Coenzymes & Cofactors: Definitions & Distinctions - Enzyme's Entourage
- Cofactor: Non-protein molecule essential for enzyme activity. Can be inorganic (e.g., metal ions like $Mg^{2+}$, $Zn^{2+}$) or organic.
- Coenzyme: Organic cofactor, often derived from vitamins (e.g., NAD+, FAD).
- Cosubstrate: Loosely bound coenzyme; binds and releases like a substrate.
- Prosthetic Group: Tightly or covalently bound coenzyme (e.g., heme in cytochromes).
- Apoenzyme: Inactive protein portion of an enzyme, requires a cofactor for activity.
- Holoenzyme: Catalytically active enzyme; formed by Apoenzyme + Cofactor.
⭐ Holoenzyme = Apoenzyme + Cofactor (or Coenzyme).
Metal Ion Cofactors - Metal Mavericks
Metal ions: vital inorganic cofactors.
- Metalloenzymes: Tightly bind ions (e.g., $Fe^{2+}$, $Zn^{2+}$, $Cu^{2+}$) for structure/catalysis.
- Metal-activated enzymes: Loosely bind ions (e.g., $Mg^{2+}$, $K^{+}$, $Ca^{2+}$) for activation/substrate binding.
| Metal Ion | Enzyme Examples | Key Function/Role |
|---|---|---|
| $Zn^{2+}$ | Carbonic Anhydrase, Alcohol Dehydrogenase, Carboxypeptidases | Catalytic, structural (zinc fingers) |
| $Mg^{2+}$ | Kinases, ATPases, Polymerases | Binds ATP ($Mg^{2+}$-ATP), enzyme activation |
| $Fe^{2+}/Fe^{3+}$ | Cytochromes, Catalase, Aconitase | Redox, $O_2$ transport (Heme) |
| $Cu^{+}/Cu^{2+}$ | Cytochrome c Oxidase, Tyrosinase, SOD (Cu-Zn) | Redox, $O_2$ activation |
| $Mn^{2+}$ | Arginase, SOD (Mn-SOD), Pyruvate Carboxylase | Redox, Lewis acid |
| $K^{+}$ | Pyruvate Kinase | Enzyme activation |
| Mo | Xanthine Oxidase | Redox (O-atom transfer) |
| Se | Glutathione Peroxidase | Antioxidant (selenocysteine) |
Vitamin-Derived Coenzymes (B1, B2, B3) - B-Vitamin Buzz
| Vitamin (Name & Bx) | Active Coenzyme Form(s) | Key Biochemical Rxn/Pathway | Clinically Relevant Deficiency Symptoms/Syndrome |
|---|---|---|---|
| Thiamine (B1) | Thiamine Pyrophosphate (TPP) | $\alpha$-keto acid decarboxylation (PDH, $\alpha$-KGDH); Transketolase (HMP shunt) | Beriberi (Wet: CVS; Dry: PNS), Wernicke (Confusion, Ophthalmoplegia, Ataxia), Korsakoff (Amnesia, Confabulation) |
| Riboflavin (B2) | Flavin Mononucleotide (FMN), Flavin Adenine Dinucleotide (FAD) | Redox rxns (e.g., Succinate Dehydrogenase $FAD \rightarrow FADH_2$); ETC | Ariboflavinosis: Cheilosis, glossitis (magenta tongue), seborrheic dermatitis, corneal vascularization |
| Niacin (B3) | Nicotinamide Adenine Dinucleotide ($NAD^+$), Nicotinamide Adenine Dinucleotide Phosphate ($NADP^+$) | Redox rxns ($NAD^+ \leftrightarrow NADH$, $NADP^+ \leftrightarrow NADPH$); Dehydrogenases | Pellagra (📌 3Ds: Dermatitis, Diarrhea, Dementia; can be 4Ds with Death); Hartnup disease, Carcinoid syndrome |
Vitamin-Derived Coenzymes (B5, B6, B7, B9, B12) - More B-Power Ups
| Vitamin (Name & Bx) | Active Coenzyme Form(s) | Key Biochemical Rxn/Pathway | Clinically Relevant Deficiency Symptoms/Syndrome |
|---|---|---|---|
| B5 (Pantothenic Acid) | Coenzyme A (CoA), Acyl Carrier Protein (ACP) | Acyl transfer (TCA, FA metab.) | Rare; Burning feet syndrome, dermatitis, alopecia |
| B6 (Pyridoxine) | Pyridoxal Phosphate (PLP) | Transamination, decarboxylation (AA metab.), heme syn., glycogen phosphorylase | Sideroblastic anemia, peripheral neuropathy, cheilosis, convulsions (Isoniazid can induce) |
| B7 (Biotin) | Biotin (bound to carboxylases) | Carboxylation (📌 'ABC' enzymes: Pyruvate Carboxylase, Acetyl-CoA Carboxylase, Propionyl-CoA Carboxylase) | Rare; Dermatitis, alopecia, enteritis. Avidin (raw egg white) antagonism. |
| B9 (Folic Acid) | Tetrahydrofolate (THF) derivatives | 1-C metab. (purine, $dTMP$ syn.); $Homocysteine \rightarrow Methionine$ (w/ B12) | Megaloblastic anemia (NO neurological sx), neural tube defects (fetus), glossitis. ↑Homocysteine. |
| B12 (Cobalamin) | Methylcobalamin, Adenosylcobalamin | $Homocysteine \rightarrow Methionine$ (Methionine Syn.); $Methylmalonyl-CoA \rightarrow Succinyl-CoA$ (Methylmalonyl-CoA Mutase) | Megaloblastic anemia, pernicious anemia (IF def.), neurological sx (SCD). ↑Homocysteine, ↑MMA. |
High‑Yield Points - ⚡ Biggest Takeaways
- Cofactors: Non-protein helpers for enzyme activity. Coenzymes are organic cofactors, often vitamin-derived.
- Apoenzyme (protein) + Cofactor = Holoenzyme (active enzyme).
- Key B-vitamin coenzymes: NAD⁺ (Niacin), FAD (Riboflavin), TPP (Thiamine), CoA (Pantothenate), PLP (B6).
- Metal ions (Mg²⁺, Zn²⁺, Fe²⁺) are common inorganic cofactors, aiding catalysis or binding.
- Prosthetic groups are tightly bound cofactors (e.g., Heme, Biotin in carboxylases).
- Coenzymes act as group transfer reagents or electron carriers (e.g., ATP, NADH).
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