Q.64 Which of the following reactions in plants is/are catalyzed by the malic enzymes?
(A) Malate + NAD+ → Pyruvate + CO2 + NADH
(B) Malate + NAD+ ⇌ Oxaloacetate + NADH
(C) Malate ⇌ Fumarate
(D) Malate + NADP+ → Pyruvate + CO2 + NADPH

Malic enzymes in plants catalyze the oxidative decarboxylation of malate to pyruvate, CO₂, and reduced NAD(P)H, playing key roles in C4 photosynthesis, TCA cycle regulation, and NADPH supply. Options (A) and (D) represent the correct reactions catalyzed by NAD-ME and NADP-ME isoforms, respectively.​

Option Analysis

Option (A): Malate + NAD⁺ → Pyruvate + CO₂ + NADH
This reaction is catalyzed by NAD-dependent malic enzyme (NAD-ME, EC 1.1.1.38/39), primarily mitochondrial in plants. It supports C4 photosynthesis in NAD-ME-type species, TCA cycle anaplerosis, and metabolite exchange.​

Option (B): Malate + NAD⁺ ⇌ Oxaloacetate + NADH
Incorrect; this is catalyzed by malate dehydrogenase (MDH, EC 1.1.1.37), not malic enzyme. MDH operates in the TCA cycle and malate-aspartate shuttle without decarboxylation.​

Option (C): Malate ⇌ Fumarate
Incorrect; fumarase (EC 4.2.1.2) catalyzes this reversible hydration/dehydration in the TCA cycle. Malic enzymes require NAD(P)⁺ and produce pyruvate, not fumarate.​

Option (D): Malate + NADP⁺ → Pyruvate + CO₂ + NADPH
Correct; NADP-dependent malic enzyme (NADP-ME, EC 1.1.1.40) in chloroplasts/cytosol drives this. Essential for C4/CAM photosynthesis, NADPH generation in lipid synthesis, and stress responses.​

Malic enzymes in plants are vital for malate decarboxylation, converting malate to pyruvate and CO₂ while generating reducing power (NADH/NADPH). These reactions support photosynthesis in C4/CAM plants and metabolic flexibility.​

Plant Malic Enzyme Types

Plants feature NAD-ME (mitochondrial) and NADP-ME (chloroplastic/cytosolic) isoforms. NAD-ME handles Malate + NAD⁺ → Pyruvate + CO₂ + NADH in NAD-ME C4 subtypes and TCA flux. NADP-ME drives Malate + NADP⁺ → Pyruvate + CO₂ + NADPH, supplying NADPH for biosynthesis.​

• Concentrates CO₂ in bundle sheath cells of C4 plants.

• Provides NADPH under stress (drought, salt).​

Physiological Roles

Beyond C4/CAM, malic enzymes regulate malic acid metabolism, fatty acid synthesis, and ROS scavenging. NADP-ME boosts lipid accumulation in oilseeds; both isoforms aid stress tolerance via NADPH.​

Exam Relevance (CSIR NET)

For CSIR NET Life Sciences, recognize both (A) and (D) as correct. Distinguish from MDH (option B) and fumarase (C). Focus on EC numbers and photosynthetic subtypes.​