Q.33 In an in vitro dehydrogenation reaction of succinate catalyzed by succinate dehydrogenase, malonate
is added. Which one of the following curves represents the effect of malonate on the catalysis of
succinate dehydrogenase?
Malonate acts as a competitive inhibitor of succinate dehydrogenase in the in vitro dehydrogenation of succinate. This inhibition occurs because malonate structurally resembles succinate, competing for the enzyme’s active site and altering kinetic parameters in a characteristic manner.
Succinate dehydrogenase malonate inhibition is a classic example of competitive inhibition in enzyme kinetics, vital for CSIR NET Life Sciences preparation. This SEO-optimized guide details the in vitro dehydrogenation reaction of succinate, malonate’s impact on catalysis, typical curve options, and graphical analysis for exam success.
Reaction Overview
Succinate dehydrogenase catalyzes succinate to fumarate in the TCA cycle, with FADH2 production during dehydrogenation. In vitro, adding malonate—a succinate analog (HOOC-CH2-COOH vs. HOOC-CH2-CH2-COOH)—blocks the active site without reaction. This competitive inhibition raises apparent Km but leaves Vmax unchanged, as excess substrate displaces malonate.
Typical Curve Options Explained
CSIR NET-style questions (like Q.33) present 4 graphs (A-D), often Michaelis-Menten or Lineweaver-Burk plots. No images here, but standard options include:
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Option A: Michaelis-Menten (v vs. [S]) – Curves with same Vmax, inhibited reaches Vmax slower (higher Km). Wrong if Vmax decreases.
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Option B: Lineweaver-Burk (1/v vs. 1/[S]) – Lines intersect y-axis (same Vmax), different x-intercepts (higher Km for +malonate). Correct for competitive inhibition.
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Option C: Non-competitive pattern – Same Km (x-intercept), lower Vmax (higher y-intercept). Lines intersect x-axis. Wrong—malonate is competitive.
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Option D: Uncompetitive or mixed – Parallel lines or altered intercepts differently. Wrong for malonate.
The correct curve shows competitive inhibition signatures: unchanged Vmax, increased Km.
| Plot Type | Competitive (+Malonate) Effect | Key Feature |
|---|---|---|
| Michaelis-Menten | Same Vmax, higher [S] for half Vmax | Slower rise to plateau |
| Lineweaver-Burk | Same y-intercept, more positive x-intercept | Lines cross at y-axis |
| Non-competitive | Lower Vmax, same Km | Lines cross at x-axis |
