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Enzymes and their Cofactors Source: Biochemistry: An Illustrated Review Table 1.4 Water-soluble Vitamins: Sources and Functional Forms Vitamin Source Functional Form B2 (riboflavin) Milk, eggs Flavin mononucleotide and flavin adenine dinucleotide (FMN, FAD) B3 (niacin) Meat, yeast products, fruits, vegetables, synthesized from tryptophan Nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate (NAD+ and NADP+) C (ascorbic acid) Fruits, vegetables Ascorbate B1 (thiamine) Grain, yeast products, pork Thyamine pyrophosphate (TPP) B5 (pantothenic acid) Widely distributed Coenzyme A B6 (pyridoxine) Meat, vegetables, grains Pyridoxal phosphate B12 (cobalamin) Meat, liver, milk, eggs Adenosylcobalamin methylcobalamin Biotin Yeast products, legumes, nuts Biotin Folate Fresh green vegetables, liver Tetrahydrofolate (THF) Involved in redox reactions Involved in nonredox reactions Table 1.6 Vitamins Involved in Nonredox Reactions, Their Associated Enzymes and Functions Vitamin Cofactor and Its Associated Enzyme and Function B1 (thiamine) Thyamine pyrophosphate (TPP) is a cofactor for: -- Pyruvate dehydrogenase complex: This enzyme decarboxylates pyruvate to generate acetyl coenzyme A (CoA) -- a-ketoglutarate dehydrogenase complex: This tricarboxylic acid (TCA) cycle enzyme decarboxylates a-ketoglutarate to generate succinyl CoA 1 -- Branched chain a-keto acid dehydrogenase: This enzyme plays a role in the degradation of branched chain amino acids isoleucine, leucine and valine -- Transketolase: This enzyme transfers twocarbon fragments between molecules in the pentose phosphate pathway B5 (pantothenic acid) Pantothenic acid is a component of: -- Coenzyme A: This molecule forms high-energy thioester bonds with other molecules to "activate" them and is involved in reactions that transfer fatty acyl groups between molecules -- Fatty acid synthase (FAS) complex: This enzyme catalyzes the synthesis of fatty acids B6 (pyridoxine) Pyridoxal phosphate is a cofactor for: -- Transaminases: These enzymes transfer amino groups between molecules to interconvert amino acids and alfa-keto acids -- Decarboxylases: These enzymes catalyze the removal of carboxyl groups. Specific decarboxylases play a critical role in the synthesis of neurotransmitters (e.g., dopamine) -- Glycogen phosphorylase: This enzyme is involved in the breakdown of glycogen (glycogenolysis) -- Aminolevulinic acid (ALA) synthase: The ratelimiting step in heme and porphyrin synthesis is catalyzed by this enzyme B12 (cobalamin) Adenosylcobalamin and methylcobalamin are cofactors for: -- Methylmalonyl CoA mutase: This enzyme isomerizes methylmalonyl CoA into succinyl CoA in a reaction that is part of the pathway that degrades odd-numbered fatty acids -- Methionine synthase/homocysteine methyltransferase: This enzyme transfers a methyl group from 5-methyltetrahydrofolate (N5-methylTHF) onto homocysteine to form methionine. Methionine reacts with adenosine triphosphate (ATP) to generate S-adenosyl methionine (SAM), the latter of which regenerates homocysteine when SAM donates a methyl group to target molecules. Biotin Biotin serves as a cofactor for: 2 -- Pyruvate carboxylase: This enzyme irreversibly carboxylates pyruvate to produce oxaloacetate -- Acetyl CoA carboxylase: This enzyme irreversibly carboxylates acetyl CoA to form malonyl CoA Folate Methyltetrahydrofolate (methyl-THF) is used as a source of methyl groups, which are transferred to various molecules. Methyl-THF is used in the biosynthesis of the purine nucleotides and thymidylate. Table 1.14 Enzymes that Utilize Zn as a Cofactor Enzyme Action Superoxide dismutase This antioxidant enzyme binds the free radical of molecular oxygen Collagenases Belongs to the family of matrix metalloproteinases. They are the only enzymes capable of degrading triple helical collagen. They play a major role in tissue repair and remodeling. Alcohol dehydrogenase Converts alcohol to acetaldehyde Alkaline phosphatase Mineralizes bone salts Transcription factors Several proteins that contain Zn-finger domains bind to specific sequences on DNA and regulate transcription Carbonic anhydrase Interconverts CO2 and bicarbonate to balance pH Table 1.15 Enzymes that Utilize Cu as a Cofactor Enzyme Action Cytochrome-c oxidase Accepts electrons from cytochrome-c in the electron transport chain Ferroxidase (ceruloplasmin) Oxidizes ferrous iron (Fe2+ ) into ferric iron (Fe3+) Superoxide dismutase This antioxidant enzyme binds the free radical of molecular oxygen Lysyl oxidase Cross-links collagen and elastic tissue during wound healing Tyrosinase Synthesizes melanin 3 Table 1.18 Trace Elements and Their Uses Microelements Use Mo A cofactor for oxidases (xanthine oxidase, aldehyde oxidase, and sulfite oxidase); antagonistic to Cu2+ Mn A cofactor for pyruvate carboxylase, which converts pyruvate to oxaloacetate A cofactor for superoxide dismutase, which binds the free radical of molecules oxygen A cofactor for arginase, which catalyzes the formation of urea from arginine A cofactor for malic enzyme, which catalyzes the oxidative decarboxylation of malate to pyruvate and CO2 Co Found in cobalamin (vitamin B12) Cr Involved in the activity of insulin to reduce blood glucose levels S Found in coenzyme A, glutathione and Sadenosylmethionine I Required for the synthesis of thyroid hormones such as circulating thyroxine (T4) and the intracellular active hormone triiodothyronine (T3). They play an important role in growth, development, and metabolic processes. Insufficient dietary intake of iodine leads to endemic goiter. Se A cofactor of glutathione peroxidase, an important antioxidant enzyme that detoxifies hydrogen peroxide; required for the conversion of the thyroid hormone T4 to T3 F Fluorine is not a proper bone salt, but it prevents demineralization of teeth (dental caries) 4