Chapter 6: Enzymes – The Catalysts of Life (Vocabulary Flashcards)

Vocabulary flashcards covering core concepts from the enzyme chapter, including activation energy, enzyme structure, classes, kinetics, inhibition, regulation, and example processes in digestion.

Activation energy

The minimum amount of energy required to start a chemical reaction; enzymes lower this barrier.

Transition state

The point in a reaction where reactants have their highest energy and are on the verge of becoming products.

Active site

The region of an enzyme where the substrate binds.

Substrate

The substance that an enzyme acts upon.

Enzyme-substrate complex

The temporary complex formed when a substrate binds to the enzyme’s active site.

Cofactor

Non-protein helpers required by some enzymes to function.

Coenzyme

An organic cofactor, often derived from vitamins or minerals, that assists enzyme activity.

Enzyme

A biological catalyst, most are proteins, that speeds up chemical reactions.

Oxidoreductases

Enzymes that catalyze oxidation–reduction (electron transfer) reactions.

Transferases

Enzymes that transfer functional groups between molecules.

Hydrolases

Enzymes that break bonds using water (hydrolysis).

Lyases

Enzymes that break bonds without water or redox reactions; often form double bonds or rings.

Decarboxylase

An example of a lyase that removes a carboxyl group as CO2.

Ligases

Enzymes that join two molecules together, often using ATP.

Isomerases

Enzymes that rearrange atoms within a molecule (isomerization).

Enzyme specificity

Enzymes act on specific substrates and at a specific active site.

Cofactor vs coenzyme

Cofactors are non-protein helpers; coenzymes are organic cofactors derived from vitamins/minerals.

Optimal conditions sensitivity

Enzymes are sensitive to temperature and pH, affecting activity and stability.

Denaturation

Loss of an enzyme’s native structure due to factors like high temperature, leading to loss of function.

Activation energy strategies

Raising temperature or adding a catalyst to lower the energy barrier for a reaction.

Temperature effect on enzymes

Higher temperatures raise reaction rate but can cause denaturation; moderate temperatures preserve structure.

pH effect on enzymes

Changes in pH can alter amino acid charges at the active site and affect substrate binding.

Inhibitors

Molecules that reduce or prevent enzyme activity; can be reversible or irreversible.

Activators

Molecules that increase enzyme activity or enable activity that is otherwise limited.

Competitive inhibition

Inhibitor binds to the enzyme’s active site, blocking substrate binding; Km increases, Vmax unchanged.

Noncompetitive inhibition

Inhibitor binds to an allosteric site, changing enzyme shape; Vmax decreases, Km unchanged.

Uncompetitive inhibition

Inhibitor binds only to the enzyme–substrate complex, typically decreasing both Vmax and Km.

Mixed inhibition

Inhibitor affects both Km and Vmax, not binding exclusively to active site.

Irreversible inhibitors

Inhibitors that permanently bind to an enzyme and inactivate it.

Reversible inhibitors

Inhibitors that bind temporarily and can dissociate to restore activity.

Allosteric enzymes

Enzymes regulated by molecules binding at sites other than the active site, causing conformational changes.

Feedback inhibition

Product of a pathway inhibits an enzyme early in the pathway to regulate production.

Covalent modification

Regulation by adding or removing chemical groups (e.g., phosphate) to alter activity.

Proteolytic cleavage

Activation or inactivation of a protein by cutting its peptide chain (e.g., zymogens).

Zymogen

An inactive enzyme precursor that requires proteolytic activation.

Pepsinogen → Pepsin

Pepsinogen (inactive) is activated by cleavage to pepsin (active) in the stomach.

Parietal cells

Gastric cells that secrete HCl into the stomach.

Chief cells

Gastric cells that secrete pepsinogen, an inactive enzyme precursor.

Gastric gland

Gland in the stomach where parietal and chief cells reside.

Vmax

The maximum rate of an enzyme-catalyzed reaction when all active sites are saturated with substrate.

Km (Michaelis constant)

The substrate concentration at which an enzyme-catalyzed reaction proceeds at half its Vmax; reflects substrate affinity.

Lineweaver–Burk plot

A double reciprocal plot of 1/v versus 1/[S] to linearize Michaelis–Menten kinetics and determine Km and Vmax.

Initial velocity (V0)

The reaction rate measured at the beginning of the reaction when substrate is abundant.

Turnover number (kcat)

The number of substrate molecules converted to product per enzyme molecule per unit time when the enzyme is fully saturated.

kcat formula

kcat = Vmax / [E] (enzyme concentration) under saturating substrate conditions.

Michaelis–Menten equation

Relationship describing how reaction velocity depends on substrate concentration: v = (Vmax [S]) / (Km + [S]).

Vmax units

Units of velocity, e.g., M/s or µM/min.

Km units

Concentration units, e.g., M or µM.

Lineweaver–Burk equation

1/v = (Km/Vmax)(1/[S]) + 1/Vmax, used to plot and extract Km and Vmax.

Saturation

Condition when increasing [S] no longer increases reaction rate because all enzymes are bound to substrate.

Competitive inhibitors effect on Km

Increase in Km (lower affinity) with Vmax unchanged.

Noncompetitive inhibitors effect on Vmax

Decrease in Vmax with Km unchanged.

Uncompetitive inhibitors effect on Km and Vmax

Typically decrease both Km and Vmax by binding to the enzyme–substrate complex.

Allosteric site

A site on an enzyme other than the active site where regulatory molecules bind.

Allosteric regulation

Regulation of enzyme activity through binding at allosteric sites leading to conformational change.

Covalent modification examples

Addition/removal of chemical groups (e.g., phosphorylation) to regulate enzyme activity.

Proteolytic cleavage example

Activation of enzymes by cleavage of inhibitory segments (e.g., pepsinogen to pepsin).