Enzyme Kinetics: Vmax and the Michaelis Constant
Rate plotted against substrate concentration flattens to a maximum because the active sites become saturated. Vmax and Km are read off that curve and used to compare enzymes.
What a learner can do afterwards
- Reads Vmax and Km from a rate against substrate concentration curve.
- Explains the shape of the curve in terms of how many active sites are occupied.
- Compares two enzymes by their Km and says which binds its substrate more tightly.
1 · Read
Plot reaction rate against substrate amount and the same curve appears every time. Rate climbs steeply at first, then bends and levels off toward a ceiling called Vmax. That ceiling is the fastest the enzyme can go, when every active site is busy. Read Vmax straight off that flat top.
Km comes from the same curve. Find half of Vmax on the rate axis, move across to the curve, then drop down to read the substrate amount. That amount is Km. A low Km means half speed arrives with little substrate, so the enzyme grips tightly and works well even when little is around.
The curve bends because active sites fill up. At low substrate most sites sit empty, so each extra molecule quickly finds a home and the rate jumps. At high substrate nearly every site is occupied, so extra molecules must queue and the rate barely moves. Once all sites work full time, adding more cannot speed anything up.
Compare enzymes by Km when substrates are scarce. The enzyme with the smaller Km reaches half its top speed sooner, which means it binds its substrate more tightly. Tight binding wins when food is short. Never read Km as the ceiling: Vmax is the height, Km is the grip.
Vmax is the flat top where sites saturate, and Km is the substrate that buys half of it.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.