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
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
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
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)
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 (E 1 ) in the sequence, and production is stopped When product concentration is low, it dissociates from E 1 and production is resumed Feedback control allows products to be formed only when needed
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
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
A Zinc Carboxypeptidase A Zn 2+ 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 Zn 2+ activates the carbonyl in a similar way as an acid catalyst
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
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
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
Thiamin (Vitamin B 1 ) 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 α-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
Riboflavin (Vitamin B 2 ) 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 O H 3C N N H H 3 C N N O D-Ribitol CH 2 CH CH CH CH 2 OH OH OH OH
Niacin (Vitamin B 3 ) 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 O OH N Niacin (Nicotinic Acid)
Pantothenic Acid (Vitamin B 5 ) 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 CH 3 OH O O HO CH 2 C CH C N CH 2 CH 2 C OH CH 3 H
Pyridoxine (Vitamin B 6 ) Pyridoxine and pyridoxal are two forms of vitamin B 6 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
Cobalamin consists of four pyrrole rings with a Co 2+ It is a coenzyme involved in the transfer of methyl groups, acetyl choline synthesis and red blood cell production A deficiency in vitamin B 12 can lead to pernicious anemia and nerve damage Dietary sources include beef, chicken, fish and milk products (strict vegans should take B 12 supplements) Cobalamin (Vitamin B 12 )
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 O O CH 2 OH CHOH HO OH
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
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 H 3 C CH 3 Beta-carotene CH 3 CH 3 H 3 C CH 3 CH 3 CH 3 H 3 C CH 3 H 3 C CH 3 CH 3 CH 3 CH 2 OH CH 3 Retinol (vitamin A)
Vitamin D Vitamin D (D 3 ) 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)
Vitamin E Vitamin E (α-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 HO H 3 C CH 3 CH 3 O CH 3 CH 3 CH 3 CH 3 CH 3
Vitamin K Vitamin K 1 (in plants) has a saturated side chain Vitamin K 2 (in animals) has a long unsaturated side chain Vitamin K 2 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 O CH 3 O CH 3 O CH 3 CH 3 CH 3 Vitamin K 1 (phylloquinone) O CH 3 CH 3 CH 3 Vitamin K 2 (menaquinone)