Enzyme Biology, Kinetics, Regulation, and SNAREs

Vocabulary flashcards covering core concepts about enzymes, kinetics, regulation, and SNARE-mediated vesicle fusion based on the lecture notes.

Enzymes

Biological catalysts that act on substrates to yield products; they lower activation energy to speed up reactions without changing the overall free energy difference (ΔG) of the reaction.

Active site (catalytic site)

The region of an enzyme shaped to fit its substrate; determines specificity; may undergo induced fit to better accommodate the substrate.

Specificity

Enzymes recognize and act on only particular substrates; they can distinguish between isomers (stereo-specificity).

Isoenzymes (isozymes)

Different enzymes that catalyze the same chemical reaction in different tissues or contexts.

Proteases / Peptidases

Enzymes that hydrolyze peptide bonds in proteins to produce smaller peptides or amino acids.

Kinases

Enzymes that add phosphate groups to molecules, playing key roles in signaling and energy regulation.

Phospholipases

Enzymes that break down phospholipids, which are major components of biological membranes.

Synthases

Enzymes that build molecules by joining smaller units into larger ones.

Induced fit

The enzyme undergoes a conformational change to better fit the substrate, rather than the substrate fitting a rigid lock-and-key pocket.

Activation energy (Ea)

The energy barrier that must be overcome for a reaction to proceed; enzymes lower Ea to speed up reactions.

ΔG (Gibbs free energy change)

The overall energy difference between reactants and products; enzymes do not change ΔG, only the rate.

Transition state

Top of the energy barrier that must be reached for a reaction to proceed; stabilized by enzyme interactions.

Vmax

Maximum reaction rate when an enzyme is fully saturated with substrate.

Km (Michaelis constant)

Substrate concentration at which the reaction rate is half of Vmax; reflects enzyme affinity (low Km = high affinity).

Michaelis–Menten kinetics

Describes how reaction rate depends on substrate concentration, with Vmax and Km defining the curve.

Competitive inhibition

Inhibitor binds to the enzyme’s active site, increasing Km while leaving Vmax unchanged; high substrate can overcome inhibition.

Noncompetitive inhibition

Inhibitor binds to an allosteric site, reducing Vmax with Km unchanged; substrate cannot fully restore activity.

Allosteric regulation

Regulation of enzyme activity by a molecule binding at a site other than the active site, causing a conformational change.

Cofactor

Non-protein helpers (often metal ions) required for enzyme activity (e.g., Zn2+ or Mg2+).

Coenzyme

Organic molecule cofactors (e.g., biotin) that assist in enzyme-catalyzed reactions.

Zymogen (proenzyme)

An inactive enzyme precursor that contains a prodomain blocking the active site; activation occurs when the prodomain is removed by proteolytic cleavage.

Prodomain

An extra segment in zymogens that blocks the active site until removed during activation.

Protease cleavage

The proteolytic removal of a prodomain to activate a zymogen into an active enzyme.

Cooperativity

A phenomenon where binding of substrate to one subunit increases activity of other subunits; often yields a sigmoidal (S-shaped) kinetic curve.

Domains

Distinct structural/functional regions within a single protein, each with a specific role (e.g., binding, catalysis).

Subunits

Separate polypeptide chains that come together to form a larger protein complex (e.g., hemoglobin with 4 subunits).

SNARE proteins

Membrane proteins that drive vesicle fusion by pulling two membranes together; essential for vesicle trafficking and secretion.

V-SNARE

Vesicular SNARE; located on the vesicle membrane and pairs with target SNAREs to mediate fusion.

T-SNARE

Target SNARE; located on the target membrane (e.g., plasma membrane) to interact with v-SNAREs.

Synaptobrevin

A vSNARE involved in neurotransmitter vesicle fusion with the plasma membrane.

Vesicle fusion

The process by which a vesicle membrane fuses with a target membrane to release contents (e.g., neurotransmitters or hormones) outside the cell or into the membrane.

Allosteric site

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

Denaturation

Loss of an enzyme’s structure and function due to factors like excessive heat, which disrupts its active site.

Optimal temperature

Moderate temperatures that increase enzyme activity; too high temperatures cause denaturation and loss of function.