Regulation of Enzyme Activity

1. Regulation of Enzyme Activity • Enzyme activity must be regulated so that the proper levels of products are produced at all times and places • This control occurs in several ways: - biosynthesis at the genetic level - covalent modification after biosynthesis - regulatory enzymes - feedback inhibition • A common covalent enzyme modification is the addition or removal of a phosphate group - under high-energy conditions (high ATP and low ADP), phosphorylation is favored - under low-energy conditions (low ATP and high ADP), dephosphorylation is favored - this regulates the balance between biosynthesis and catabolism

2. Zymogens • Zymogens (proenzymes) are inactive forms of enzymes • They are activated by removal of peptide sections • For example, proinsulin is converted to insulin by removing a 33-amino acid peptide chain

3. Digestive Enzymes • Digestive enzymes are produced as zymogens, and are then activated when needed • Most of them are synthesized and stored in the pancreas, and then secreted into the small intestine, where they are activated by removal of small peptide sections • The digestive enzymes must be stored as zymogens because otherwise they would damage the pancreas

4. Allosteric Enzymes • An allosteric enzyme binds a regulator molecule at a site other than the active site (an allosteric site) • Regulators can be positive or negative: - a positive regulator enhances the binding of substrate and accelerates the rate of reaction. - a negative regulator prevents the binding of the substrate to the active site and slows down the rate of reaction (non-competitive inhibition)

5. Feedback Control • In feedback control, a product acts as a negative regulator • When product concentration is high, it binds to an allosteric site on the first enzyme (E1) in the sequence, and production is stopped • When product concentration is low, it dissociates from E1 and production is resumed • Feedback control allows products to be formed only when needed

6. Enzyme Cofactors • A simple enzyme consists only of protein in its active form • Other enzymes are active only when they combine with cofactors such as metal ions or small molecules - a cofactor that is a small organic molecule, such as a vitamin, is called a coenzyme

7. Metal Ions as Cofactors • Many enzymes require a metal ion to carry out catalysis • Metal ions in the active site are attached to one or more amino acid side-chains • The metal ions have various functions, such as electron exchange and substrate stabilization

8. A Zinc Carboxypeptidase • A Zn2+ ion in the active site of carboxypeptidase A promotes hydrolysis of a C-terminal amino acid from a polypeptide by interacting with the carbonyl oxygen • The Zn2+ activates the carbonyl in a similar way as an acid catalyst

9. Functions of Coenzymes • Coenzymes are small organic molecules that are often required to prepare the active site for proper substrate binding and/or participate in catalysis • Because they are not destroyed during the reaction, coenzymes are only required in small quantities

10. Water Soluble Vitamins • Vitamins are organic molecules that are essential for metabolism, but can not be biosynthesized; they must be consumed in the diet • Many coenzymes come from water-soluble vitamins • Water soluble vitamins are not stored in the body, and so should be consumed daily

11. Fat Soluble Vitamins • Fat soluble vitamins are not used as coenzymes • However, they are important in vision, bone formation, antioxidants, and blood clotting • Fat soluble vitamins are stored in the body, so should not be consumed in excess, as they can be toxic at high levels

12. Thiamin (Vitamin B1) • Thiamin was the first B vitamin identified, and is part of the coenzyme thiamin pyrophosphate (TPP) • TPP coenzyme is required by enzymes for decarboxylation of -keto carboxylic acids • A deficiency of thiamin results in beriberi (fatigue, weight loss, and nerve degeneration) • Dietary sources include whole grains, milk products and yeast

13. Riboflavin (Vitamin B2) • Riboflavin is made of the sugar alcohol ribitol and flavin • It is part of the coenzymes flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) • FAD and FMN are used in redox reactions involving carbohydrates, proteins and fats • Riboflavin is needed for good vision and healthy skin, and a deficiency can lead to cataracts and dermatitis • Dietary sources include green leafy vegetables, whole grains, milk products, chicken, eggs and peanuts

14. Niacin (Vitamin B3) • Niacin is part of the coenzyme nicotinamide adenine dinucleotide (NAD+) and NADP+ (P = phosphate) • NAD+ and NADP+ are used in redox reactions involving carbohydrates, proteins and fats • A deficiency of niacin can result in dermatitis, muscle fatigue and loss of appetite • Dietary sources include meats, rice, and whole grains

15. Pantothenic Acid (Vitamin B5) • Pantothenic acid is part of coenzyme A • Coenzyme A is involved in energy production, conversion of lipids and amino acids to glucose and synthesis of cholesterol and steroid hormones • A deficiency of pantothenic acid can result in fatigue, retarded growth, cramps, and anemia • Dietary sources include salmon, meat, eggs, whole grains, and vegetables

16. Pyridoxine (Vitamin B6) • Pyridoxine and pyridoxal are two forms of vitamin B6 • They are converted to the coenzyme pyridoxal phosphate (PLP) • PLP is involved in the transamination of amino acids and the decarboxylation of carboxylic acids • A deficiency of pyridoxine may lead to dermatitis, fatigue and anemia • Dietary sources include fish, meat, nuts, whole grains and spinach

17. Cobalamin (Vitamin B12) • Cobalamin consists of four pyrrole rings with a Co2+ • It is a coenzyme involved in the transfer of methyl groups, acetyl choline synthesis and red blood cell production • A deficiency in vitamin B12 can lead to pernicious anemia and nerve damage • Dietary sources include beef, chicken, fish and milk products (strict vegans should take B12 supplements)

18. Ascorbic Acid (Vitamin C) • Ascorbic acid is a very polar hydroxy ester that is a weak acid • It is involved in the synthesis of hydroxyproline and hydroxylysine, two modified amino acids that are required for collagen synthesis • A deficiency of vitamin C can lead to slow-healing wounds, weakened connective tissue, bleeding gums and anemia • Dietary sources include berries, citrus fruits, tomatoes, bell peppers, broccoli and cabbage

19. Folic Acid (Folate) • Folic acid (folate) consists of pyrimidine, p-aminobenzoic acid (PABA) and glutamate • It forms the coenzyme THF used in the synthesis of nucleic acids • A deficiency can lead to abnormal red blood cells, anemia, poor growth, hair loss and depression • Dietary sources include green leafy vegetables, beans, meat, seafood, yeast, asparagus and whole grains • Some derivatives of folic acid, such as methotrexate, are inhibitors of the enzyme that converts folic acid to THF - these are used as anti-cancer drugs, especially for leukemias

20. Vitamin A • Vitamin A can exist as an alcohol (retinol), an aldehyde (retinal) or a carboxylic acid (retinoic acid) • In the retina of the eye, retinol undergoes cis-trans isomeration as part of photoreception • Vitamin A is also involved in synthesis of RNA and glycoproteins • A deficiency in vitamin A can lead to night blindness, depressed immune response and growth inhibition • Dietary sources include yellow and green fruits and vegetables • Beta-carotenes are converted to vitamin A in the liver

21. Vitamin D • Vitamin D (D3) is synthesized from 7-dehydrocholesterol in skin exposed to sunlight • It regulates the absorption of phosphorus and calcium during bone growth • A deficiency in vitamin D can result in weakened bones • Dietary sources include cod liver oil, egg yolk, and vitamin D enriched foods (such as milk)

22. Vitamin E • Vitamin E (-tocopherol) acts as an antioxidant in cells • Not much is know about its mechanism, but it may prevent the oxidation of unsaturated fatty acids • A deficiency of vitamin E can lead to anemia • Dietary sources include meat, nuts, vegetable oils, whole grains, and vegetables • Synthetic vitamin E is a mixture of the alpha and beta forms (enantiomers) - only the alpha form can be utilized by our cells

23. Vitamin K • Vitamin K1 (in plants) has a saturated side chain • Vitamin K2 (in animals) has a long unsaturated side chain • Vitamin K2 is needed for the synthesis of zymogens for blood clotting • A deficiency of vitamin K can lead to extended bleeding from small cuts and increased bruising • Dietary sources include meat, spinach and cauliflower