Coenzymes

Coenzymes

Coenzymes are small, non-protein organic molecules that are essential for the activity of certain enzymes. They act as carriers of specific chemical groups or electrons between enzymatic reactions. Most coenzymes are derived from B vitamins.

Classification

  • Cosubstrates: Loosely bound, act as second substrate; regenerated by another enzyme. Example: NAD+ (oxidized) ↔ NADH (reduced). Released after each reaction.
  • Prosthetic groups: Tightly (covalently) bound to enzyme; not released. Example: Biotin, FAD in some enzymes.

Major Coenzymes

  • NAD+ / NADH (from Niacin/B3): Electron carriers in catabolism. NAD+ accepts hydride (H⁻) → NADH. Used in glycolysis, TCA cycle, β-oxidation. Reoxidized by ETC Complex I. NADPH (from NADP+) used in reductive synthesis (FA synthesis, steroid synthesis) and antioxidant defense.
  • FAD / FADH₂ (from Riboflavin/B2): Electron carrier. FAD accepts 2 electrons → FADH₂. Tightly bound prosthetic group. Reoxidized by ETC Complex II. Examples: Succinate dehydrogenase (TCA), Acyl-CoA dehydrogenase (β-oxidation).
  • Coenzyme A (CoA-SH) (from Pantothenic acid/B5): Carrier of acyl groups (acetyl-CoA, succinyl-CoA, fatty acyl-CoA). Central to fatty acid metabolism, TCA cycle, cholesterol synthesis. The thioester bond (-CO-S-CoA) is a high-energy bond (activated acyl group).
  • Thiamine Pyrophosphate (TPP) (from Thiamine/B1): Required for oxidative decarboxylation reactions. Key enzymes: Pyruvate dehydrogenase (Pyruvate → Acetyl-CoA), α-Ketoglutarate dehydrogenase (TCA cycle), Transketolase (HMP shunt).
  • Pyridoxal Phosphate (PLP) (from Pyridoxine/B6): Most versatile coenzyme. Involved in ALL amino acid metabolism reactions: Transamination (Aminotransferases), Decarboxylation (decarboxylases), Racemization. Also for glycogen phosphorylase and heme synthesis (ALA synthase).
  • Biotin (B7): Prosthetic group of carboxylases. CO₂ carrier (activated by ATP). Key enzymes: Acetyl-CoA Carboxylase (→Malonyl-CoA, rate-limiting in FA synthesis), Pyruvate Carboxylase (→OAA), Propionyl-CoA Carboxylase.
  • Tetrahydrofolate (THF) (from Folate/B9): Carrier of one-carbon groups (methyl, methylene, formyl, etc.). Essential for: dTMP synthesis (thymidylate synthase), Purine synthesis, Amino acid interconversions (Ser→Gly). Methotrexate → DHF reductase inhibitor → blocks folate metabolism → stops rapidly dividing cells.
  • Cobalamin (B12): Cofactor for: Methionine Synthase (remethylates homocysteine, requires B12 and THF → methyl trap in B12 deficiency) and Methylmalonyl-CoA Mutase (odd-chain fatty acid and amino acid catabolism).
  • Lipoic Acid: Prosthetic group of pyruvate and α-ketoglutarate dehydrogenase complexes. Carries acyl groups and electrons between active sites in multienzyme complex.

Key Clinical Tips

  • TPP, PLP, Biotin, Lipoic acid, FAD are prosthetic groups (tightly bound)
  • NAD+, CoA, THF are cosubstrates (loosely bound, released)
  • Vitamin B12 deficiency → methyl trap (THF trapped as N5-methyl-THF) → folate deficiency symptoms even with adequate folate

Quiz - Exam Preparation Strategy

When studying Quiz for your final board exams, it is critical to focus on the core concepts and fundamental formulas. Relying strictly on NCERT textbook solutions and practicing previous year questions (PYQs) is the proven methodology for scoring high marks. Avoid rote memorization and instead focus on the logical application of the theories presented in this chapter.

⚠️ Common Mistakes to Avoid

❓ Frequently Asked Questions

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Board exams tend to favor conceptual application questions and direct formula-based derivations from the NCERT syllabus. Ensure you have solved every single exercise in the official textbook.

Is reading the NCERT book enough for this chapter?

Yes, the NCERT textbook is the absolute gold standard for board exams. However, to improve your speed and accuracy during the actual exam, you must supplement your reading by solving timed mock tests and objective questions.