Enzymes – Structure, Function & Regulation

40 vocabulary flashcards covering enzyme structure, function, regulation, inhibition, classification, and key terminology to aid exam preparation.

Enzyme

A protein (or RNA) biocatalyst that accelerates a specific biochemical reaction without being consumed.

Ribozyme

A catalytic RNA molecule that functions as an enzyme, discovered by Thomas Cech and Sidney Altman.

Endo-enzyme

An enzyme that acts inside the cell in which it is produced.

Exo-enzyme

An enzyme secreted to function outside the cell that produced it.

Active Site

The three-dimensional region of an enzyme where the substrate binds and the reaction occurs.

Binding Site

Portion of an enzyme’s active site whose amino acids form temporary bonds with the substrate.

Catalytic Site

Region within the active site whose amino acids perform the chemical transformation of the substrate.

Cofactor

A non-protein component (metal ion or organic molecule) required for enzyme activity.

Coenzyme

A detachable organic cofactor, often vitamin-derived, that assists enzymes (e.g., NAD, FAD).

Prosthetic Group

A permanently bound organic cofactor (e.g., Mg-porphyrin in chlorophyll).

Apoenzyme

The inactive protein portion of a conjugated enzyme, lacking its cofactor.

Holoenzyme

The active form of a conjugated enzyme consisting of apoenzyme plus its cofactor.

Activator

An ion or small molecule that increases enzyme activity, often a metal such as Mn²⁺ or Mg²⁺.

Inhibitor

A substance that decreases or stops enzyme activity.

Enzyme–Substrate Complex (ES)

Transient complex formed when a substrate binds to an enzyme’s active site.

Lock-and-Key Model

Emil Fischer’s 1898 idea that the rigid active site is exactly complementary to its substrate.

Induced-Fit Model

Koshland’s 1959 model proposing that substrate binding induces a conformational change in the enzyme to facilitate catalysis.

Activation Energy

The minimum energy required for reactants to reach the transition state; lowered by enzymes.

Optimum Temperature

Temperature at which a given enzyme catalyzes a reaction at its maximum rate (≈37 °C for most human enzymes).

Optimum pH

The pH at which an enzyme shows maximal activity (e.g., pepsin ≈1.4; pancreatic amylase ≈8.5).

Allosteric Site

A site on an enzyme other than the active site where regulators bind, altering activity.

Competitive Inhibition

Reversible inhibition where substrate analogs compete for the active site (e.g., penicillin).

Non-competitive Inhibition

Inhibition in which a molecule binds at an allosteric site, changing enzyme shape and reducing activity, independent of substrate concentration.

Uncompetitive Inhibition

Inhibition where the inhibitor binds only to the enzyme–substrate complex, preventing product release.

Feedback Inhibition

Regulatory mechanism where a pathway’s end product inhibits an early enzyme, often at an allosteric site.

Oxidoreductase

Enzyme class that catalyzes oxidation–reduction reactions (electron or hydrogen transfer).

Transferase

Enzyme that transfers a functional group (e.g., phosphate) from one molecule to another.

Hydrolase

Digestive enzyme class that cleaves bonds by adding water (hydrolysis).

Lyase

Enzyme that breaks specific covalent bonds without hydrolysis or oxidation, often forming double bonds.

Isomerase

Enzyme that catalyzes intramolecular rearrangements, converting a molecule into one of its isomers.

Ligase (Synthetase)

Enzyme that joins two molecules via condensation, usually using ATP (e.g., DNA polymerase).

Protease

Hydrolase that breaks peptide bonds in proteins (e.g., pepsin, trypsin).

Lipase

Enzyme that hydrolyzes fats into glycerol and fatty acids.

Glycosidase

General term for enzymes that hydrolyze carbohydrates, such as amylase, cellulase, lactase.

Nuclease

Enzyme that hydrolyzes nucleic acids (e.g., DNase, RNase, ATPase).

Thermophilic Enzyme

Enzyme from heat-loving bacteria, displaying optimum activity around 70 °C or higher.

Substrate Saturation

Condition in which increasing substrate concentration no longer raises reaction rate because all enzyme active sites are occupied.

Pepsinogen

Inactive zymogen form of pepsin; activated by HCl via removal of a blocking peptide chain.