Question

Assertion (A): TCA cycle requires continuous replenishment of oxaloacetic acid.
Reason (R): TCA cycle requires regeneration of NAD\textsuperscript{+} and FAD\textsuperscript{+} from NADH and FADH.

• A. Both (A) and (R) are correct, R is correct explanation for (A)
• B. Both (A) and (R) are correct. But (R) is not correct explanation for (A)
• C. (A) is true, but (R) is false
• D. (A) is false but (R) is true

Hint:

Distinguish between two critical requirements for the continuous operation of the TCA cycle: \begin{itemize} \item \textbf{Anaplerotic reactions:} Replenish cycle intermediates (like OAA) when they are used in biosynthetic pathways. \item \textbf{Regeneration of oxidized coenzymes:} NADH and FADH\textsubscript{2} must be reoxidized to NAD\textsuperscript{+} and FAD in the electron transport chain to ensure a continuous supply of electron acceptors for the cycle's dehydrogenation steps. \end{itemize} These are distinct, though equally vital, aspects.

Answer 1

The Correct Option is B

Step 1: Analyze Assertion (A)
Assertion (A) states: "TCA cycle requires continuous replenishment of oxaloacetic acid."
\begin{itemize} \item The TCA (Tricarboxylic Acid) cycle, also known as the Krebs cycle, is a central metabolic pathway. Oxaloacetic acid (OAA) is a 4-carbon compound that condenses with acetyl-CoA (2-carbon) to initiate the cycle, forming citrate. OAA is regenerated at the end of each turn of the cycle. \item However, many intermediates of the TCA cycle (e.g., $\alpha$-ketoglutarate, succinyl CoA, malate, fumarate, OAA itself) serve as precursors for various anabolic pathways (e.g., synthesis of amino acids, porphyrins, glucose). When these intermediates are drawn off for biosynthesis, the concentration of OAA would decrease, potentially halting the cycle. \item To maintain the cycle's continuous operation and compensate for the withdrawal of intermediates, OAA must be replenished through anaplerotic reactions (e.g., conversion of pyruvate to OAA by pyruvate carboxylase). \item Therefore, Assertion (A) is True. \end{itemize} Step 2: Analyze Reason (R)
Reason (R) states: "TCA cycle requires regeneration of NAD\textsuperscript{+} and FAD\textsuperscript{+} from NADH and FADH." \begin{itemize} \item During the TCA cycle, NAD\textsuperscript{+} is reduced to NADH and FAD is reduced to FADH\textsubscript{2} at several steps. These reduced coenzymes carry high-energy electrons. \item For the TCA cycle to continue functioning, these electron acceptors (NAD\textsuperscript{+} and FAD) must be continuously regenerated from their reduced forms (NADH and FADH\textsubscript{2}). \item This regeneration primarily occurs through oxidative phosphorylation, where NADH and FADH\textsubscript{2} donate their electrons to the electron transport chain, getting reoxidized to NAD\textsuperscript{+} and FAD. \item Without this regeneration, the supply of NAD\textsuperscript{+} and FAD would become limiting, and the TCA cycle (as well as glycolysis) would come to a halt. \item Therefore, Reason (R) is True. \end{itemize} Step 3: Evaluate the relationship between (A) and (R) \begin{itemize} \item Both Assertion (A) and Reason (R) are correct statements about the TCA cycle. \item Assertion (A) addresses the need to maintain the carbon skeleton of the cycle by replenishing OAA when intermediates are siphoned off for other pathways. \item Reason (R) addresses the need to maintain the redox balance of the cycle by regenerating the oxidized forms of the coenzymes (NAD\textsuperscript{+} and FAD) that are required for the cycle's enzymatic reactions. \item While both are crucial for the continuous operation of the TCA cycle, the regeneration of NAD\textsuperscript{+} and FAD does not explain why OAA needs to be replenished. The need for OAA replenishment arises from the anaplerotic functions of the cycle intermediates. \item Therefore, Reason (R) is not the correct explanation for Assertion (A). \end{itemize} Step 4: Choose the correct option
Based on the analysis, both (A) and (R) are correct, but (R) is not the correct explanation for (A). This matches Option (2).