STEPTRACKERBIOCHEMISTRY + PHARMACOLOGY | METABOLISM PRINCIPLES | HIGH-YIELD REVIEW

Enzyme Kinetics & Inhibition

Read Michaelis-Menten and Lineweaver-Burk plots, and tell competitive from noncompetitive inhibition at a glance.

FOCUS ON
Km = substrate concentration at ½Vmax; low Km = high affinity. Vmax ∝ enzyme amount. Lineweaver-Burk: x-intercept = −1/Km, y-intercept = 1/Vmax, slope = Km/Vmax. Competitive inhibitor: ↑ Km, Vmax same (lines cross on the y-axis), overcome by more substrate. Noncompetitive: ↓ Vmax, Km same (lines cross on the x-axis). Allosteric/cooperative enzymes give a sigmoid curve.
CONCEPT
DEFINITION
ON THE GRAPH
CLUE · EXAMPLE
Vmax
Maximum rate when enzyme is saturated
Plateau of Michaelis-Menten curve · 1/Vmax = y-intercept on Lineweaver-Burk
↑ Enzyme amount → ↑ Vmax
Km
[S] giving half-maximal velocity; inverse of affinity
x-value at ½Vmax · −1/Km = x-intercept
Hexokinase low Km vs glucokinase high Km (8.01)
Competitive inhibitor (reversible)
Resembles substrate, binds active site
↑ Km, Vmax unchanged; LB lines intersect on y-axis
Methotrexate on DHFR · overcome with ↑ [S] · ↓ potency, same efficacy
Noncompetitive inhibitor
Binds an allosteric site (not active site)
↓ Vmax, Km unchanged; LB lines intersect on x-axis
Not overcome by substrate · ↓ efficacy
Uncompetitive inhibitor
Binds only the enzyme–substrate complex
↓ Vmax and ↓ Km; parallel LB lines
Rarely tested
Irreversible inhibitor
Covalent binding destroys activity
Acts like noncompetitive (↓ Vmax)
Aspirin (COX), organophosphates (AChE), omeprazole, clopidogrel
Cooperativity
Multiple subunits: binding one ligand changes affinity of others
Sigmoid curve
Hemoglobin–O₂ (not an enzyme but same shape), allosteric enzymes (PFK-1)
Zero-order kinetics
Constant amount eliminated per time (enzyme saturated)
Linear decline of concentration
Ethanol, phenytoin, aspirin at high dose ("PEA")
First-order kinetics
Constant fraction eliminated per time
Exponential decline; fixed half-life
Most drugs
Allosteric regulation
Effector binds a regulatory site → changes activity fast
Shifts the sigmoid curve
ATP, citrate inhibit PFK-1; AMP, F-2,6-BP activate

Picture it — kinetics plots

[S]vVmax½VmaxKm
Michaelis-MentenVelocity rises to Vmax; Km is the [S] giving half of Vmax.
1/[S]1/v1/Vmax same+ inhibitorno inhibitor−1/Km
Competitive inhibitorLines meet on the y-axis: same Vmax, higher Km.
1/[S]1/v−1/Km same+ inhibitorno inhibitor
Noncompetitive inhibitorLines meet on the x-axis: same Km, lower Vmax.
[S]vhyperbolicsigmoid(cooperative)
Cooperative enzymeSigmoid curve of allosteric enzymes (and hemoglobin).

Schematic diagrams, not to scale.

Mechanism — how to read an inhibited plot

Lines cross on y-axisSame 1/Vmax
→
CompetitiveKm ↑
Lines cross on x-axisSame −1/Km
→
NoncompetitiveVmax ↓
Parallel lines
→
UncompetitiveBoth ↓

On Lineweaver-Burk, a line further to the right on the x-axis means lower affinity (higher Km); higher on the y-axis means lower Vmax.

Inhibitor summary

TYPE
Km
Vmax
MORE SUBSTRATE?
Competitive
↑
—
Overcomes
Noncompetitive
—
↓
No effect
Uncompetitive
↓
↓
No effect
Irreversible
—
↓
No effect

How it's tested

COMMON CONFUSIONCompetitive inhibitors change potency (EC50 shifts right) but not efficacy; noncompetitive inhibitors lower maximal efficacy.
THE TRAPOn a Lineweaver-Burk plot higher y-intercept means LOWER Vmax (it's 1/Vmax).
YOUR APPROACHFind where the lines meet → which parameter stayed the same → name the inhibitor → whether extra substrate rescues it.

If you see… think…

Lines meet on the y-axis → competitive inhibition
Lines meet on the x-axis → noncompetitive inhibition
Enzyme with lower Km → higher affinity
Doubling enzyme concentration → Vmax doubles, Km same
Ethylene glycol treated with ethanol → competitive inhibition of ADH
Aspirin acetylating COX → irreversible inhibition
Sigmoid substrate curve → cooperative/allosteric enzyme
Drug eliminated at a constant amount per hour → zero-order (ethanol, phenytoin, aspirin)
Inhibitor overcome by raising substrate → competitive
x-intercept of Lineweaver-Burk → −1/Km