I can rearrange Michaelis Menten and read a Lineweaver Burk plot, but I could not explain to someone what Km is a measure of. Calling it the substrate concentration at half Vmax is a definition rather than a meaning.
What are these two quantities telling me about an enzyme?
Accepted Answer
Vmax is a capacity. Km is an affinity, read backwards.
Vmax is the rate when every active site is occupied, so it tells you how fast this enzyme can possibly go at the amount of enzyme present. Doubling the enzyme doubles Vmax.
Km is the substrate concentration at which half the sites are filled. A low Km means the enzyme reaches half capacity at low substrate, so it grabs substrate readily. A high Km means it needs a lot of substrate to get going.
So low Km equals high affinity, which is the part that reads backwards and is worth drilling, because the exam tests exactly that inversion.
Km does not change with enzyme concentration, which is the other half of the contrast with Vmax.
One correction, because it is the standard overstatement and gets asked about.
Km is only an affinity measure when the catalytic step is slow relative to the substrate letting go. Formally Km is (k₋₁ + k₂)/k₁, and it reduces to the dissociation constant only when k₂ is small.
For a very fast enzyme, Km can be much larger than the true dissociation constant, and calling it affinity will mislead you. The clean statement is that Km is the substrate concentration for half maximal velocity, and that it approximates affinity under an assumption worth remembering.
The classic application is alcohol metabolism. Alcohol dehydrogenase has a low Km and saturates quickly, which is why elimination is roughly zero order: the enzyme is already flat out at ordinary concentrations.
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