Enzyme Kinetics, Mechanisms, and Regulation

Comprehensive vocabulary flashcards generated from lecture notes covering enzyme kinetics, inhibition models, enzyme classification, allosteric regulation, and clinical biochemical applications.

Alcohol Dehydrogenase (ADH)

An enzyme that oxidizes alcohol to an aldehyde while reducing $NAD^+$ to $NADH$; in cases of ethylene glycol poisoning, it is competitively inhibited by ethanol.

Coenzyme-Cosubstrate

A coenzyme, such as $NAD^+$ for alcohol dehydrogenase, that functions catalytically during a reaction and dissociates from the enzyme upon completion.

Noncompetitive Inhibitor

An inhibitor that binds to a site other than the active site on both free enzyme and enzyme-substrate complex, leaving $K_m$ unchanged while decreasing $V_{max}$.

Kinase

A class of transferase enzymes that catalyzes the transfer of a phosphate group from a high-energy donor molecule (such as ATP) to a substrate.

Sulfa Drugs

Antibacterial agents that act as competitive inhibitors of an enzyme required by bacteria but not present in human cells.

Lyase

An enzyme class that catalyzes the cleavage of C-C, C-O, C-N, or other bonds by elimination, leaving double bonds or adding groups to double bonds without hydrolysis.

Turnover Number ($k_{cat}$)

A measure of the catalytic efficiency of an enzyme, defined as the maximum number of substrate molecules converted to product per enzyme molecule per unit time.

Allosteric Enzyme

An oligomeric enzyme that does not follow Michaelis-Menten kinetics, displays a sigmoidal rate curve ($V_0$ versus $[S]$), and undergoes conformational changes upon binding effectors.

Transition State Analog

A stable molecule designed to resemble the transition state structure of a substrate in an enzyme-catalyzed reaction, binding much more tightly to the enzyme than the normal substrate.

Competitive Inhibitor

An inhibitor that competes directly with the substrate for binding at the active site, increasing the apparent $K_m$ without affecting the $V_{max}$.

Isoenzymes (Isozymes)

Physically distinct forms of an enzyme, often with different amino acid sequences and tissue localizations, that catalyze the exact same chemical reaction.

Lineweaver-Burk Plot

A double-reciprocal plot ($1/V_0$ versus $1/[S]$) used to determine kinetic parameters, where the Y-intercept equals $1/V_{max}$, the X-intercept equals $-1/K_m$, and the slope equals $K_m/V_{max}$.

Apoenzyme

An inactive protein moiety of an enzyme that requires a bound co-substrate, coenzyme, or cofactor to form a catalytically active holoenzyme.

Suicide Inhibitor

Also known as a mechanism-based inhibitor; a substrate analog that is converted by the enzyme's active site into a reactive species that covalently and irreversibly inactivates the enzyme.

Glucokinase vs. Hexokinase $K_m$

Glucokinase has a high $K_m$ ($10\,mM$) and operates at approximately $0.5\,V_{max}$ at $10\,mM$ blood glucose, whereas hexokinase has a low $K_m$ ($0.1\,mM$) and operates near $V_{max}$ at the same glucose concentration.

Cytosolic Aldehyde Dehydrogenase

An isozyme of aldehyde dehydrogenase with a higher $K_m$ (lower affinity) for acetaldehyde ($CH_3CHO$) than the mitochondrial isozyme, leading to elevated acetaldehyde levels and physiological flushing when mitochondrial ALDH is deficient.

Ligase

An enzyme class that catalyzes the joining together of two large molecules with the concomitant hydrolysis of a diphosphate bond in ATP or a similar triphosphate.

Organophosphate Inactivation

An irreversible inhibition process where organophosphate compounds form stable covalent ester bonds with a critical serine hydroxyl group in acetylcholine esterase.

Serine Racemase

An enzyme that catalyzes the reversible interconversion between L-serine and D-serine, demonstrating a lack of stereospecificity.

Induced Fit Model

A mechanism of enzyme action where substrate binding induces flexible conformational changes in the enzyme, bringing catalytic groups into proper spatial alignment.

Cooperativity

A characteristic of multi-subunit allosteric enzymes where the binding of a substrate molecule to one subunit alters the affinity of remaining subunits for substrate binding.

$$K_{0.5}$$

The substrate concentration required for an allosteric enzyme to achieve half of its maximal velocity ($0.5\,V_{max}$), serving as the allosteric equivalent to $K_m$.