SN1
SHORT NOTE
5M
★ High-yield
Define coenzymes. Enumerate the coenzymes derived from vitamins with their functions.
Definition
Coenzymes = small, heat-stable, non-protein organic molecules that are essential for the activity of certain enzymes, acting as carriers of atoms, groups or electrons.
- Most coenzymes are derived from water-soluble B-complex vitamins — the reason these vitamins are required in the diet
- They are loosely bound; a tightly (covalently) bound cofactor is a prosthetic group
- Coenzymes are regenerated, so they act catalytically and are needed only in small amounts
Coenzymes Derived from Vitamins
| Vitamin | Coenzyme | Function | Deficiency |
|---|---|---|---|
| B1 Thiamine | Thiamine pyrophosphate (TPP) | Oxidative decarboxylation; transketolase | Beri-beri, Wernicke encephalopathy |
| B2 Riboflavin | FMN, FAD | Hydrogen transfer (2H) | Angular stomatitis, glossitis |
| B3 Niacin | NAD+, NADP+ | Hydrogen transfer (2 electrons + H+) | Pellagra |
| B5 Pantothenate | Coenzyme A | Acyl group transfer | Burning feet syndrome |
| B6 Pyridoxine | Pyridoxal phosphate (PLP) | Transamination, decarboxylation, ala synthase | Peripheral neuropathy, sideroblastic anaemia, convulsions |
| B7 Biotin | Biocytin | CO2 fixation (carboxylation) | Dermatitis; caused by raw egg white (avidin) |
| B9 Folate | Tetrahydrofolate | One-carbon transfer | Megaloblastic anaemia, neural tube defect |
| B12 Cobalamin | Methylcobalamin, deoxyadenosylcobalamin | Methyl transfer; methylmalonyl-CoA mutase | Megaloblastic anaemia, subacute combined degeneration |
Non-vitamin Coenzymes
- ATP — phosphate and energy transfer
- S-adenosylmethionine (SAM) — the chief methyl donor
- UDP-glucose — glycosyl transfer
- Lipoic acid — acyl transfer in the PDH complex
- Coenzyme Q — electron transfer in the respiratory chain
Applied Aspects
- Isoniazid antagonises pyridoxine → peripheral neuropathy; hence pyridoxine is given prophylactically with antitubercular treatment
- Methotrexate blocks dihydrofolate reductase → folate coenzymes cannot be regenerated; rescued with folinic acid
- Thiamine must be given before glucose in a malnourished or alcoholic patient — a glucose load without thiamine can precipitate Wernicke encephalopathy
- Multiple carboxylase deficiency responds to large doses of biotin
All questions in Enzymes
- Q1Define enzymes. Classify them according to the IUBMB system and describe the mechanism of enzyme action. Add a note on enzyme specificity. ★
- Q2Describe the effect of substrate concentration on enzyme activity. Explain the Michaelis–Menten equation, the significance of Km and the Lineweaver–Burk plot. ★
- Q3Classify enzyme inhibition. Describe competitive, non-competitive and uncompetitive inhibition with their effects on Km and Vmax, and give examples. ★
- Q4Describe the mechanisms regulating enzyme activity. Explain allosteric regulation, covalent modification and enzyme induction with examples. ★
- Q5Describe the diagnostic importance of serum enzymes. Give the enzyme profile in myocardial infarction and liver disease. Add a note on isoenzymes. ★
- SN1Define coenzymes. Enumerate the coenzymes derived from vitamins with their functions. ★
- SN2Define isoenzymes. Describe the isoenzymes of lactate dehydrogenase and creatine kinase with their clinical significance. ★
- SN3Describe allosteric enzymes with their characteristics and examples. Add a note on feedback inhibition. ★
- SN4Define zymogens. Give examples and explain their physiological importance. ★
- SN5Define Km. Describe its significance with the example of hexokinase and glucokinase. ★
- SN6Describe competitive inhibition with its characteristics and medically important examples. ★
- SN7Describe the types of enzyme specificity with examples.