Chapter 2 - Enzymes Practice Flashcards

Comprehensive practice questions covering enzyme classification, kinetics, environmental impacts, and regulatory mechanisms based on the lecture notes.

How do enzymes affect the kinetics and thermodynamics of a reaction?

Enzymes affect kinetics by making reactions go faster (reaching equilibrium faster), but they do not change the thermodynamics, meaning they do not alter the overall free energy ($\Delta G$) or the equilibrium constant.

What is the definition of activation energy?

The energy required to reach the transition state of a chemical reaction.

What are the six classifications of enzymes according to the LIL HOT mnemonic?

Ligases, Isomerases, Lyases, Hydrolases, Oxidoreductases, and Transferases.

What is the function of Oxidoreductases?

They catalyze oxidation-reduction reactions, which involves the transfer of electrons between biological molecules (e.g., dehydrogenase, oxidase).

How do Hydrolases differ from Lyases?

Hydrolases break bonds by adding a water molecule (hydrolysis), whereas Lyases catalyze the cleavage of a single molecule into two products without the use of water or redox chemistry.

What is the difference between an endergonic and an exergonic reaction?

An endergonic reaction requires energy input ($\Delta G > 0$), while an exergonic reaction gives off energy ($\Delta G < 0$).

Contrast the Lock & Key Theory with the Induced Fit Model.

The Lock & Key Theory suggests the enzyme's active site is already in the correct conformation for the substrate, while the Induced Fit Model suggests the enzyme changes shape upon binding to create a more precise fit.

What are apoenzymes and holoenzymes?

Apoenzymes are enzymes without their cofactors (inactive), while holoenzymes are enzymes containing their cofactors (active).

What is the difference between cofactors and coenzymes?

Cofactors are generally inorganic molecules or metal ions (e.g., $Mg^{2+}$, $Zn^{2+}$); coenzymes are small organic molecules, often derived from vitamins (e.g., $NAD^{+}$, $FAD$, $CoA$).

What is the Michaelis-Menten equation?

$$V = \frac{V_{max}[S]}{K_m + [S]}$$

How is $K_m$ defined and what does it indicate about enzyme affinity?

$K_m$ is the substrate concentration at which half of the enzyme's active sites are full (half of $V_{max}$). A low $K_m$ reflects high affinity for the substrate, while a high $K_m$ reflects low affinity.

What is the turnover number ($K_{cat}$) and its relationship to $V_{max}$?

$K_{cat}$ is the number of substrate molecules one enzyme converts per second when saturated. The relationship is $V_{max} = [E]K_{cat}$.

In a Lineweaver-Burk plot, what do the x-intercept, y-intercept, and slope represent?

The x-intercept is $-1/K_m$, the y-intercept is $1/V_{max}$, and the slope is $K_m/V_{max}$.

What is cooperative binding and how does it appear on a graph?

It occurs when the binding of one substrate facilitates the binding of others, transitioning the enzyme from a low-affinity (T) state to a high-affinity (R) state, resulting in a sigmoidal (S-shaped) curve.

Explain the significance of Hill's coefficient values.

If Hill's coefficient $> 1$, there is positive cooperativity; if $< 1$, there is negative cooperativity; and if $= 1$, there is no cooperativity (Michaelis-Menten kinetics).

How does temperature affect enzyme-catalyzed reactions?

Reaction speed tends to double for every $10^{\circ}\text{C}$ increase until the optimal temperature is reached, after which the enzyme begins to denature and activity decreases.

What are the effects of pH and salinity on enzyme activity?

Changes in pH can alter the ionization of amino acids in the active site or cause denaturation. Altering salt concentrations (in vitro) can disrupt hydrogen and ionic bonds, changing enzyme activity.

What is feedback inhibition (Negative Feedback)?

A mechanism where a product formed later in a metabolic pathway inhibits an enzyme that acts earlier in the pathway to prevent unnecessary production.

How does competitive inhibition affect $K_m$ and $V_{max}$?

The inhibitor binds to the active site; $K_m$ increases (lower affinity), but $V_{max}$ remains unchanged because the inhibition can be overcome by adding more substrate.

How does noncompetitive inhibition affect $K_m$ and $V_{max}$?

The inhibitor binds to an allosteric site; $V_{max}$ decreases because fewer functional enzymes are available, but $K_m$ stays the same because substrate binding affinity is unaffected.

How does uncompetitive inhibition affect $K_m$ and $V_{max}$?

The inhibitor binds only to the enzyme-substrate complex; both $K_m$ and $V_{max}$ decrease.

What defines irreversible inhibition?

The active site is made unavailable for a prolonged period or the enzyme is permanently altered, usually via a covalent bond, requiring the cell to synthesize new enzymes to regain activity.

What are zymogens?

Enzymes that are secreted in an inactive form and must be activated by cleavage.