydrogen bonds between the backbone amide nitrogen and carbonyl oxygen, independent of side chain identity. The other interactions stabilize tertiary or quaternary structure.

Question 27Biomolecules and the chemistry of living systems

Which functional group gives a molecule its acidic character by donating a proton?

  1. Amino group
  2. Amide group
  3. Carboxyl group Correct answer.
  4. Hydroxyl group on an alcohol

Explanation

Carboxyl groups (-COOH) readily donate a proton to form a carboxylate, making them the principal acidic groups in biomolecules. Amino groups act as bases, and amides are essentially neutral.

Question 28Biomolecules and the chemistry of living systems

A triglyceride is hydrolyzed completely. Which products are formed?

  1. Glycerol and three phosphate groups
  2. Three amino acids and one glycerol
  3. Glucose and three fatty acid molecules
  4. Glycerol and three fatty acid molecules Correct answer.

Explanation

A triglyceride is a triester of glycerol with three fatty acids, so complete hydrolysis releases one glycerol and three fatty acid molecules. Phosphates and amino acids are not components of triglycerides.

Question 29Biomolecules and the chemistry of living systems

Which base pairing is correct for complementary strands of DNA?

  1. Adenine pairs with thymine via two hydrogen bonds Correct answer.
  2. Adenine pairs with guanine via three hydrogen bonds
  3. Guanine pairs with adenine via two hydrogen bonds
  4. Cytosine pairs with thymine via two hydrogen bonds

Explanation

In DNA, adenine pairs with thymine through two hydrogen bonds and guanine pairs with cytosine through three hydrogen bonds, following complementary base-pairing rules.

Question 30Biomolecules and the chemistry of living systems

Which class of biological macromolecule is built from amino acids linked by peptide bonds?

  1. Proteins Correct answer.
  2. Nucleic acids
  3. Triglycerides
  4. Polysaccharides

Explanation

Proteins are polymers of amino acids joined by peptide bonds formed between the carboxyl group of one amino acid and the amino group of the next. The other macromolecules are built from different monomers.

Question 31Biomolecules and the chemistry of living systems

Which statement best describes the role of an enzyme in lowering the activation energy of a reaction?

  1. It stabilizes the transition state more than the substrate Correct answer.
  2. It permanently consumes the substrate
  3. It raises the free energy of the products
  4. It increases the equilibrium constant of the reaction

Explanation

Enzymes lower activation energy by binding and stabilizing the transition state preferentially relative to the substrate, providing an alternative lower-energy pathway. They do not change the overall free energy change or the equilibrium constant.

Question 32Biomolecules and the chemistry of living systems

Which property of the peptide bond explains the planar geometry of the polypeptide backbone?

  1. Partial double-bond character from resonance Correct answer.
  2. Complete rotation around the C-N bond
  3. Ionic attraction between adjacent residues
  4. Hydrogen bonding between side chains

Explanation

The peptide bond has partial double-bond character due to resonance between the carbonyl oxygen and the amide nitrogen, restricting rotation and forcing a planar arrangement.

Question 33Biomolecules and the chemistry of living systems

Which molecule serves as the primary short-term energy currency in cells?

  1. NADH
  2. ATP Correct answer.
  3. Pyruvate
  4. Glucose

Explanation

ATP is the principal energy currency, transferring a phosphoryl group to drive endergonic reactions. NADH and glucose are energy carriers or fuels but not the direct currency for most cellular work.

Question 34Biomolecules and the chemistry of living systems

A protein has a high content of alpha helix. Which amino acid would most disrupt this helix if placed in the middle of the helical segment?

  1. Leucine
  2. Alanine
  3. Glutamate
  4. Proline Correct answer.

Explanation

Proline's side chain is covalently linked to the backbone nitrogen, creating a kink that breaks helical hydrogen bonding and is known as a helix breaker. Alanine, leucine, and glutamate readily accommodate helical geometry.

Question 35Biomolecules and the chemistry of living systems

A polypeptide has a net charge of +3 at pH 5 and a net charge of -1 at pH 9. Approximately where is its isoelectric point?

  1. Exactly at pH 7
  2. Between pH 5 and pH 9, closer to pH 9 Correct answer.
  3. Below pH 5
  4. Above pH 9

Explanation

The isoelectric point lies between the two pH values where the net charge changes sign, and its position is weighted toward the pH where the net charge is closer to zero. Since the net charge at pH 9 (-1) is closer to zero than at pH 5 (+3), the pI is between them but nearer pH 9.

Question 36Biomolecules and the chemistry of living systems

An enzyme has a Michaelis constant of 0.5 mM. At a substrate concentration of 0.5 mM, what fraction of the maximum velocity is observed?

  1. One quarter
  2. One half Correct answer.
  3. All of the maximum velocity
  4. One tenth

Explanation

When substrate concentration equals the Michaelis constant, the Michaelis-Menten equation gives v = Vmax/2, so half of maximum velocity is observed. This is the defining relationship of the Michaelis constant.

Question 37Biomolecules and the chemistry of living systems

A molecule of DNA contains 30 percent adenine. What percentage of its bases is guanine?

  1. 40 percent
  2. 70 percent
  3. 30 percent
  4. 20 percent Correct answer.

Explanation

By complementary base pairing, adenine equals thymine, so thymine is also 30 percent, leaving 40 percent for guanine plus cytosine. Since guanine equals cytosine, guanine is 20 percent.

Question 38Biomolecules and the chemistry of living systems

Two solutions are separated by a membrane permeable only to water. Solution A contains 0.2 M solute and solution B contains 0.5 M solute. In which direction does water move?

  1. From A to B Correct answer.
  2. Equally in both directions with no net movement
  3. Water cannot cross the membrane
  4. From B to A

Explanation

Water moves by osmosis from the region of lower solute concentration (higher water concentration) to the region of higher solute concentration. Solution A is more dilute, so water flows from A to B.

Question 39Biomolecules and the chemistry of living systems

A reaction has a negative standard free energy change. Which statement about the reaction is correct?

  1. It cannot occur in a living cell
  2. It has a positive equilibrium constant Correct answer.
  3. It is spontaneous under standard conditions
  4. It must be catalyzed by an enzyme to proceed

Explanation

A negative standard free energy change corresponds to a positive equilibrium constant, since the two are related through the standard free energy equation. The reaction is thermodynamically favorable under standard conditions.

Question 40Biomolecules and the chemistry of living systems

Which structural feature of a phospholipid allows it to form bilayers in water?

  1. A hydrophilic head group and two hydrophobic tails Correct answer.
  2. Two hydrophilic head groups and no tails
  3. A single hydrophobic head and a hydrophilic tail
  4. Only hydrophobic groups throughout the molecule

Explanation

Phospholipids are amphipathic, combining a polar, water-loving head with nonpolar, water-fearing tails. In water, the tails associate away from water while the heads face the aqueous environment, producing a bilayer.

Question 41Biomolecules and the chemistry of living systems

A researcher measures the rate of an enzyme-catalyzed reaction at increasing substrate concentrations and observes a hyperbolic curve that plateaus. What does the plateau value represent?

  1. The rate at which the enzyme is denatured
  2. The rate when substrate concentration is zero
  3. The Michaelis constant of the enzyme
  4. The maximum velocity when all enzyme active sites are saturated Correct answer.

Explanation

Hyperbolic saturation kinetics indicate that all active sites become occupied with substrate, so the rate approaches a maximum velocity. The Michaelis constant is the substrate concentration at half of this maximum, not the plateau itself.

Question 42Biomolecules and the chemistry of living systems

Which change would most increase the fluidity of a cell membrane at low temperature?

  1. Increasing the proportion of long saturated fatty acid tails
  2. Increasing the proportion of unsaturated fatty acid tails Correct answer.
  3. Decreasing the number of phospholipid molecules
  4. Adding cholesterol at very high concentrations

Explanation

Cis double bonds in unsaturated tails create kinks that prevent tight packing, maintaining membrane fluidity at low temperatures. Long saturated tails pack tightly and reduce fluidity.

Question 43Biomolecules and the chemistry of living systems

Amino acids are joined into a polypeptide. Which atoms form the peptide bond?

  1. The carboxyl carbon of one amino acid and the amino nitrogen of the next Correct answer.
  2. The carboxyl carbon of one amino acid and the carboxyl carbon of the next
  3. The side chain of one amino acid and the backbone of the next
  4. The amino nitrogen of one amino acid and the amino nitrogen of the next

Explanation

A peptide bond forms by condensation between the alpha-carboxyl group of one amino acid and the alpha-amino group of the next, releasing water. Side chains are not directly involved in the linkage.

Question 44Biomolecules and the chemistry of living systems

A protein is treated with urea, which disrupts hydrogen bonding, and beta-mercaptoethanol, which reduces disulfide bonds. Which outcome is expected?

  1. Conversion of all alpha helices into beta sheets
  2. Loss of tertiary and quaternary structure with retention of the primary sequence Correct answer.
  3. Loss of the primary structure only
  4. Complete hydrolysis of the polypeptide into free amino acids

Explanation

Denaturants that disrupt hydrogen bonds and disulfide linkages unfold the protein, abolishing tertiary and quaternary structure while leaving peptide bonds and the primary sequence intact.

Question 45Biomolecules and the chemistry of living systems

A solution of a weak acid with a pKa of 6.0 is at pH 6.0. Which statement about the solution is correct?

  1. The conjugate base concentration is ten times the acid concentration
  2. The solution has no buffering capacity
  3. The concentrations of the acid and its conjugate base are equal Correct answer.
  4. The acid is fully deprotonated

Explanation

By the Henderson-Hasselbalch relationship, when pH equals pKa, the ratio of conjugate base to acid is one, so their concentrations are equal. This is also the point of maximum buffering capacity.

Question 46Biomolecules and the chemistry of living systems

A competitive inhibitor binds the active site of an enzyme. How does it affect the apparent Michaelis constant and maximum velocity?

  1. It increases the apparent Michaelis constant while leaving maximum velocity unchanged Correct answer.
  2. It decreases the apparent Michaelis constant and decreases maximum velocity
  3. It increases both the Michaelis constant and maximum velocity
  4. It leaves the Michaelis constant unchanged and decreases maximum velocity

Explanation

A competitive inhibitor competes with substrate for the active site, so more substrate is needed to reach half-maximal velocity, raising the apparent Michaelis constant. Because inhibition can be overcome at high substrate, maximum velocity is unchanged.

Question 47Biomolecules and the chemistry of living systems

A mutation replaces a buried hydrophobic leucine with a charged aspartate in the core of a globular protein. What is the most likely consequence?

  1. No effect, since all amino acids are interchangeable
  2. Enhanced hydrophobic packing that stabilizes the core
  3. Destabilization of the folded structure due to unfavorable desolvation of the charge Correct answer.
  4. Improved disulfide bonding in the core

Explanation

Buried charged groups are energetically costly because the low-dielectric protein interior cannot stabilize the charge as water does, so folding is destabilized. Hydrophobic residues are normally favored in the core.

Question 48Biomolecules and the chemistry of living systems

An enzyme follows Michaelis-Menten kinetics with a Michaelis constant of 2 mM and a maximum velocity of 100 micromolar per second. At a substrate concentration of 6 mM, what is the velocity?

  1. 50 micromolar per second
  2. 33 micromolar per second
  3. 100 micromolar per second
  4. 75 micromolar per second Correct answer.

Explanation

Using the Michaelis-Menten equation, v = Vmax times substrate divided by the sum of the Michaelis constant and substrate. Here v = 100 times 6 divided by 8, which equals 75 micromolar per second.

Question 49Biomolecules and the chemistry of living systems

A double-stranded DNA segment has the sequence 5'-GATTACA-3' on one strand. What is the sequence of the complementary strand written 5' to 3'?

  1. 5'-CTAATGT-3'
  2. 5'-TGTAATC-3' Correct answer.
  3. 5'-GATTACA-3'
  4. 5'-CUUACUG-3'

Explanation

The complementary strand is antiparallel, so it is formed by pairing each base and reversing orientation. Pairing GATTACA gives CTAATGT read antiparallel, which written 5' to 3' is TGTAATC.

Question 50Biomolecules and the chemistry of living systems

A reaction is at equilibrium. Which change would alter the equilibrium constant of the reaction?

  1. Increasing the concentration of reactants
  2. Changing the temperature Correct answer.
  3. Adding a catalyst
  4. Increasing the concentration of products

Explanation

The equilibrium constant depends on temperature because it is related to the standard free energy change, which is temperature dependent. Catalyst and concentration changes shift position or rate but not the constant.