Unit 3: Enzymes and Kinetics
How a protein speeds up a reaction by a million-fold without being consumed, and the math that turns a saturating rate curve into two numbers.
7 topics · 30 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Activation Energy and Enzyme Catalysis
- How it worksHow does an enzyme lower the energy barrier a reaction has to climb over?
- Common mistakeWhy doesn't an enzyme change whether a reaction happens, only how fast it happens?
- Common mistakeWhy isn't an enzyme used up or permanently changed by the reaction it catalyzes?
- Going furtherWhat happens to a reaction's rate once the temperature climbs past an enzyme's optimum?
Your turn1 of 2
A reaction releases energy overall and is thermodynamically favorable, yet it proceeds so slowly at body temperature that almost nothing happens over an hour. Adding the right enzyme makes it proceed rapidly. What did the enzyme actually change?The Active Site, Binding, and Specificity
- How it worksHow does an active site's shape and chemistry select for one substrate over all the rest?
- How it worksWalk me through how induced fit lets the active site close around a substrate as it binds.
- Worked exampleFind how many distinct six-residue binding pockets are possible if each of the six positions can hold any of 20 amino acids.
- Common mistakeWhy isn't the lock-and-key model the whole story of how substrates actually bind an enzyme?
- Going furtherWhat happens to an enzyme's specificity when a single amino acid in its active site gets mutated?
Your turn1 of 2
Find how many distinct binding pockets are possible if each of 6 positions in the pocket can independently hold any of 20 amino acids.Michaelis-Menten Kinetics
- How it worksHow does the Michaelis-Menten equation turn a substrate concentration into a predicted reaction rate?
- Worked exampleFind the reaction rate of an enzyme with a Vmax of 100 micromoles per minute and a Km of 5 millimolar at a substrate concentration of 5 millimolar.
- Common mistakeWhy doesn't doubling the substrate concentration double the reaction rate once an enzyme nears saturation?
- Going furtherWhat happens to the rate curve when substrate concentration is pushed far above an enzyme's Km?
Your turn1 of 2
An enzyme has a Vmax of 154 micromoles per minute and a Km of 8 millimolar. Find the reaction rate at a substrate concentration of 23 millimolar.Finding Km and Vmax from a Lineweaver-Burk Plot
- How it worksHow does taking the reciprocal of both substrate concentration and rate turn the Michaelis-Menten curve into a line?
- Worked exampleFind Km and Vmax from a Lineweaver-Burk plot with a y-intercept of 0.01 and a slope of 0.1.
- Worked exampleFind the x-intercept of a Lineweaver-Burk plot for an enzyme with a Km of 4 millimolar.
- Common mistakeWhy isn't the y-intercept of a Lineweaver-Burk plot equal to Km instead of 1 over Vmax?
Your turn1 of 2
On a Lineweaver-Burk plot (1/rate versus 1/substrate concentration), the y-intercept is 0.0363 and the slope is 0.202. Find Vmax.Competitive, Noncompetitive, and Uncompetitive Inhibition
- How it worksHow does a competitive inhibitor block the active site without changing an enzyme's maximum speed?
- How it worksHow does a noncompetitive inhibitor lower an enzyme's maximum speed while leaving its Km untouched?
- Worked exampleFind how a competitive inhibitor shifts an enzyme's apparent Km from 2 millimolar to 6 millimolar while Vmax stays fixed.
- Common mistakeWhy doesn't every inhibitor that lowers a reaction's rate work by blocking the active site?
- Going furtherWhat happens to the lines on a Lineweaver-Burk plot as a competitive inhibitor's concentration increases?
Your turn1 of 2
A competitive inhibitor raises an enzyme's apparent Km from 3.59 millimolar to 10 millimolar while Vmax stays unchanged. Find the fold increase in apparent Km.Allosteric Regulation and Cooperativity
- How it worksHow does a ligand binding one subunit of an allosteric enzyme change the shape of the other subunits?
- How it worksWalk me through how cooperative binding produces an S-shaped rather than a hyperbolic rate curve.
- Common mistakeWhy isn't allosteric inhibition just a slower version of competitive inhibition at the active site?
- Ties togetherHow does this same cooperative logic behind allosteric enzymes also explain hemoglobin's oxygen-binding curve?
Your turn1 of 2
A student argues that allosteric inhibition is really just a slower version of competitive inhibition, since both processes reduce an enzyme's activity. What is the actual difference between them?Regulation by Phosphorylation and Covalent Modification
- How it worksHow does a kinase attaching a phosphate group change an enzyme's shape and activity?
- How it worksHow does a phosphatase reverse that switch by removing the phosphate group again?
- Common mistakeWhy isn't phosphorylation a permanent modification the way the amino acid sequence itself is?
- Going furtherWhat happens to a signaling pathway when the phosphatase that normally reverses it gets deleted?
Your turn1 of 2
A kinase attaches a phosphate group to a target enzyme, switching it from inactive to active. A student assumes this phosphorylation is a permanent change to the protein, similar to a mutation in its amino acid sequence. Why is this wrong?