Enzymes: The Catalysts of Life and Biological Regulation

These flashcards cover the fundamental vocabulary of enzyme biology, including catalytic properties, activation energy, enzyme classification, structural sites, inhibition types, and regulatory mechanisms as described in the lecture notes.

Cellulose

A linear polymer of monomers of $\beta\text{-D-glucose}$ that belongs to the polysaccharide group of macromolecules.

Enzymes

Biological catalysts that make the difference between a reaction that can take place and one that will take place by lowering activation energy ($E_A$).

Metastable State

A state in which many thermodynamically feasible reactions do not proceed at an appreciable rate because they lack the required activation energy.

Activation Energy ($E_A$)

The minimum amount of energy that reactants must contain before collisions between them will be successful and lead to a reaction.

ATP Hydrolysis

A reaction where $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + P_i$ with a $\Delta G = -7.3\,kcal/mol$, which remains stable in water for several days due to high activation energy.

Isothermal

A condition of being constant in temperature, which is characteristic of cells maintaining homeostasis.

Catalyst

A substance that enhances the rate of a reaction by providing a surface that lowers the activation energy ($E_A$) while remaining unaltered by the reaction.

Ribozymes

Specifically discovered RNA molecules that possess catalytic activity, such as Ribonuclease P and peptidyl transferase on the large ribosomal subunit.

Oxidoreductases

A class of enzymes that catalyze oxidation-reduction reactions (electron transfer), such as alcohol dehydrogenase.

Transferases

A class of enzymes responsible for the transfer of functional groups from one molecule to another, exemplified by hexokinase.

Hydrolases

A class of enzymes that perform the hydrolytic cleavage of one molecule into two molecules, such as glucose-6-phosphatase.

Lyases

Enzymes that catalyze the removal of a group from, or the addition of a group to, a molecule, such as pyruvate decarboxylase.

Isomerases

Enzymes that catalyze the movement of a functional group within a molecule, such as maleate isomerase.

Ligases

Enzymes that join two molecules to form a single molecule, such as pyruvate carboxylase.

Active Site

A groove or pocket formed by the three-dimensional folding of a protein, containing a cluster of amino acids where substrates bind and catalysis occurs.

Prosthetic Groups

Nonprotein cofactors needed for catalytic activity, usually metal ions or small organic molecules called coenzymes which are derivatives of vitamins.

Denaturation

The rapid inactivation of an enzyme that occurs when temperatures rise above the optimal point, destroying the enzyme's functional structure.

Optimal pH of Pepsin

Approximately $2.0$, which corresponds to the acidic environment of the stomach.

Optimal pH of Trypsin

Near $8.0$, which corresponds to the environment of the intestines.

Induced-Fit Model

A model where substrate binding induces a conformational change in the enzyme, positioning amino acid side chains for optimal catalysis and holding the substrate via noncovalent interactions.

Substrate Activation Mechanisms

Three common methods: bond distortion (making bonds more susceptible to attack), proton transfer (increasing reactivity), and electron transfer (forming temporary covalent bonds).

Irreversible Inhibitors

Generally toxic molecules, such as heavy metal ions or nerve gas, that bind covalently to an enzyme and cause permanent loss of catalytic activity.

Competitive Inhibition

A form of inhibition where the inhibitor and substrate both bind to the active site; its effect depends on the concentration of the substrate.

Noncompetitive Inhibition

A form of inhibition where the inhibitor binds to a site other than the active site, distorting the enzyme's shape and reducing activity independent of substrate concentration.

Substrate-Level Regulation

Regulation where increases in substrate levels increase reaction rates, while increased product levels lead to lower rates.

Allosteric Regulation

The most important control mechanism where enzymes shift between high-affinity and low-affinity conformations based on molecules binding to an allosteric site.

Feedback Inhibition

Also called end-product inhibition, where the final product of an enzyme pathway negatively regulates an earlier step in that pathway.

Covalent Modification

The regulation of enzyme activity by the addition or removal of chemical groups, such as phosphorylation/dephosphorylation or methylation/demethylation.

Zymogens

Inactive forms of enzymes synthesized and stored by cells, which are later activated by irreversible proteolytic cleavage.