Concept 04 / 05Biomolecules
Enzymes, Vitamins and Hormones
Enzymes as specific biological catalysts (lock-and-key model, examples and Michaelis-Menten behaviour), fat-soluble and water-soluble vitamins with their deficiency diseases, and hormones (peptide, amino acid derivatives and steroids) with their functions.
Enzymes are biological catalysts, almost all globular proteins, that speed up reactions in living cells. They are named after the substrate or reaction, usually ending in -ase.
Examples: invertase (sucrose → glucose + fructose); maltase (maltose → glucose); diastase (starch → maltose); zymase (glucose → ethanol + ); urease (urea → ); pepsin (proteins → peptides); oxidoreductases catalyse redox reactions.
Features. Highly specific (each enzyme acts on one substrate or a narrow class); very efficient (turnover numbers up to millions per second); act under mild conditions (optimum temperature about 298-310 K and optimum pH, e.g. about 2 for pepsin and 7 to 8 for many others); lower the activation energy (sucrose hydrolysis: from about 107 to 36 kJ mol⁻¹ with invertase); activity is increased by activators and co-enzymes and reduced by inhibitors and poisons.
Mechanism. In the lock-and-key model, the substrate fits the active site of the enzyme like a key in a lock, forming an enzyme-substrate complex that then gives the products: . This leads to Michaelis-Menten kinetics, (see figure). Drugs often work by inhibiting a particular enzyme.
Vitamins are organic compounds needed in small amounts for normal growth and health, which the body (mostly) cannot synthesise.
Fat-soluble: A, D, E and K. They are stored in the liver and adipose tissue.
Water-soluble: the B group and C. They are excreted in urine and so must be supplied regularly in the diet (except , which is stored in the liver).
Deficiency diseases: A (retinol): xerophthalmia (hardening of the cornea) and night blindness. (thiamine): beriberi. (riboflavin): cheilosis (cracking at the corners of the mouth), digestive disorders. (pyridoxine): convulsions. (cyanocobalamin, contains cobalt): pernicious anaemia. C (ascorbic acid): scurvy (bleeding gums). D: rickets in children and osteomalacia in adults. E (tocopherols): increased fragility of red blood cells and muscular weakness. K: increased blood-clotting time.
Hormones are molecules produced by endocrine glands and carried by the blood to target cells; they act as intercellular messengers.
Polypeptides and proteins: insulin and glucagon (pancreas) control blood glucose; insulin lowers it, glucagon raises it.
Amino acid derivatives: epinephrine and norepinephrine (fight-or-flight response); thyroxine (an iodinated derivative of tyrosine) controls metabolic rate; low levels cause hypothyroidism and goitre, excess causes hyperthyroidism.
Steroids: glucocorticoids (carbohydrate metabolism, anti-inflammatory, stress), mineralocorticoids (renal excretion of water and salts; deficiency leads to Addison's disease), and sex hormones (testosterone, estradiol, progesterone), which control sexual characteristics and the reproductive cycle.
Key Takeaways:
- Enzymes are specific, efficient protein catalysts that lower activation energy; they work best at an optimum temperature and pH.
- Rate is when .
- Fat-soluble: A, D, E, K (stored); water-soluble: B and C (must be taken regularly).
- Pair each vitamin with its disease: C scurvy, D rickets, beriberi, pernicious anaemia, K clotting.
- Pitfall: vitamin is water-soluble but, unlike the others, is stored in the liver.
Brain-Teaser 1: An enzyme follows Michaelis-Menten kinetics. At what substrate concentration, in terms of , is the rate of ?
Solution: gives , so . Saturation is approached slowly: going from to needs nine times the substrate.
Brain-Teaser 2: Why must vitamin C be supplied regularly, while vitamin A need not be?
Solution: Vitamin C is water-soluble, so any excess is excreted in urine and cannot be stored. Vitamin A is fat-soluble and is stored in the liver and adipose tissue, so reserves last for some time (and very large doses can accumulate to toxic levels).
Brain-Teaser 3: Enzymes lower the activation energy of sucrose hydrolysis from to . By roughly what factor does the rate increase at 310 K, assuming the same pre-exponential factor?
Solution: . Enzymes speed up this reaction by about a trillion times.