ABT Week 10: Enzymes, Kinetics, and Receptor Pharmacology

A set of comprehensive vocabulary flashcards covering the definitions of enzyme classifications, kinetics models, measurement assays, and receptor pharmacology from Week 10.

Enzymes

Biological catalysts that accelerate biochemical reactions by lowering the activation energy required for a reaction to occur without altering the overall free-energy difference or equilibrium position.

Ribozymes

RNA molecules that possess catalytic activity.

Bond specificity

A type of enzyme specificity where the enzyme recognises particular chemical bonds and surrounding functional groups (for example, peptidases and esterases).

Group specificity

A type of enzyme specificity where the enzyme acts on substrates sharing particular structural characteristics (for example, kinases).

Stereospecificity

A type of enzyme specificity where the enzyme distinguishes between different optical or geometrical isomers, recognising only one stereoisomer.

Oxidoreductases (EC 1)

An enzyme class that catalyses oxidation–reduction reactions involving electron or hydrogen transfer (for example, dehydrogenases, oxidases, catalase).

Transferases (EC 2)

An enzyme class that transfers chemical groups between molecules (for example, kinases, aminotransferases).

Hydrolases (EC 3)

An enzyme class that breaks chemical bonds through the addition of water (for example, peptidases, esterases, phosphatases).

Lyases (EC 4)

An enzyme class that removes or adds groups without hydrolysis, often forming or acting on double bonds (for example, aldolase, enolase).

Isomerases (EC 5)

An enzyme class that rearranges atoms within a molecule to produce an isomer (for example, glucose-6-phosphate isomerase).

Ligases (EC 6)

An enzyme class that joins molecules through covalent bond formation, usually coupled to ATP hydrolysis (for example, DNA ligase).

Translocases (EC 7)

An enzyme class included in modern enzyme classification alongside the six traditional Enzyme Commission classes.

Isoenzymes (isozymes)

Different molecular forms of enzymes that catalyse the same chemical reaction.

Multi-enzyme complexes

Groups of enzymes physically associated to perform one or several reactions within a biochemical pathway.

Enzyme kinetics

The study of the rates of enzyme-catalysed reactions and how those rates change under different conditions, such as substrate concentration, enzyme concentration, pH, temperature, and inhibitors.

$$V_{max}$$

The maximum reaction velocity of an enzyme-catalysed reaction when the enzyme is completely saturated with substrate.

$K_m$ (Michaelis constant)

The substrate concentration at which the reaction velocity equals half ($50\%$) of $V_{max}$.

Thermal denaturation

A process occurring above the optimum temperature where thermal energy disrupts stabilising interactions (such as hydrogen bonding and ionic interactions), causing secondary and tertiary structure changes that distort the active site and reduce catalytic activity.

Optimum pH

The specific pH at which an enzyme's activity is greatest, where the ionisation of amino acid side chains optimally supports active-site charge, substrate binding, catalytic activity, and protein stability.

Pre-steady-state kinetics

The initial transient period immediately after mixing an enzyme and substrate, before enzyme–substrate intermediates reach approximately constant concentrations, used to investigate early catalytic steps.

Steady-state kinetics

The reaction phase during which the concentrations of enzyme-bound intermediates remain approximately constant because their formation and breakdown are balanced, used to determine kinetic parameters such as $K_m$ and $V_{max}$.

Burst kinetics

A kinetic phenomenon where an enzyme rapidly produces an initial amount of product during an early step before continuing at a slower steady-state rate limited by a subsequent step.

Burst amplitude

The y-axis intercept of the extrapolated steady-state rate line on a product-versus-time graph, which provides information regarding the amount of functional enzyme present.

Fixed-time assay

An analytical assay method that measures product formation or remaining substrate after a predetermined incubation period, allowing many samples to be analysed simultaneously.

Continuous assay

An analytical assay method that monitors reaction progress repeatedly over time (such as through spectrophotometric absorbance changes) to determine reaction rates from the slope of the signal-versus-time graph.

Isothermal titration calorimetry (ITC)

An analytical method that measures the heat absorbed or released during biomolecular interactions to investigate binding thermodynamics.

Receptors

Proteins that recognise and bind specific signalling molecules (ligands), often undergoing a conformational change to initiate or modify a cellular response.

Dose–response curve

A graph illustrating how the magnitude of a biological response changes as the dose or concentration of a drug or agonist increases.

$$E_{max}$$

The maximum biological response that an agonist can produce in an experimental system.

$$EC_{50}$$

The agonist concentration producing $50\%$ of its maximal response.

Potency

The amount or concentration of a drug required to produce a specified effect, where a lower $EC_{50}$ (left-shifted curve) generally indicates greater potency.

Efficacy

The ability of a drug to produce a biological response, with higher maximal efficacy indicated by a higher $E_{max}$.

Full agonist

A ligand that binds to and activates a receptor to produce the maximum response possible in the experimental system (maximum efficacy).

Partial agonist

A ligand that binds to and activates a receptor but produces a lower maximal response than a full agonist, even at high receptor occupancy.

Inverse agonist

A ligand that binds to a receptor and reduces its constitutive (basal) activity below baseline, producing negative efficacy.

Partial inverse agonist

A ligand that reduces constitutive receptor activity but produces a smaller maximal inverse effect than a full inverse agonist.

Protean agonist

A ligand that can behave as an agonist or an inverse agonist depending on the receptor's basal activity level and the cellular context.

Biased agonist

A ligand that preferentially activates certain signalling pathways over others through the same receptor.

Antagonist

A substance that binds to a receptor and prevents or reduces agonist-mediated activation without activating the receptor itself, possessing zero intrinsic efficacy.