MCAT BIO/BIOCHEM
Enzyme Kinetics Flashcards
40 enzyme kinetics terms the MCAT expects you to define, not just recognise
40 cards~10 min
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All 40 MCAT BIO/BIOCHEM Enzyme Kinetics flashcards
The full deck in writing, listed A to Z. Read it through if the topic is new, then use the cards above to test yourself without looking.
- Alpha Factor
- The multiplier 1 + [I]/Ki that appears in inhibition rate laws, quantifying how far a given inhibitor concentration distorts the apparent kinetic constants of an enzyme.For example: When inhibitor concentration equals Ki the factor is 2, so a competitive inhibitor doubles the apparent Km.
- Apoenzyme
- The bare protein part of an enzyme with its required non-protein partner missing, and therefore catalytically dead until that partner is supplied.For example: A transaminase stripped of pyridoxal phosphate is an apoenzyme and converts nothing.
- Catalytic Efficiency (kcat/Km)
- The ratio used to rank enzymes against one another, blending how fast one works when fully loaded with how well it captures substrate when substrate is scarce.For example: Judging two enzymes on kcat alone misleads, because inside a dilute cell the one with the lower Km usually wins.
- Coenzyme
- A small organic helper molecule, most often built from a vitamin, that associates loosely with a protein and ferries chemical groups or electrons away between reactions.For example: NAD+ from niacin, FAD from riboflavin, and coenzyme A from pantothenate.
- Concerted Model
- The MWC account of cooperativity in which every subunit holds the same conformation at once, the whole protein flipping between a low-affinity tense form and a high-affinity relaxed form. Mixed states are forbidden.For example: Ligand binding tips a pre-existing equilibrium toward the relaxed form rather than creating that form.
- Cooperativity
- Behaviour of a multi-subunit protein in which occupying one subunit changes how readily the remaining subunits bind, turning a gradual response into a switch-like one.For example: Haemoglobin is the classic teaching case, although it transports oxygen rather than catalysing a reaction.
- Covalent Modification
- Reversible control in which a chemical group is attached to or stripped from an enzyme, changing its activity. Adding a phosphate is by far the commonest version in human cells.For example: One hormonal signal phosphorylates glycogen phosphorylase on and glycogen synthase off at the same time.
- Diffusion Limit
- The physical ceiling on catalytic efficiency, near 10^8 to 10^9 per molar per second, imposed by how fast two molecules can find each other in solution. Enzymes at this ceiling are called catalytically perfect.For example: Triose phosphate isomerase and catalase both work here, so faster chemistry could not make them any faster.
- Eadie-Hofstee Plot
- A linearisation of saturation data graphing velocity against velocity divided by substrate concentration, giving a slope equal to negative Km and a vertical intercept equal to Vmax.For example: Both axes carry the measured rate, so error spreads across the whole line instead of blowing up at the dilute end.
- First-Order Region
- The steep early stretch of a saturation curve where substrate sits far below Km, most enzyme molecules are unoccupied, and velocity climbs in direct proportion to how much substrate is added.For example: Doubling substrate in this stretch roughly doubles the measured rate.
- Heterotropic Regulation
- Regulation in which a molecule other than the substrate occupies a separate regulatory site and shifts activity up or down, without itself being converted into product.For example: 2,3-BPG lowering haemoglobin's oxygen affinity, or ATP damping phosphofructokinase-1.
- Hill Coefficient
- A single number summarising cooperativity: greater than one signals positive cooperativity, exactly one signals independent non-cooperative binding, and less than one signals negative cooperativity.For example: Haemoglobin sits near 2.8 despite holding four oxygen molecules, because cooperation is strong but not perfect.
- Holoenzyme
- The complete working assembly of an enzyme's protein portion together with the non-protein partner it needs. Only this combined form is capable of catalysis.For example: Transaminase plus its bound pyridoxal phosphate is the holoenzyme that actually moves amino groups.
- Homotropic Regulation
- Regulation in which the substrate itself doubles as the modulator, binding one site and changing the protein's affinity at the others. Positive cooperativity is the usual outcome.For example: Each oxygen taken up by haemoglobin raises the appetite of the subunits still empty.
- Hydrolase
- The enzyme class that severs a bond by inserting a molecule of water across it. Digestive proteases, lipases and phosphatases are all members.For example: Pancreatic lipase splitting a triglyceride into fatty acids and monoglyceride.
- Inhibition Constant (Ki)
- The dissociation constant of the enzyme-inhibitor pair, numerically the inhibitor concentration producing half-maximal inhibition. Smaller values mean a tighter grip and therefore a more potent inhibitor.For example: A compound with Ki of 5 nanomolar binds about two hundred times more tightly than one at 1 micromolar.
- Initial Velocity (V0)
- The rate recorded in the opening moments of an assay, before product builds up enough to drive the reverse reaction and before substrate is measurably consumed, so one rate maps to one substrate concentration.For example: Taking the slope of absorbance over the first thirty seconds, rather than after the progress curve begins to bend.
- Irreversible Inhibition
- Inactivation in which the blocking molecule forms a covalent or effectively permanent attachment, removing that enzyme from the working pool for good. Dilution and dialysis do not bring activity back.For example: Aspirin acetylates a serine of cyclooxygenase, so platelet activity returns only as new protein is made.
- Isomerase
- The enzyme class that rearranges atoms inside a single molecule, producing an isomer with the same molecular formula but a different arrangement.For example: Phosphoglucose isomerase converting glucose-6-phosphate into fructose-6-phosphate.
- Isozyme
- One of several distinct proteins encoded by different genes that run the same reaction but differ in kinetic constants or tissue location, letting each tissue tune identical chemistry to its own job.For example: Hexokinase works at low glucose in most tissues, while liver glucokinase only engages once glucose is plentiful.
- Ligase
- The enzyme class that welds two molecules into one, paying for the new bond with energy released from ATP. Synthetases and carboxylases sit in this family.For example: DNA ligase sealing a nick in the backbone; pyruvate carboxylase attaching carbon dioxide to pyruvate.
- Lyase
- The enzyme class that breaks a bond without using water and without oxidation, usually leaving a double bond behind, and which can equally add a group across an existing double bond.For example: Aldolase cleaving fructose-1,6-bisphosphate into two three-carbon sugars.
- Michaelis-Menten Equation
- The rate law v0 = Vmax[S] / (Km + [S]), which predicts initial velocity from substrate concentration and generates the hyperbolic curve that flattens as an enzyme approaches full occupancy.For example: At [S] = 4 Km the equation returns 0.8 Vmax, so four times the half-saturation point still leaves the enzyme short of its ceiling.
- Mixed Inhibition
- Inhibition by a molecule that can attach to the unoccupied enzyme and to the enzyme already carrying substrate, but with unequal affinity for the two, so Vmax always falls and Km shifts either direction.For example: Favouring the unoccupied form pushes apparent Km up; favouring the loaded form pulls it down.
- Oxidoreductase
- The enzyme class that shuttles electrons between molecules, oxidising one partner while reducing the other. Dehydrogenases, oxidases and reductases all belong to it.For example: Lactate dehydrogenase moving a hydride between pyruvate and NADH.
- pH Optimum
- The acidity at which a given enzyme runs fastest, set by the charge state its catalytic and binding residues must carry. Rate falls away on either side well before any unfolding happens.For example: Pepsin peaks near pH 2 in the stomach while pancreatic trypsin peaks near pH 8 in the duodenum.
- Prosthetic Group
- A non-protein helper held tightly, frequently by a covalent bond, and kept on the protein permanently rather than released once a catalytic cycle finishes.For example: The haem of catalase, or the biotin covalently anchored to pyruvate carboxylase.
- Protein Kinase
- An enzyme that moves the terminal phosphate of ATP onto a serine, threonine or tyrosine of a target protein, switching that target on or off depending on which protein it is.For example: Protein kinase A acts once cyclic AMP rises behind glucagon or adrenaline.
- Protein Phosphatase
- An enzyme that removes a phosphate group from a protein by hydrolysis, undoing a kinase's work and returning that target to its unphosphorylated activity state.For example: Protein phosphatase 1 strips glycogen synthase, switching glucose storage back on after insulin signals.
- Sequential Model
- The KNF account of cooperativity in which each subunit changes shape individually as it binds, and that change nudges its neighbours, so intermediate proteins carrying mixed conformations are permitted.For example: It is the only one of the two standard models able to explain negative cooperativity.
- Sigmoidal Kinetics
- The S-shaped rate curve produced by cooperative multi-subunit enzymes. Because that curve is not hyperbolic, these enzymes fall outside Michaelis-Menten treatment and report K0.5 rather than Km.For example: Phosphofructokinase-1 shows this shape, letting glycolysis swing sharply on small concentration changes.
- Specific Activity
- A purity measure reported as units of catalytic activity per milligram of total protein in a sample. It climbs through a purification while total activity can only fall.For example: A step that loses half the total activity but raises this figure tenfold has still produced a cleaner preparation.
- Steady-State Assumption
- The premise behind Michaelis-Menten kinetics that the enzyme-bound intermediate is formed and consumed at equal rates, holding its concentration constant while free substrate is still in large excess.For example: It is valid only over the first small percentage of a reaction, which is why kinetic runs are read early rather than to completion.
- Suicide Inhibition
- Irreversible inactivation in which a harmless-looking compound is processed by the enzyme's own catalytic machinery into a reactive species that then destroys the very enzyme that made it.For example: Thymidylate synthase converts a fluorouracil metabolite into a trapped complex it cannot release.
- Transferase
- The enzyme class that carries a functional group from one molecule and hands it to another. Kinases are the phosphate-carrying members of this family.For example: Aspartate aminotransferase passing an amino group from aspartate to a keto acid.
- Transition State
- The highest-energy atomic arrangement along the path from reactant to product, surviving less than a picosecond and never isolable, which distinguishes it from a genuine reaction intermediate.For example: Molecules built to mimic this arrangement bind their enzyme far more tightly than substrate does, a standard route to drug design.
- Transition-State Stabilization
- The core catalytic strategy: gripping the fleeting high-energy arrangement more tightly than the ground-state reactant, which shrinks the barrier without shifting where the reaction's equilibrium finally sits.For example: Because equilibrium is untouched, the reverse direction is accelerated by exactly the same factor as the forward one.
- Turnover Number (kcat)
- How many substrate molecules one active site converts into product each second under saturating conditions, obtained by dividing Vmax by the total enzyme concentration present in the assay.For example: Carbonic anhydrase reaches roughly a million per second, while most enzymes fall between one and ten thousand.
- Zero-Order Region
- The flat plateau of a saturation curve where velocity has stopped responding to substrate concentration entirely and the only remaining limit is how quickly each enzyme molecule can cycle.For example: Pouring in more substrate at the plateau changes nothing, whereas adding more enzyme lifts the plateau itself.
- Zymogen
- An inactive precursor that becomes a working enzyme only once specific peptide bonds are cut, letting an organism build something destructive safely and switch it on at the right place.For example: Trypsinogen leaves the pancreas harmless and is cleaved to trypsin only in the small intestine.