L7 Vitamins in One Carbon Metabolism PDF

Vitamins in One Carbon Metabolism Tetrahydrofolate, Vitamin B12, and SAM Marks’ Basic Medical Biochemistry , 6th Ed chapter 38 (pages: 849-866) Biochemistry, Cell and Molecular Biology, and Genetics Part V Chapter 38 (324-335) Zeynep Gromley, Ph.D....

Vitamins in One Carbon Metabolism Tetrahydrofolate, Vitamin B12, and SAM Marks’ Basic Medical Biochemistry , 6th Ed chapter 38 (pages: 849-866) Biochemistry, Cell and Molecular Biology, and Genetics Part V Chapter 38 (324-335) Zeynep Gromley, Ph.D. MANS 429 [email protected] 1  Learning Objectives Describe the roles of one-carbon metabolism by reactions involved tetrahydrofolate (FH4), vitamin B12, and S-adenosyl methionine (SAM) Evaluate biochemical basis for using methotrexate and 5-fluoro uracil in chemotherapy. Explain the importance of formyl-FH4 and methylene-FH4 in DNA synthesis, megaloblastic anemia, and neural tube defects. Explain the absorption of vitamin B12. Describe the function of B12 for the SAM biosynthesis and for the regeneration of functional folate. Explain the basic pathophysiology of both B12 and folate deficiency. 2  Functions of one-carbon metabolism Many reactions in human metabolism involve the transfer of an activated one-carbon from a donor molecule to an acceptor molecule. Biotin is the coenzyme of carboxylases, which transfers CO2 Tetra hydro folate (FH4) is the carrier of one-carbon groups as methyl (CH3), methylene (CH2), formyl (CHO) dUMP forms dTMP by accepting a one-carbon group from methylene-FH4, this reaction produces thymine required for DNA synthesis Formyl-FH4 is required for purine synthesis Vitamin B12, as a coenzyme, transfers a methyl group from methyl-FH4 to homocysteine to form methionine, which is used for the synthesis of S-adenosylmethionine (SAM) SAM is the main donor of methyl (CH3) groups for the synthesis or degradation of many molecules/hormones/compounds 3 Folate 4  Folate and its active Tetrahydrofolate (FH4) form Folate consist of three components. Not folate, but FH4 can be synthesized in the body. Dihydrofolate reductase (DHFR) converts folate to FH2 and then FH4. NADPH is the hydrogen donor for the reaction. FH4 can carry one-carbon units as Methyl (CH3) Methylene (CH2) Formyl (CHO) 5 Importance of FH4 in biosynthetic reactions One-carbon groups can be transferred to FH4 from serine, glycine, histidine, formaldehyde, formate. These one-carbon groups are attached to FH4 are known as one-carbon pool. The one-carbon groups carried by FH4 are used for many biosynthetic reactions: Synthesis of purine ring Synthesis of dTMP from dUMP Vitamin B12 obtains a methyl group from 5-methyl-FH4 6  The synthesis of all purines and thymidine require THF as one-carbon donor. 7  thymidylate synthase converts dUMP to dTMP by receiving a methyl group from formyl-FH4 is used in the synthesis of purine ring methylene-FH4 8  Deficiency of folate can cause megaloblastic anemia Deficiency of folate one-carbon metabolism preferentially impacts rapidly impairs DNA synthesis dividing cells, including stem cells, reticulocytes, enterocytes, and immune system cells. Lack of adequate nucleic acid synthesis results in decreased red cell number, and release of large reticulocytes into circulation: macrocytic, normochromic red blood cells (megaloblasts) due to impaired cell division. 9 10  Folate metabolism is target for chemotherapeutic drugs and antibacterial drugs Folate analog methotrexate inhibits DHFR thus, depletes functional (active) folate DHFR DHFR Folate FH2 FH4 (FH4) pool NADPH NADP+ NADPH NADP+ Trimethoprim is an inhibitor of bacterial but not human dihydrofolate reductase. Sulfonamides inhibits the synthesis of pteroic acid in bacteria, inhibits FH4 formation in bacteria. Therefore, inhibits the bacterial growth. 11  Both 5-Flourouracil and Methotrexate DHFR DHFR are used for chemotherapy Folate FH2 FH4 NADPH NADP+ NADPH NADP+ Rapidly dividing cells require high levels of DNA and RNA synthesis. Drugs that reduces the availability of FH4 for one carbon metabolism are useful chemotherapeutic agents for treating cancer. Methotrexate, a folate analog, competitively inhibits DHFR 5-Fluorouracil, a pyrimidine analog, competitively inhibits thymidylate synthase Both drugs causes “thymineless death” 12 B12 (cobalamin) coenzyme for two reactions in our metabolism 13  Functions of Vitamin B12 Vitamin B12 is involved in 2 reactions in the body: 1. Rearrangement of L-methylmalonyl-CoA to form succinyl CoA by the methylmalonyl-CoA mutase enzyme requires B12 14  Functions of Vitamin B12 Vitamin B12 is involved in 2 reactions in the body: 1. Rearrangement of L-methylmalonyl-CoA to form succinyl CoA by the methylmalonyl-CoA mutase enzyme requires B12 2. Transfer of methyl group from FH4-CH3 to homocysteine to form methionine by methionine synthase requires B12 15  Vitamin B12 obtains a methyl group from FH4·CH3 B12 is required for SAM transferring it to homocysteine to form methionine by biosynthesis and methionine synthase. regeneration of FH4 Methionine can be activated to S-adenosylmethionine (SAM) to transfer the methyl group to other compounds. Methylenetetrahydrofolate reductase (MTHFR) Methylene-FH4 16 17  SAM is another one-carbon Methylenetetrahydrofolate carrier and involved in reductase (MTHFR) biosynthesis or inactivation of other compounds Methylene-FH4 SAM is also required for inactivation of catecholamines and serotonin. ~ 35 reactions require methyl donations from SAM. 18  Methionine Folate Cycle B12 Cycle Homocysteine 19  If the CH3 cannot be removed from FH4-CH3, then FH4-CH3 Deficiency of accumulates. This can be due to either B12 deficiency or methionine synthase B12 causes deficiency, most folate becomes trapped as FH4-CH3 form. Trapping most folate as FH4-CH3 causes deficiency of functional folate-trap folate. Deficiency of functional folate due to B12 deficiency is known as methyl-trap hypothesis 20 Functional folate deficiency due to B12 deficiency causes DNA synthesis deficiency, megaloblastic anemia, and neural tube defects 21  Hyperhomocysteinemia Hyperhomocysteinemia is a multifactorial condition associated with variety of nutritional and genetic factors. Deficiency of folate, or B12, or B6 can cause hyperhomocysteinemia Genetic deficiency of methionine synthase, or cystathionine synthase, or methylenetetrahydrofolate reductase can cause hyperhomocysteinemia. Regardless of etiology even mild hyperhomocysteinemia confers an increased risk of adverse cardiovascular events. Most likely, due to homocysteine causing endothelial cell dysfunction, because homocysteine is a potent oxidizing agent, inactivates vasoprotective agent nitric oxide, alters the coagulant properties of blood. 1. Methionine Synthase 2. Methylene FH4 reductase (MTHFR) 3. Cystathionine synthase 22 B12 deficiency causes pernicious anemia, neurological dysfunctions, and hyperhomocysteinemia Pernicious anemia is a severe form of megaloblastic anemia Neurological dysfunctions due to demyelination (because SAM is required for choline (phosphatidyl choline) biosynthesis, B12 deficiency causes neurologic symptoms) symmetric numbness of hands and feet diminishing vibratory and position sense spastic gait disturbance patient may become extremely irritable “megaloblastic madness” vision problems, loosing taste and smell functions 23  Sources of Vitamin B12 The major source of B12 is dietary meat, fish, egg, milk, poultry. Absorption is very complex process. Average daily intake is ~ 5-30 μg. of which ~ 1-5 μg is absorbed. RDA is 2.4 μg/day. Total body content 2-5 mg, of which 1 mg is in the liver. Deficiency is observed after several years on a diet that is deficient in this vitamin. 24  Vitamin B12 requires intrinsic factor for its absorption Free B12 binds R-binders proteins, which are secreted by salivary glands and gastric mucosa. In the small intestine R-binders are digested, and released B12 binds to intrinsic factor, which is also secreted by gastric mucosa. The intrinsic factor-B12 binds specific receptor in the ileum and complex is internalized. Transcobalamin II-B12 complex delivers B12 to the tissues. 25  Deficiency of B12 (cobalamin) can develop for many reasons Dietary deficiency of B12 Deficiency of intrinsic factor: - production of intrinsic factor declines by age, - partial resection of the stomach - insufficient gastric acid secretion (anti-acids medicine) Absent or diseased gastric or ileal mucosa Autoimmune disease (damaged gastric mucosa that produces gastric acid) Inflammatory bowel disease (celiac disease) Pancreatic insufficiency (absence of proteases from pancreas) Impaired translocation of B12 due to inherited mutations of transcobalamin II or its receptors. 26 27