MCAT BIO/BIOCHEM: Enzyme Kinetics Flashcards

Question 1

Define Michaelis-Menten Equation.

Accepted Answer

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.

Question 2

Define Steady-State Assumption.

Accepted Answer

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.

Question 3

Define Initial Velocity (V0).

Accepted Answer

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.

Question 4

Define First-Order Region.

Accepted Answer

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.

Question 5

Define Zero-Order Region.

Accepted Answer

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.

Question 6

Define Turnover Number (kcat).

Accepted Answer

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.

Question 7

Define Catalytic Efficiency (kcat/Km).

Accepted Answer

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.

Question 8

Define Diffusion Limit.

Accepted Answer

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.

Question 9

Define Transition State.

Accepted Answer

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.

Question 10

Define Transition-State Stabilization.

Accepted Answer

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.

Question 11

Define Inhibition Constant (Ki).

Accepted Answer

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.

Question 12

Define Mixed Inhibition.

Accepted Answer

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.

Question 13

Define Alpha Factor.

Accepted Answer

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.

Question 14

Define Irreversible Inhibition.

Accepted Answer

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.

Question 15

Define Suicide Inhibition.

Accepted Answer

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.

Question 16

Define Eadie-Hofstee Plot.

Accepted Answer

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.

Question 17

What is Cooperativity?

Accepted Answer

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.

Question 18

Define Sigmoidal Kinetics.

Accepted Answer

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.

Question 19

Define Hill Coefficient.

Accepted Answer

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.

Question 20

Define Homotropic Regulation.

Accepted Answer

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.

Question 21

Define Heterotropic Regulation.

Accepted Answer

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.

Question 22

Define Concerted Model.

Accepted Answer

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.

Question 23

Define Sequential Model.

Accepted Answer

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.

Question 24

What is Apoenzyme?

Accepted Answer

The bare protein part of an enzyme with its required non-protein partner missing, and therefore catalytically dead until that partner is supplied.

Question 25

What is Holoenzyme?

Accepted Answer

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.

Question 26

What is Coenzyme?

Accepted Answer

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.

Question 27

Define Prosthetic Group.

Accepted Answer

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.

Question 28

What is Zymogen?

Accepted Answer

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.

Question 29

Define Covalent Modification.

Accepted Answer

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.

Question 30

Define Protein Kinase.

Accepted Answer

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.

Question 31

Define Protein Phosphatase.

Accepted Answer

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.

Question 32

What is Oxidoreductase?

Accepted Answer

The enzyme class that shuttles electrons between molecules, oxidising one partner while reducing the other. Dehydrogenases, oxidases and reductases all belong to it.

Question 33

What is Transferase?

Accepted Answer

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.

Question 34

What is Hydrolase?

Accepted Answer

The enzyme class that severs a bond by inserting a molecule of water across it. Digestive proteases, lipases and phosphatases are all members.

Question 35

What is Lyase?

Accepted Answer

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.

Question 36

What is Isomerase?

Accepted Answer

The enzyme class that rearranges atoms inside a single molecule, producing an isomer with the same molecular formula but a different arrangement.

Question 37

What is Ligase?

Accepted Answer

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.

Question 38

Define pH Optimum.

Accepted Answer

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.

Question 39

Define Specific Activity.

Accepted Answer

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.

Question 40

What is Isozyme?

Accepted Answer

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.