Enzymes live life in the fast lane! Enzyme kinetics measures how fast an enzyme turns substrate into product, and that speed depends on more than the enzyme itself.
How much enzyme there is, how much substrate is around and how tightly the two bind all change the rate.
We’ll cover enzyme kinetics on the MCAT from the ground up here, with practice questions at the end. It’ll also set you up for enzyme inhibitors, which build on everything here. Let’s get started!
What Is Enzyme Kinetics on the MCAT?
Enzyme kinetics looks at the rate (v) of an enzyme-catalyzed reaction, usually the initial rate, measured before much product builds up. You’ll see it as curves, straight-line plots and tables of constants.
The AAMC’s content outline lists enzyme kinetics, Michaelis-Menten and cooperativity under both Bio/Biochem and Chem/Phys, and it names all four inhibition types. Our AAMC outline checklist lays the whole list out!
Questions often ask you to read a graph or predict how a change shifts Km or Vmax. A quick plug-in like kcat = Vmax/[E]T can show up too, so we’ll walk through the math as well 🙂
If rate laws feel rusty, our chemical kinetics notes make a good warm-up, and our enzyme structure and function notes cover active sites!
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Enzyme Kinetics Equations for the MCAT
Every equation on this page comes from one model. Enzyme (E) binds substrate (S) to form the enzyme-substrate complex (ES), which either falls apart or turns over into product (P).
Each arrow has a rate constant, with k1 for E and S binding, k−1 for ES falling back apart and kcat for ES turning into product.
[S] is the substrate concentration, and [E]T is the total enzyme, free E plus ES.
The Michaelis-Menten Equation
The Michaelis-Menten equation gives the reaction rate at any substrate concentration.
It assumes there’s far more substrate than enzyme and that you measure the rate early, before product builds up. With the amount of enzyme fixed, you’ll get a hyperbola when you plot the rate against [S].
What Km tells you
Plug in [S] = Km, and the equation gives v = ½Vmax. So Km is the substrate concentration that fills half the active sites and gives half the maximum rate!
The MCAT treats Km as a measure of affinity. A low Km means the enzyme reaches half speed with only a little substrate, so its affinity is high. A high Km means low affinity!
Low substrate and high substrate
When [S] is far below Km, the rate climbs almost in a straight line with [S]. It’s roughly first order in substrate.
When [S] is far above Km, nearly every active site is full and the rate levels off near Vmax. Extra substrate barely changes it, so it’s close to zero order.
Here’s a handy check. [S] = 3Km gives 75% of Vmax, since 3Km / (Km + 3Km) = 3/4.
The kcat Equation (Turnover Number)
kcat is the turnover number. It’s the number of substrate molecules one enzyme molecule turns into product each second when it’s saturated, so its unit is s−1.
The kcat formula comes from the saturated enzyme. When every active site is full, all the enzyme’s in the ES form and the rate hits its ceiling. Rearranged, that’s Vmax = kcat[E]T.
The name comes from the catalytic step, ES → E + P. In the full model, Km includes kcat too, since Km = (k−1 + kcat) / k1.
An enzyme sample at a total concentration of 0.010 µM has a Vmax of 5.0 µM/s.
So kcat = 5.0 µM/s ÷ 0.010 µM = 500 s−1. Each enzyme turns over 500 substrate molecules every second at saturation!
Catalytic Efficiency Formula (kcat/Km)
Catalytic efficiency combines speed and affinity into one number. A high kcat (fast turnover) and a low Km (high affinity) both raise it!
Its unit is M−1s−1. It’s also the rate constant you see at low substrate, where the Michaelis-Menten equation shrinks to v ≈ (kcat/Km)[E]T[S].
Back to the enzyme above. If its Km is 25 µM, its catalytic efficiency is 500 s−1 ÷ 25 µM = 20 µM−1s−1, or 2.0 × 107 M−1s−1.
A high kcat paired with a very high Km can still give a low efficiency. Work out kcat/Km for each enzyme and compare those numbers!
Reading a Lineweaver-Burk Plot
A Lineweaver-Burk plot is a double reciprocal plot. Take 1 over both sides of the Michaelis-Menten equation, and the curve turns into a straight line!
Plot 1/v on the y-axis against 1/[S] on the x-axis. Each part of the line gives you a constant!
- The y-intercept is 1/Vmax
- The x-intercept is −1/Km
- The slope is Km/Vmax
![Lineweaver-Burk plot of 1/v against 1/[S]: the y-intercept is 1/Vmax, the x-intercept is -1/Km and the slope is Km/Vmax](https://medlifemastery.com/wp-content/uploads/2023/01/lineweaver-burk-plot-intercepts.png "<strong>Enzyme Kinetics on the MCAT</strong> 3")
Your measured points all sit right of the y-axis, since 1/[S] can’t be negative. Extend the line to the left to read the x-intercept.
A quick rule for intercepts
The closer an intercept sits to the origin, the larger the value it stands for. An x-intercept near the origin means a large Km (low affinity), and a y-intercept near the origin means a large Vmax!
Enzyme Inhibition on a Lineweaver-Burk Plot
Inhibitors change Km, Vmax or both, and each reversible type leaves its own fingerprint on a Lineweaver-Burk plot. You’ll often get the plot and be asked to name the inhibitor!
Competitive
A competitive inhibitor looks enough like the substrate to sit in the active site. Add enough substrate and it gets outcompeted, so Vmax doesn’t change. You’ll need more substrate to reach half speed, so Km goes up.
Noncompetitive
A noncompetitive inhibitor binds an allosteric site on E or ES with the same affinity. Extra substrate can’t win that enzyme back, so Vmax drops.
Any enzyme left working binds substrate as well as ever, so Km holds steady.
Uncompetitive
An uncompetitive inhibitor binds only the ES complex and locks the substrate in. Km and Vmax both drop by the same factor, so the slope (Km/Vmax) stays the same and the lines run parallel!
Mixed
A mixed inhibitor binds both E and ES but prefers one. If it prefers free enzyme, Km rises, and if it prefers ES, Km falls. Vmax drops either way!
The lines cross left of the y-axis. The crossing sits above the x-axis when the inhibitor prefers E and below it when the inhibitor prefers ES.
Inhibitor strength is often given as Ki, the dissociation constant of the enzyme-inhibitor complex. A lower Ki means tighter binding, so the inhibitor is more potent!
Our reversible inhibition notes go deeper on each type, and our other forms of enzyme inhibition notes pick up from there!
To see how top scorers pull these plots apart under time pressure, our Chem/Phys & Bio/Biochem Mastery Course has 100+ strategy video lessons from 130+ C/P and B/B scorers, including 70+ AAMC passage walkthroughs.
Cooperativity, Sigmoidal Curves and the Hill Coefficient
Some enzymes have several subunits, each with its own active site. When substrate binds one site, it can change how easily the other sites bind. That’s cooperativity, and it bends the rate curve into a sigmoidal (S-shaped) curve!
The Hill coefficient puts a number on it. Above 1 means positive cooperativity, so one bound substrate raises the other sites’ affinity and the rate jumps over a narrow range of [S]. Below 1 means negative cooperativity, and a value of 1 means none.
The Michaelis-Menten equation doesn’t describe a sigmoidal curve. The [S] at half Vmax still tells you about affinity, though it’s usually written K0.5 for these enzymes.
Allosteric enzymes that control a pathway, like phosphofructokinase-1 in glycolysis, often show this curve. Hemoglobin’s the classic example of positive cooperativity, even though it carries oxygen and isn’t an enzyme!
See a sigmoidal curve of rate against [S]? Think multiple subunits, cooperativity and a Hill coefficient above 1.
Enzyme Kinetics MCAT Practice Problems
You’ll get the most out of these if you try each one before you open the answer. Reading the graph or table first is half the battle!
Question 1
The Lineweaver-Burk plot below shows two enzymes, A and B, acting on the same substrate. Which statement best describes how enzyme A differs from enzyme B?
- Enzyme B has a higher binding affinity for the substrate.
- Enzyme A has a lower maximum reaction rate.
- Enzyme B has a lower Km and a lower Vmax.
- Enzyme A has a higher Km and a higher Vmax.
Show the answer
The answer is B. Enzyme A’s y-intercept sits farther from the origin, so its 1/Vmax is larger and its Vmax is lower.
Its x-intercept also sits farther from the origin, so its Km is lower and its affinity is higher. Enzyme B has the higher Km and the higher Vmax, so A, C and D don’t fit.
Question 2
The table gives Km and kcat for a wild-type enzyme and four mutants, each with one amino acid swapped. Which mutation lowers the enzyme’s catalytic efficiency the most?
- Y228F
- E162L
- A73M
- H90K
Show the answer
The answer is B. Catalytic efficiency is kcat/Km, so look for the mutant that raises Km and lowers kcat the most.
E162L raises Km from 26 to 218 and drops kcat from 4,245 to 0.020. Its kcat/Km falls from about 163 to about 0.00009, and none of the other mutants comes close!
Question 3
A drug raises an enzyme’s apparent Km and leaves Vmax unchanged. On a Lineweaver-Burk plot with and without the drug, where do the two lines cross?
- On the y-axis
- On the x-axis
- Nowhere, since the lines are parallel
- Left of the y-axis and below the x-axis
Show the answer
The answer is A. A higher Km with the same Vmax is competitive inhibition. Both lines share the same 1/Vmax, so they cross at the y-intercept!
Question 4
A 2.0 nM enzyme solution has a Vmax of 1.0 µM/s. What is kcat?
- 0.50 s−1
- 2.0 s−1
- 500 s−1
- 2,000 s−1
Show the answer
The answer is C. Convert first, so 2.0 nM is 0.0020 µM. Then kcat = Vmax/[E]T = 1.0 µM/s ÷ 0.0020 µM = 500 s−1.
Skip the unit change and you’ll get 0.50, choice A. Watch those units, since they’re easy to miss!
The Supplemental Resources at the bottom of this page list official AAMC questions on this topic too!
(Optional) If you’d like to work through passage-style kinetics questions with a top scorer beside you, our 6-Week Live MCAT Accelerator runs 12 live strategy classes taught by a 520+ scorer.
Enzyme Kinetics Terms to Know
Enzyme Kinetics MCAT FAQs
What is the kcat equation?
The kcat equation is kcat = Vmax / [E]T, where [E]T is the total enzyme concentration. It’s the turnover number, the substrate molecules each enzyme converts per second at saturation, so its unit is s−1.
What is the formula for catalytic efficiency?
Catalytic efficiency = kcat / Km, in M−1s−1. A higher kcat or a lower Km raises it!
Does a low Km mean high affinity?
Yes, on the MCAT. Km is the substrate concentration at half of Vmax, so an enzyme with a low Km gets to half speed with less substrate.
What does a Hill coefficient greater than 1 mean?
Positive cooperativity. Binding one substrate makes the other sites bind more easily, and the rate curve turns sigmoidal!
How do you tell inhibitors apart on a Lineweaver-Burk plot?
Find where the lines cross. On the y-axis means competitive, on the x-axis means noncompetitive, parallel lines mean uncompetitive and a crossing off both axes means mixed!
Is enzyme kinetics on the MCAT?
Yes! The AAMC’s content outline lists enzyme kinetics, Michaelis-Menten and cooperativity in both Bio/Biochem and Chem/Phys, along with the four inhibition types.
Final Thoughts on Enzyme Kinetics on the MCAT
Enzyme kinetics on the MCAT comes down to a handful of relationships. Km is the [S] at half Vmax, kcat is Vmax per enzyme, and the Lineweaver-Burk lines tell you which inhibitor you’re looking at!
If you’d like help turning these into points, our 6-Week Live MCAT Accelerator pairs live strategy classes with seven 1:1 sessions with a 515+ scorer.
If the cohort dates don’t line up with your test, 1:1 MCAT tutoring with a 515+ scorer works at your own pace.
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More Enzyme Study Notes
- Enzyme Models & Classification on the MCAT
- Enzyme Structure & Function on the MCAT
- Enzyme Reversible Inhibition on the MCAT
- Enzyme Inhibition: Other Forms on the MCAT
- Enzymes on the MCAT (chapter overview)
- The Enzyme Kinetics & Inhibitors cheat sheet, free with our MCAT Cheat Sheets
Supplemental Resources
Want to go deeper on this topic? Here's where to read, watch and practice it next.
Read
- Kaplan MCAT Biochemistry Review 2026-2027, §2.3
Watch
- “Steady states and the Michaelis Menten equation” (Khan Academy)
- “An introduction to enzyme kinetics” (Khan Academy)
Practice passages
- “Phenylalanine hydroxylase and phenylketonuria” (5 of its 6 questions test this topic)
- “NTKase and NTKase deficiency” (3 of its 6 questions test this topic)
- “PMM/PGM and its substrates” (question 2 of 6)
- “Acetylcholinesterase catalysis and inhibition” (question 6 of 6)
Official AAMC practice that tests this topic
- Bio/Biochem Section Bank, Q73
- Chem/Phys Section Bank, Passage 4, Q30
- Chem/Phys Section Bank, Q48
- Chem/Phys Section Bank, Q49
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