Biochemistry - Metabolic Pathways & Enzyme Regulation Flashcards

Question 1

What is glycolysis, where does it occur, and what are its net products?

Accepted Answer

Glycolysis is the metabolic pathway that breaks down glucose into pyruvate. It occurs in the cytoplasm. Net products per glucose molecule are 2 ATP, 2 NADH, and 2 pyruvate.

Question 2

Identify the three main regulatory enzymes in glycolysis and their irreversible steps.

Accepted Answer

1.  **Hexokinase/Glucokinase**: Glucose $\rightarrow$ Glucose-6-phosphate
2.  **Phosphofructokinase-1 (PFK-1)**: Fructose-6-phosphate $\rightarrow$ Fructose-1,6-bisphosphate (major regulatory step)
3.  **Pyruvate Kinase**: Phosphoenolpyruvate $\rightarrow$ Pyruvate

Question 3

What is the function of the Pyruvate Dehydrogenase Complex (PDC) and its key products?

Accepted Answer

PDC links glycolysis to the citric acid cycle by converting pyruvate into acetyl-CoA. It occurs in the mitochondrial matrix. Products per pyruvate are 1 acetyl-CoA, 1 NADH, and 1 $CO_2$.

Question 4

What is the Citric Acid Cycle (Krebs Cycle), where does it occur, and what are its net products per acetyl-CoA?

Accepted Answer

The Citric Acid Cycle (CAC) is a central metabolic pathway that oxidizes acetyl-CoA to $CO_2$, generating ATP precursors. It occurs in the mitochondrial matrix. Net products per acetyl-CoA are 3 NADH, 1 $FADH_2$, 1 GTP (which converts to ATP), and 2 $CO_2$.

Question 5

Name the primary regulatory enzymes of the Citric Acid Cycle.

Accepted Answer

1.  **Citrate Synthase**: Regulated by ATP, NADH, succinyl-CoA.
2.  **Isocitrate Dehydrogenase**: Activated by ADP, inhibited by ATP, NADH.
3.  **$\alpha$-Ketoglutarate Dehydrogenase Complex**: Inhibited by succinyl-CoA, NADH.

Question 6

What is gluconeogenesis, where does it primarily occur, and what are its main precursors?

Accepted Answer

Gluconeogenesis is the metabolic pathway that synthesizes glucose from non-carbohydrate precursors. It primarily occurs in the liver (and kidney). Main precursors include lactate, amino acids (glucogenic), and glycerol.

Question 7

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

Accepted Answer

1.  **Pyruvate Carboxylase**: Converts pyruvate to oxaloacetate.
2.  **PEP Carboxykinase (PEPCK)**: Converts oxaloacetate to phosphoenolpyruvate.
3.  **Fructose-1,6-bisphosphatase**: Converts fructose-1,6-bisphosphate to fructose-6-phosphate.
4.  **Glucose-6-phosphatase**: Converts glucose-6-phosphate to glucose.

Question 8

Describe the two main components of oxidative phosphorylation and their overall function.

Accepted Answer

Oxidative phosphorylation is the process where ATP is formed as a result of the transfer of electrons from NADH and $FADH_2$ to $O_2$ by a series of electron carriers. It consists of:
1.  **Electron Transport Chain (ETC)**: A series of protein complexes that transfer electrons, pumping protons across the inner mitochondrial membrane.
2.  **ATP Synthase**: Uses the proton gradient (proton-motive force) to synthesize ATP.

Question 9

Briefly describe the function of the four main complexes in the Electron Transport Chain.

Accepted Answer

*   **Complex I (NADH Dehydrogenase)**: Oxidizes NADH, transfers electrons to CoQ, pumps 4 protons.
*   **Complex II (Succinate Dehydrogenase)**: Oxidizes $FADH_2$ (from succinate), transfers electrons to CoQ, no proton pumping.
*   **Complex III (Cytochrome bc1 complex)**: Transfers electrons from $CoQH_2$ to cytochrome c, pumps 4 protons.
*   **Complex IV (Cytochrome c Oxidase)**: Transfers electrons from cytochrome c to $O_2$ (final electron acceptor), pumps 2 protons.

Question 10

How does ATP Synthase produce ATP?

Accepted Answer

ATP Synthase utilizes the proton-motive force (electrochemical gradient of protons) generated by the ETC. Protons flow back into the mitochondrial matrix through the $F_0$ subunit, causing conformational changes in the $F_1$ subunit that drive the synthesis of ATP from ADP and $P_i$.

Question 11

What is beta-oxidation, where does it occur, and what are its products per cycle?

Accepted Answer

Beta-oxidation is the catabolic process that breaks down fatty acids into acetyl-CoA. It occurs in the mitochondrial matrix. Each cycle shortens the fatty acid by two carbons, producing 1 acetyl-CoA, 1 NADH, and 1 $FADH_2$.

Question 12

What is fatty acid synthesis, where does it occur, and what is its key enzyme?

Accepted Answer

Fatty acid synthesis is the anabolic process of creating fatty acids from acetyl-CoA. It primarily occurs in the cytoplasm (liver, adipose tissue). The key enzyme is **Fatty Acid Synthase**, a multi-enzyme complex.

Question 13

State the Michaelis-Menten equation and define its variables.

Accepted Answer

The Michaelis-Menten equation describes the rate of enzymatic reactions: $V_0 = \frac{V_{max}[S]}{K_m + [S]}$
*   $V_0$: Initial reaction velocity
*   $V_{max}$: Maximum reaction velocity
*   $[S]$: Substrate concentration
*   $K_m$: Michaelis constant

Question 14

Define the Michaelis constant ($K_m$) and explain its significance.

Accepted Answer

$K_m$ is the substrate concentration at which the reaction velocity ($V_0$) is half of $V_{max}$ ($V_0 = \frac{1}{2}V_{max}$). It reflects the affinity of an enzyme for its substrate; a lower $K_m$ indicates higher affinity.

Question 15

Define $V_{max}$ in enzyme kinetics and explain what it represents.

Accepted Answer

$V_{max}$ is the maximum rate of an enzymatic reaction when the enzyme is saturated with substrate. It represents the turnover number ($k_{cat}$) multiplied by the total enzyme concentration ($V_{max} = k_{cat}[E]_T$).

Question 16

What is the purpose of a Lineweaver-Burk plot, and what do its axes and intercepts represent?

Accepted Answer

The Lineweaver-Burk plot (double reciprocal plot) is a graphical representation of the Michaelis-Menten equation, used to determine $K_m$ and $V_{max}$ and analyze enzyme inhibition.
*   **Axes**: x-axis is $1/[S]$, y-axis is $1/V_0$.
*   **Intercepts**: x-intercept is $-1/K_m$, y-intercept is $1/V_{max}$.

Question 17

Describe competitive inhibition, its effect on $K_m$ and $V_{max}$, and its appearance on a Lineweaver-Burk plot.

Accepted Answer

Competitive inhibition occurs when an inhibitor binds reversibly to the enzyme's active site, competing with the substrate.
*   **Effect**: Increases apparent $K_m$ (lower affinity), $V_{max}$ remains unchanged.
*   **Lineweaver-Burk**: Lines intersect at the y-axis (same $1/V_{max}$).

Question 18

Describe non-competitive inhibition, its effect on $K_m$ and $V_{max}$, and its appearance on a Lineweaver-Burk plot.

Accepted Answer

Non-competitive inhibition occurs when an inhibitor binds to an allosteric site on the enzyme (or enzyme-substrate complex), changing its conformation and reducing its catalytic efficiency.
*   **Effect**: $K_m$ remains unchanged, decreases apparent $V_{max}$.
*   **Lineweaver-Burk**: Lines intersect at the x-axis (same $-1/K_m$).

Question 19

Describe uncompetitive inhibition, its effect on $K_m$ and $V_{max}$, and its appearance on a Lineweaver-Burk plot.

Accepted Answer

Uncompetitive inhibition occurs when an inhibitor binds only to the enzyme-substrate (ES) complex, preventing product formation.
*   **Effect**: Decreases apparent $K_m$ and decreases apparent $V_{max}$ proportionally.
*   **Lineweaver-Burk**: Lines are parallel.

Question 20

What is allosteric regulation in enzymes?

Accepted Answer

Allosteric regulation involves the binding of a molecule (allosteric effector) to a site on the enzyme distinct from the active site. This binding causes a conformational change that alters the enzyme's activity (either activating or inhibiting it). Often seen in multi-subunit enzymes.

Question 21

Explain how covalent modification, specifically phosphorylation, regulates enzyme activity.

Accepted Answer

Covalent modification involves the formation or cleavage of covalent bonds to alter enzyme activity. Phosphorylation, the most common type, involves the addition of a phosphate group (by kinases) or its removal (by phosphatases) to specific amino acid residues (Ser, Thr, Tyr). This can activate or inactivate an enzyme by inducing conformational changes.

Question 22

What are zymogens (proenzymes) and how are they activated?

Accepted Answer

Zymogens are inactive precursors of enzymes, typically proteases, that require proteolytic cleavage to become active. This irreversible activation mechanism allows for tight control over enzyme activity, often preventing premature or inappropriate enzyme action (e.g., digestive enzymes, clotting factors).

Biochemistry - Metabolic Pathways & Enzyme Regulation

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