Km and Vmax
Km and Vmax are the two parameters you will be asked to read from kinetics plots and interpret in passages. They carry different information, and the MCAT will test whether you can tell them apart.
Vmax is About Enzyme Amount
Vmax is the ceiling - the maximum velocity the enzyme can achieve when substrate is so abundant that every active site is occupied at all times. Double the amount of enzyme and Vmax doubles. Half the enzyme and Vmax is halved. Vmax is proportional to [E]total.
Vmax is also set by how fast one enzyme molecule can turn over substrates - the turnover number kcat.
- kcat = number of substrate molecules converted per active site per second, at saturation
- [E]total = total enzyme concentration
Km is About Affinity, Not Amount
Km depends only on the identity of the enzyme and the substrate. It does NOT change when you add more enzyme. A passage that says “doubling [E] increased both Km and Vmax” is wrong - only Vmax responds to enzyme concentration.
Km changes only when:
- You change the enzyme (mutation, isoform).
- You change the substrate.
- You add something that changes the effective affinity, like a competitive inhibitor (which raises apparent Km) or a noncompetitive inhibitor (which does not).
Reading the Curve
As substrate rises, rate climbs then plateaus at Vmax — every enzyme is busy. Km is the [S] giving half-Vmax (lower Km = tighter binding). Competitive inhibitors raise Km (more substrate beats them); noncompetitive inhibitors lower Vmax (substrate can't).
To estimate Vmax by eye, look for where the curve flattens - that horizontal asymptote is Vmax. Drop down to half that value, then read left to the x-axis - that substrate concentration is Km.
Hexokinase vs. Glucokinase - A Classic Comparison
| Property | Hexokinase (muscle) | Glucokinase (liver) |
|----------|---------------------|---------------------|
| Km for glucose | ~0.1 mM (low, high affinity) | ~10 mM (high, low affinity) |
| Works at | Any blood glucose level | Only after a meal (high glucose) |
| Feedback | Inhibited by glucose-6-P (product) | Not inhibited by glucose-6-P |
| Role | Ensures muscle always has fuel | Buffers blood glucose, stores it |
kcat and the Specificity Constant
- kcat / Km is called the specificity constant. It measures how well an enzyme distinguishes between two competing substrates.
- A catalytically perfect enzyme has kcat/Km near M-1 s-1, meaning the rate is limited only by how fast substrate and enzyme can bump into each other (diffusion limit).
You do not need to calculate kcat/Km on the MCAT, but you should recognize that a bigger kcat/Km means the enzyme strongly prefers that substrate.