Biochemistry - Metabolism & Enzyme Kinetics Flashcards

Question 1

What is the primary purpose of glycolysis?

Accepted Answer

To break down glucose into two molecules of pyruvate, generating a small amount of ATP and NADH, providing precursors for further metabolic pathways.

Question 2

List the net products of glycolysis from one molecule of glucose.

Accepted Answer

2 Pyruvate, 2 ATP (net), 2 NADH.

Question 3

Identify the three irreversible enzymes in glycolysis that are key regulatory points.

Accepted Answer

Hexokinase (or Glucokinase), Phosphofructokinase-1 (PFK-1), and Pyruvate Kinase.

Question 4

What is gluconeogenesis, and why is it essential?

Accepted Answer

The metabolic pathway that synthesizes glucose from non-carbohydrate precursors (e.g., lactate, amino acids, glycerol). It's essential to maintain blood glucose levels, especially during fasting or starvation, to supply glucose to tissues like the brain and red blood cells.

Question 5

Name the four key enzymes in gluconeogenesis that bypass the irreversible steps of glycolysis.

Accepted Answer

Pyruvate Carboxylase, PEP Carboxykinase, Fructose-1,6-bisphosphatase, and Glucose-6-phosphatase.

Question 6

What are the main inputs and outputs of one turn of the Citric Acid Cycle (TCA cycle)?

Accepted Answer

**Input:** Acetyl-CoA. **Outputs:** 3 NADH, 1 FADH$_2$, 1 GTP (or ATP), 2 CO$_2$.

Question 7

Describe the role of the Electron Transport Chain (ETC) in oxidative phosphorylation.

Accepted Answer

The ETC consists of protein complexes that accept electrons from NADH and FADH$_2$, passing them sequentially to oxygen. This electron flow pumps protons from the mitochondrial matrix to the intermembrane space, creating an electrochemical proton gradient.

Question 8

How does ATP Synthase produce ATP?

Accepted Answer

ATP Synthase utilizes the proton motive force (the electrochemical gradient generated by the ETC) to drive the phosphorylation of ADP to ATP. Protons flow back into the mitochondrial matrix through the enzyme, causing conformational changes that synthesize ATP.

Question 9

What is $\beta$-oxidation, and what are its primary products?

Accepted Answer

$\beta$-oxidation is the catabolic process by which fatty acid molecules are broken down in the mitochondria to generate Acetyl-CoA, NADH, and FADH$_2$. Each cycle removes two carbons from the fatty acyl chain.

Question 10

Briefly explain the overall purpose of the Urea Cycle.

Accepted Answer

The Urea Cycle is the primary mechanism for the excretion of excess nitrogen from amino acid catabolism in the form of urea, a less toxic compound, preventing ammonia toxicity.

Question 11

Differentiate between light-dependent and light-independent reactions in photosynthesis.

Accepted Answer

**Light-dependent reactions:** Occur in thylakoid membranes, use light energy to produce ATP and NADPH, releasing O$_2$. **Light-independent reactions (Calvin Cycle):** Occur in the stroma, use ATP and NADPH from light reactions to fix CO$_2$ into glucose.

Question 12

Define the Michaelis-Menten equation and explain the significance of $K_m$ and $V_{max}$.

Accepted Answer

The Michaelis-Menten equation describes the rate of enzymatic reactions: $V_0 = \frac{V_{max}[S]}{K_m + [S]}$.
*   $V_{max}$ (maximum velocity): The maximum rate of reaction when the enzyme is saturated with substrate.
*   $K_m$ (Michaelis constant): The substrate concentration at which the reaction velocity is half of $V_{max}$. It reflects the enzyme's affinity for its substrate (lower $K_m$ = higher affinity).

Question 13

How do competitive inhibitors affect $K_m$ and $V_{max}$ on a Lineweaver-Burk plot?

Accepted Answer

Competitive inhibitors increase the apparent $K_m$ (shift the x-intercept closer to zero, or to the right on a standard plot of $1/V_0$ vs $1/[S]$) but do not change $V_{max}$ (the y-intercept remains the same). They compete with the substrate for the active site.

Question 14

How do non-competitive inhibitors affect $K_m$ and $V_{max}$ on a Lineweaver-Burk plot?

Accepted Answer

Non-competitive inhibitors decrease $V_{max}$ (increase the y-intercept) but do not change $K_m$ (the x-intercept remains the same). They bind to a site other than the active site, altering enzyme conformation.

Question 15

Explain allosteric regulation of enzymes.

Accepted Answer

Allosteric regulation occurs when a molecule (an allosteric effector) binds to an enzyme at a site other than the active site (the allosteric site), causing a conformational change that either increases (activator) or decreases (inhibitor) the enzyme's activity.

Question 16

What is feedback inhibition in metabolic pathways?

Accepted Answer

Feedback inhibition is a common regulatory mechanism where the end product of a metabolic pathway inhibits an enzyme earlier in the pathway. This prevents overproduction of the end product and conserves resources.

Question 17

What is the role of coenzymes and cofactors in enzyme function?

Accepted Answer

**Cofactors** are non-protein chemical components required for enzyme activity (e.g., metal ions like Mg$^{2+}$, Zn$^{2+}$). **Coenzymes** are organic cofactors, often derived from vitamins (e.g., NAD$^+$, FAD, CoA), that participate directly in the reaction, often by carrying electrons or functional groups.

Question 18

Describe the "induced fit" model of enzyme-substrate binding.

Accepted Answer

The induced fit model proposes that the active site of an enzyme is not a rigid structure but rather undergoes a conformational change upon binding to its substrate. This change optimizes the fit between the enzyme and substrate, enhancing catalysis.

Question 19

What is the primary function of the Pentose Phosphate Pathway (PPP)?

Accepted Answer

The PPP (also known as the hexose monophosphate shunt) has two main functions: to produce NADPH (essential for reductive biosynthesis and protection against oxidative stress) and to generate the 5-carbon sugar ribose-5-phosphate (a precursor for nucleotide synthesis).

Question 20

How does pyruvate dehydrogenase complex (PDC) link glycolysis to the TCA cycle?

Accepted Answer

The PDC converts pyruvate (the end product of glycolysis) into Acetyl-CoA. This irreversible reaction also produces NADH and CO$_2$, effectively linking the anaerobic process of glycolysis to the aerobic TCA cycle.

Question 21

What is the significance of Fructose-2,6-bisphosphate (F2,6BP) in regulating glycolysis and gluconeogenesis?

Accepted Answer

F2,6BP is a potent allosteric activator of PFK-1 (glycolysis) and an inhibitor of Fructose-1,6-bisphosphatase (gluconeogenesis). Its concentration is regulated by the bifunctional enzyme PFK-2/FBPase-2, ensuring reciprocal regulation of the two pathways.

Question 22

Explain the concept of uncoupling in oxidative phosphorylation.

Accepted Answer

Uncoupling agents (e.g., DNP, thermogenin) dissipate the proton gradient across the inner mitochondrial membrane by making it permeable to protons. This allows protons to flow back into the matrix without passing through ATP synthase, thus generating heat instead of ATP.

Biochemistry - Metabolism & Enzyme Kinetics

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