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Enzyme tricks that make kinetics click

Km, Vmax, inhibition, and cofactors β€” mastered.

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βš™οΈ Enzymes

Memory tricks

Proven mnemonics — fast to learn, hard to forget.

Michaelis Constant (Km)
Low Km = high affinity. High Km = low affinity. Km = [S] at Β½ Vmax.
Michaelis Constant (Km)
Km tells you how tightly the enzyme binds its substrate
Km = substrate concentration at half-maximal velocity. Low Km: enzyme achieves half-Vmax at low substrate β†’ tight binding, high affinity. Km doesn't change with enzyme concentration.
Difficulty: Intermediate
Competitive Inhibition
Competitive inhibition: same active site, Km increases, Vmax unchanged β€” overcome with substrate
Competitive Inhibition
Inhibitor competes with substrate for the active site
Add more substrate β†’ outcompete inhibitor β†’ Vmax restored. Km appears to increase. Example: methotrexate competes with folate at DHFR.
Difficulty: Advanced
Non-Competitive Inhibition
Non-competitive: binds elsewhere, Vmax decreases, Km unchanged β€” can't overcome
Non-Competitive Inhibition
Inhibitor binds allosteric site β€” more substrate won't help
Inhibitor binds separate (allosteric) site, changes enzyme shape. Vmax decreases, Km unchanged. Adding more substrate doesn't help. Example: aspirin irreversibly inhibits COX enzymes.
Difficulty: Advanced
Cofactors and Coenzymes
Cofactors = metal ions. Coenzymes = organic (often from vitamins).
Cofactors and Coenzymes
Non-protein helpers that many enzymes require to function
Metal cofactors: Zn²⁺ (carbonic anhydrase), Fe²⁺ (cytochrome), Mg²⁺ (kinases). Coenzymes: NAD⁺, FAD, CoA β€” many derived from B vitamins. Deficiency β†’ enzyme dysfunction.
Difficulty: Intermediate
Allosteric Regulation
Allosteric regulation: effector binds non-active site β†’ changes enzyme shape β†’ activates or inhibits
Allosteric Regulation
Enzymes can be turned on or off by molecules binding away from the active site
Positive allosteric effectors: bind and increase activity. Negative effectors: decrease activity. Feedback inhibition: product of a pathway inhibits an early enzyme β€” classic regulation strategy.
Difficulty: Advanced
Six Enzyme Classes
Enzyme classification: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases β€” OT HaLIL
Six Enzyme Classes
The six classes of enzymes classified by the reaction they catalyze
Oxidoreductases: catalyze oxidation-reduction. Transferases: transfer functional groups. Hydrolases: cleave bonds with water (proteases, lipases). Lyases: cleave bonds without water (non-hydrolytic). Isomerases: convert isomers. Ligases: join two molecules using ATP. Most drugs target enzymes in these classes.
Oxidoreductases
Oxidation-reduction reactions
Transferases
Transfer functional groups
Hydrolases
Cleave with water
Lyases
Cleave without water
Isomerases
Convert between isomers
Ligases
Join molecules using ATP
Difficulty: Advanced
Activation Energy
Activation energy: energy barrier a reaction must overcome. Enzymes lower it β€” don't change Ξ”G.
Activation Energy
What enzymes actually do β€” and what they don't change
Activation energy (Ea): energy needed to start a reaction. High Ea β†’ slow reaction. Enzymes provide an alternative pathway with lower Ea β†’ faster reaction. Crucially: enzymes do NOT change the equilibrium constant (K_eq) or the overall free energy change (Ξ”G). They speed up reactions that would happen anyway.
Difficulty: Intermediate
Feedback Inhibition
Feedback inhibition: the END product of a pathway inhibits an EARLY enzyme β€” classic metabolic control
Feedback Inhibition
How cells regulate metabolic pathways through end-product inhibition
Isoleucine synthesis: threonine β†’ (5 steps) β†’ isoleucine. When isoleucine accumulates, it inhibits the first enzyme in the pathway (allosterically). Efficient: stops the whole pathway when product is abundant. Avoids wasteful overproduction. Classic example of negative feedback in biochemistry.
Difficulty: Intermediate
Irreversible Inhibitors
Irreversible inhibitors: permanently inactivate enzyme by covalent bond. Aspirin, nerve agents, penicillin.
Irreversible Inhibitors
Inhibitors that permanently disable enzymes
Irreversible inhibitors form covalent bonds with the enzyme β€” permanent inactivation. Aspirin: acetylates COX enzyme β†’ blocks prostaglandin synthesis β†’ anti-inflammatory, anti-platelet. Nerve agents (sarin): covalently inhibit acetylcholinesterase β†’ nerve signals can't stop. Penicillin: covalently inhibits transpeptidase β†’ bacterial cell wall synthesis stops.
Difficulty: Advanced
Lineweaver-Burk Plot
Lineweaver-Burk plot: double reciprocal plot (1/V vs 1/[S]). Intercepts give Vmax and Km.
Lineweaver-Burk Plot
Graphical method to determine Km and Vmax from kinetic data
Plot 1/V (y-axis) vs 1/[S] (x-axis) β†’ straight line. Y-intercept = 1/Vmax. X-intercept = -1/Km. Slope = Km/Vmax. Competitive inhibitor: increases slope (higher Km), same y-intercept (same Vmax). Non-competitive: same x-intercept (same Km), increases y-intercept (lower Vmax).
Difficulty: Advanced
Prosthetic Groups
Prosthetic groups: cofactors permanently attached to enzyme. Heme in hemoglobin and cytochrome c.
Prosthetic Groups
Permanently bound cofactors essential for enzyme function
Prosthetic group: non-protein component permanently and tightly bound to the protein. Unlike coenzymes (loosely bound, can leave). Heme group: iron-containing porphyrin ring β€” in hemoglobin (Oβ‚‚ transport), myoglobin, cytochromes (ETC). FAD: covalently bound in some enzymes. Biotin: covalently bound in carboxylases.
Difficulty: Advanced
Zymogens
Zymogen (proenzyme): inactive enzyme precursor activated by cleavage. Pepsinogen β†’ pepsin, trypsinogen β†’ trypsin.
Zymogens
Inactive enzyme precursors β€” a safety mechanism
Zymogens protect cells from premature enzymatic activity. Digestive enzymes stored as zymogens in pancreas β€” activated only in the intestine. Pepsinogen (stomach) β†’ pepsin (activated by stomach acid). Trypsinogen β†’ trypsin (activated by enteropeptidase in small intestine). Blood clotting cascade: sequential zymogen activation.
Difficulty: Intermediate