Single Carbon Metabolism (Medical Biochemistry II) PDF

Medical Biochemistry II Single carbon metabolism SMU - Internal Data Tetrahydrofolate, Vitamin B12, and S Adenosylmethionine Student Learning Outcomes: Understand how single C atoms can be transferred from one compound to another Know the one-carbon sources Know the one-carbon recipients Know the ro...

Medical Biochemistry II Single carbon metabolism SMU - Internal Data Tetrahydrofolate, Vitamin B12, and S Adenosylmethionine Student Learning Outcomes: Understand how single C atoms can be transferred from one compound to another Know the one-carbon sources Know the one-carbon recipients Know the roles of FH4, vitamin B12 and SAM (SAdenosylmethionine) in the transfer of the Carbons SMU - Internal Data The single carbon donors 1. Biotin 2. TH4 3. Vitamin B12 1. S-adenylmethionine (SAM) SMU - Internal Data 1. Biotin Most oxidized form, CO2, or bicarbonate is transferred by biotin Reactions: Carboxylase: pyruvate carboxylase (to OAA) Acetyl-CoA carboxylase (to malonyl CoA) Propionyl CoA to succinyl CoA SMU - Internal Data The Vitamin Folate Within the intestinal cells, folate is converted principally to N5methyl FH4, which enters the portal vein The liver stores half of the body’s folate Within the liver, FH4 is reconjugated to the polyglutamate form A relatively large portion of the folate enters the bile and is subsequently reabsorbed N5-Methyl-FH4, the major form of folate in the blood, is loosely bound to serum albumin SMU - Internal Data Oxidation and Reduction of the One-Carbon Groups of Tetrahydrofolate One-carbon groups transferred by FH4 are attached either to nitrogen N5 or N10 or they form a bridge between N5 and N10 The collection is known as the one-carbon pool The C can be oxidized and reduced The most oxidized form is N10-formyl FH4 The most reduced form is N5-methyl-FH4 Once the methyl group is formed, it is not readily reoxidized back to N5, N10 methylene FH4, and thus N5-methyl-FH4 will tend to accumulate in the cell SMU - Internal Data Sources of One-Carbon Groups Carried by FH4 Carbon sources (1-4 below) for the one-carbon pool include serine, glycine, histidine, and formate These donors transfer the carbons to folate at different oxidation states. Serine is the major carbon source Its hydroxymethyl group is transferred to FH4 in a reversible reaction, catalyzed by the enzyme serine hydroxymethyltransferase Produces glycine and N5, N10-methylene-FH4 SMU - Internal Data Recipients of One-Carbon Groups Transfers of this sort are involved in the synthesis of glycine from serine, the synthesis of the base thymine required for DNA synthesis, the purine bases required for both DNA and RNA synthesis, and the transfer of methyl groups to vitamin B12 The nucleotide deoxythymide monophosphate (dTMP) is produced from deoxyuridine monophosphate (dUMP) by a reaction in which d UMP is methylated to form dTMP Two hydrogen atoms from FH4 are used to reduce the donated carbon to methyl level Dihydrofolate (FH2) is produced. (Recipients of C numbered 5-8) SMU - Internal Data One carbon pool overview. CO2 C is transferred by biotin. Other onecarbons less oxidized are transferred by reactions that involve: FH4, vit B12 and Sadenosylmethionine or SAM SMU - Internal Data Folate deficiency A folate-deficiency will lead to megaloblastic anemia caused by an inability of blood cell precursors to synthesize DNA and therefore to divide Folate deficiencies also have been linked to an increased incidence of neural-tube defects, such as spina bifida, in fetuses of mothers who become pregnant while folate deficient SMU - Internal Data 3. Vitamin B12 Structure and Forms of Vitamin B12 B12, also known as cobalamin, contains a corrin ring, similar to the porphyrin ring found in heme Two of the four pyrole rings are joined directly rather than by methylene bridge Its most unusual feature is the presence of cobalt, which can form a bond with a carbon atom In the body, it reacts with the carbon of a methyl group, forming methylcobalamin, or with the 5’-carbon of 5’-deoxyadenosine, forming 5’-deoxyadenosylcobalamin SMU - Internal Data Structure and Form of Vitamin B12 X (bound to cobalt) = 1. 5’ –doxyadenosine in deoxyadenosylcobalamin 2. CH3 in methylcobalamin 1. CN in cyanocobalamin (commercial form found in vitamin Tablets) SMU - Internal Data Absorption and Transport of Vitamin B12 The major source of vitamin B12 is dietary meat, eggs, dairy products, fish, poultry, and seafood; the animals that serve as the source of these foods obtain B12 mainly from bacteria in their food supply Absorption is complex because ingested B12 can exist either free or bound to proteins If free, the B12 binds to proteins known as R-binders (haptocorrins) secreted by the salivary glands and the gastric mucosa SMU - Internal Data Absorption and Transport of Vitamin B12 If the ingested B12 is bound to proteins, it must be released by digestive proteases. In the small intestine, proteases digest the haptocorrins, and B12 then binds to intrinsic factor. Intrinsic factor-B12 complex attaches to ileum, is internalized. The B12 within the enterocyte complexes with transcobalamin II and then is released into circulation. The liver takes up approximately 50%. The amount of the vitamin stored in the liver is large enough that 3 to 6 years pass before symptoms of a dietary deficiency occur. SMU - Internal Data Vitamin B12 deficiency A vitamin B12 deficiency can be brought about by the lack of intrinsic factor required for the absorption A consequence of vitamin B12 deficiency is the accumulation of methyl-FH4 and a decrease in other folate derivatives This is known as the methyl-trap hypothesis The carbons cannot be released from the folate, because of the B12 deficiency This will therefore lead to a functional folate deficiency SMU - Internal Data Functions of Vitamin B12 Two key reactions in the body use VitB12: the transfer of a methyl group from N5methyl FH4 to homocysteine to form methionine and the rearrangement of the methyl group of L-methylmalonyl CoA to form succinyl CoA Tetrahydrofolate receives a one-carbon group. This carbon is reduced to the methyl level and transferred to vitamin B12, forming methyl-B12 (or methylcobalamin) Transfers the methyl group to homocysteine, converted to methionine by methionine synthase SMU - Internal Data 4. SAM S-adenosylmethionine (SAM), which is produced from methionine and adenosine triphosphate (ATP), transfers the methyl group to precursors: creatine, phosphatidylcholine, epinephrine, Melatonin methylated nucleotides methylated DNA SMU - Internal Data Relationships Between Folate, Vitamin B12, and SAM Methyl-Trap Hypothesis For folate, equilibrium lies in the direction of the N5- methyl FH4 form (most reduced, most stable) In only one reaction can the methyl group be removed which requires vitamin B12 If B12 is deficient or methionine synthase is defective, N5methyl FH4 will accumulate Most folate will become “trapped” A functional folate deficiency results SMU - Internal Data Choline and one-carbon metabolism Choline is essential for some phospholipids Choline is oxidized to betaine and betaine can be used in the liver to donate a methyl group (one carbon group) to homocysteine to form methionine and dimethyl glycine Therefore, liver has 2 routes for homocysteine conversion whereas the nervous system only has 1 route: B12 pathway SMU - Internal Data Hyperhomocysteinemia Elevated homocysteine levels have been linked to cardiovascular and neurological disease If an individual is deficient in vitamin B12, the conversion of homocysteine to methionine by the major route is inhibited As cysteine accumulates, the pathway is also inhibited Overall, leads to an accumulation of homocysteine, which is released into the blood SMU - Internal Data Hyperhomocysteinemia Homocysteine also accumulates in the blood if a mutation is present in the enzyme that converts N5, N10 methylene FH4 to N5-methyl FH4 SMU - Internal Data Hyperhomocysteinemia A third way in which serum homocysteine levels can be elevated is by a mutated cystathinone-B-synthase or a deficiency in vitamin B6. SMU - Internal Data Neural Tube Defects Folate deficiency during pregnancy has been associated with an increased risk for neural tube defects in the developing fetus The folate deficiency and the subsequent inhibition of DNA synthesis leads to neural tube defects These findings have led to the recommendation that women considering getting pregnant begin taking folate supplements before conception occurs, and for at least 1 month after conception SMU - Internal Data Folate deficiencies and DNA synthesis Causes an increase in uracil into DNA. Uracil can be removed by DNA repair enzymes, the lack of dTTP blocks this. Resulting in fragmentation of DNA and block to normal DNA replication Clumping and polysegmentation seen in the nuclei of neutrophils in patients with megaloblastic anemia is caused by primary folate deficiency or secondary B12 deficiency Damage cannot be repaired and cells die SMU - Internal Data Summary One carbon groups at lower oxidation states than CO2 (which is carried by biotin), are transferred by reactions that involve FH4, vitamin B12, and SAM FH4 is produced from vitamin folate and obtains one carbon units from serine, glycine, histidine, formaldehyde, and formic acid Carbon attached to FH4 can be oxidized or reduced, producing several FH4 forms Once carbon on FH4 has been reduced to methyl form, it cannot be re-oxidized Carbons attached to FH4= one carbon pool SMU - Internal Data Summary Carbons carried by folate are used in a limited number of biochemical reactions, but they are very important in forming deoxythymidine monophosphate (dTMP) and purine rings Vitamin B12 participates in two reactions in the body: conversion of L-methylmalonyl-CoA to succinyl-CoA and conversion of homocysteine to methionine SMU - Internal Data