MCAT Biochemistry — Enzymes & Kinetics flashcards

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Enzyme classes, cofactors, Michaelis–Menten and Lineweaver–Burk reasoning, the four inhibition types and their effects on Km and Vmax, allostery and cooperativity, zymogens, covalent modification, feedback inhibition and the difference between spontaneity and rate.

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How does an enzyme increase the rate of a reaction without changing where its equilibrium lies?

It lowers the activation energy by binding and stabilizing the transition state. Both the forward and reverse barriers drop by the same amount, so the equilibrium constant and $\Delta G$ are untouched; equilibrium is simply reached sooner.
Enzyme Basics
What are the six classical enzyme classes, in EC-number order?

1 oxidoreductases, 2 transferases, 3 hydrolases, 4 lyases, 5 isomerases, 6 ligases.

Watch out: a seventh class, the translocases (EC 7), was added in 2018; the AAMC content outline asks only for enzyme classification by reaction type and names no list of classes.
Enzyme Classes
How do cofactors, coenzymes and prosthetic groups relate to one another?

Cofactor is the umbrella term for any non-protein helper an enzyme needs, whether a metal ion or an organic molecule. A coenzyme is an organic cofactor, often derived from a vitamin, and a prosthetic group is a cofactor that stays tightly or covalently bound to the enzyme.
Cofactors & Coenzymes
What does the Michaelis constant $K_m$ represent, and how is it related to affinity?

$K_m$ is the substrate concentration at which the rate is half of $V_{max}$ ⁠. When substrate binding is at rapid equilibrium it serves as an inverse measure of affinity: a lower $K_m$ means the enzyme is half-saturated at a lower substrate concentration, that is, higher apparent affinity.
Michaelis-Menten Kinetics
What is plotted on each axis of a Lineweaver–Burk plot, and what do the slope and intercepts give?

It plots $1/v$ against $1/[S]$ ⁠, which turns the Michaelis–Menten curve into a straight line. The y-intercept is $1/V_{max}$ ⁠, the x-intercept is $-1/K_m$ ⁠, and the slope is $K_m/V_{max}$ ⁠.
Lineweaver-Burk Plots
How does a competitive inhibitor affect $K_m$ and $V_{max}$ ⁠, and what is a classic drug example?

It binds the active site of the free enzyme, so the apparent $K_m$ rises while $V_{max}$ is unchanged; enough substrate will outcompete it.

Example: statins compete with HMG-CoA for HMG-CoA reductase, and methotrexate competes with dihydrofolate for dihydrofolate reductase.
Inhibition
What is an allosteric enzyme, and how does its rate curve differ from a Michaelis–Menten enzyme's?

An enzyme regulated by molecules binding at a site other than the active site, usually with several subunits whose conformations are coupled. Its velocity-versus-substrate curve is sigmoidal rather than hyperbolic because substrate binding is cooperative.
Allostery & Cooperativity
What is a zymogen, and why are digestive proteases produced in that form?

An inactive enzyme precursor that is switched on by proteolytic cleavage. Making pepsinogen, trypsinogen and chymotrypsinogen as zymogens lets the stomach and pancreas store protein-digesting enzymes without digesting themselves.
Zymogens & Covalent Modification
What is feedback inhibition, and which step of a pathway does it usually target?

The end product of a pathway inhibits an enzyme near the start of the pathway, usually allosterically, so production slows once enough product has accumulated. The usual target is the committed step, the first irreversible step that is unique to the pathway.
Feedback Regulation
Does an enzyme change the $\Delta G$ of the reaction it catalyzes?

No. $\Delta G$ depends only on the free energies of reactants and products, which the enzyme does not touch; it changes only the activation energy, ΔG‡, and hence the rate.
Thermodynamics vs Kinetics

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Checked against:

OpenStax Biology 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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