MCAT — Practice Questions, Part 1: Biochemistry flashcards

6 of 70 questions free to study here · 24 topics · the whole deck is built into Willys

Multiple-choice practice questions, one best answer each, on amino acid charge and pI, protein structure and separation, hemoglobin, enzyme kinetics and inhibition, and metabolism.

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Practice deck · part of MCAT · written against: AAMC MCAT content outline (2015 exam, current through 2026). Updated 2026-10-10.

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A researcher wants to replace a surface aspartate on a soluble enzyme with a different residue while keeping the side chain's charge at pH 7.4. Which substitution best meets that goal?

A) Lysine
B) Glutamine
C) Asparagine
D) Glutamate

D) Glutamate

Glutamate also has a side-chain carboxyl group (pKa about 4) that is deprotonated and negative at pH 7.4. Asparagine is tempting because it is the amide of aspartate, but its –CONH₂ side chain is uncharged.
Side-Chain Classes
On SDS-PAGE without a reducing agent, a purified protein runs as one 120 kDa band. With β-mercaptoethanol added, it runs as one 60 kDa band. What is the most likely structure?

A) Two 60 kDa chains linked by a disulfide bond
B) Four 30 kDa chains linked by disulfide bonds
C) One 120 kDa chain with an internal disulfide bond
D) Two 60 kDa chains held together only noncovalently

A) Two 60 kDa chains linked by a disulfide bond

SDS alone already separates noncovalently associated subunits, so chains that stay together until a reducing agent is added must be joined by a disulfide. Breaking an internal disulfide would not halve the apparent mass of a single chain.
Separation & Analysis
An enzyme has a $V_{max}$ of 100 µmol/min and a $K_m$ of 1 mM. What is the initial rate when the substrate concentration is 4 mM?

A) 80 µmol/min
B) 75 µmol/min
C) 50 µmol/min
D) 25 µmol/min

A) 80 µmol/min

From $v = \frac{V_{max}[S]}{K_m + [S]}$ ⁠, $v = \frac{100 \times 4}{1 + 4} = 80$ µmol/min. 50 µmol/min would be the rate only when the substrate concentration equals $K_m$ ⁠.
Michaelis-Menten Kinetics
A cell line lacking lactate dehydrogenase is grown without oxygen. Glycolysis soon stops. At which step does it stall, and why?

A) Glyceraldehyde-3-phosphate dehydrogenase, for lack of NAD⁺
B) Pyruvate kinase, for lack of ADP
C) Hexokinase, from inhibition by glucose-6-phosphate
D) Phosphofructokinase-1, for lack of AMP

A) Glyceraldehyde-3-phosphate dehydrogenase, for lack of NAD⁺

Without the electron transport chain or fermentation to reoxidize NADH, NAD⁺ runs out and the glyceraldehyde-3-phosphate dehydrogenase step, which consumes NAD⁺, halts. Fermentation exists to regenerate that NAD⁺, not to make ATP.
Glycolysis
Why can liver glycogen help maintain blood glucose during fasting while muscle glycogen cannot?

A) Muscle lacks glucose-6-phosphatase
B) Muscle glycogen has no α-1,6 branches
C) Muscle lacks any glucose transporters
D) Muscle lacks glycogen phosphorylase

A) Muscle lacks glucose-6-phosphatase

Only liver (and kidney) can remove the phosphate from glucose-6-phosphate so free glucose can leave the cell. Muscle does break down its glycogen with phosphorylase, but keeps the glucose-6-phosphate for its own glycolysis.
Glycogen
Shortly after a carbohydrate-rich meal, insulin rises. How does insulin increase glucose uptake into skeletal muscle?

A) It activates glycogen phosphorylase in the muscle
B) It switches on gluconeogenesis in the muscle
C) It moves GLUT4 transporters to the cell membrane
D) It moves GLUT2 transporters to the cell membrane

C) It moves GLUT4 transporters to the cell membrane

Muscle and fat take up glucose mainly through GLUT4, which insulin moves to the surface. GLUT2, in liver and pancreatic β cells, is present all the time and is not the insulin-regulated transporter.
Fed vs Fasting Hormones

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Review status: Second-reader reviewed, with corrections applied.

Checked against:

OpenStax Biology 2e, OpenStax Anatomy and Physiology 2e and OpenStax Chemistry 2e, consulted as references; no text copied.
Facts cross-checked against Lehninger Principles of Biochemistry, 8th ed. (reference only; no text copied).

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