Groups that contain a single-carbon atom can be transferred from one compound to another. These carbon atoms may be in a number of different oxidation states. The most oxidized form, CO2, is transferred by biotin. One-carbon groups at lower levels of oxidation than CO2 are transferred by reactions involving tetrahydrofolate (FH4), vitamin B12, and S-adenosylmethionine (SAM).
Tetrahydrofolate. Tetrahydrofolate, which is produced from the vitamin folate, is the primary one-carbon carrier in the body. This vitamin obtains one-carbon units from serine, glycine, histidine, formaldehyde, and formate (Fig. 38.1). While these carbons are attached to FH4, they can be either oxidized or reduced. As a result, folate can exist in a variety of chemical forms. Once a carbon has been reduced to the methyl level (methyl-FH4); however, it cannot be reoxidized. Collectively, these one-carbon groups attached to their carrier FH4 are known as the one-carbon pool. The term “folate” is used to represent a water-soluble B-complex vitamin that functions in transferring single-carbon groups at various stages of oxidation.
The one-carbon groups carried by FH4 are used for many biosynthetic reactions. For example, one-carbon units are transferred to the pyrimidine base of deoxyuridine monophosphate (dUMP) to form deoxythymidine monophosphate (dTMP), to the amino acid glycine to form serine, to precursors of the purine bases to produce carbons C2 and C8 of the purine ring, and to vitamin B12.
Vitamin B12. Vitamin B12 is involved in two reactions in the body. It participates in the rearrangement of the methyl group of L-methylmalonyl coenzyme A (L-methylmalonyl CoA) to form succinyl CoA, and it transfers a methyl group, obtained from FH4, to homocysteine, forming methionine.
S-adenosylmethionine. SAM, produced from methionine and adenosine triphosphate (ATP), transfers the methyl group to precursors that form a number of compounds, including creatine, phosphatidylcholine, epinephrine, melatonin, methylated nucleotides, methylated histones, and methylated DNA.
Methionine metabolism is very dependent on both FH4 and vitamin B12. Homocysteine is derived from methionine metabolism and can be converted back into methionine by using both methyl-FH4 and vitamin B12. This is the only reaction in which methyl-FH4 can donate the methyl group. If the enzyme that catalyzes this reaction is defective, or if vitamin B12 or FH4 levels are insufficient, homocysteine will accumulate. Elevated homocysteine levels have been linked to cardiovascular and neurological disease. A vitamin B12 deficiency can be brought about by the lack of intrinsic factor, a gastric protein required for the absorption of dietary B12. 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, in which, because of the B12 deficiency, most of the carbons in the FH4 pool are trapped in the methyl-FH4 form, which is the most stable. The carbons cannot be released from the folate because the one reaction in which they participate cannot occur because of the B12 deficiency. This leads to a functional folate deficiency, even though total levels of folate are normal. A folate deficiency (whether functional or actual) leads to megaloblastic anemia caused by an inability of blood cell precursors to synthesize DNA and, therefore, to divide. This leads to large, partially replicated cells being released into the blood to attempt to replenish the cells that have died. Folate deficiencies also have been linked to an increased incidence of neural tube defects, such as spina bifida, in mothers who become pregnant while folate deficient.
THE WAITING ROOM
After resection of the cancer in his large intestine and completion of a course of postoperative chemotherapy with 5-fluorouracil (5-FU) and oxaliplatin, Clark T. returned to his gastroenterologist for a routine follow-up colonoscopy. His colon was completely normal, with excellent healing at the site of the anastomosis. His physician expressed great optimism about a possible cure of Clark T.’s previous malignancy but cautioned him about the need for regular colonoscopies and surveillance CT scans over the next few years.
Beatrice T., a 75-year-old woman, went to see her physician because of numbness and tingling in her legs. A diet history indicated a normal and healthy diet, but she was not taking any supplemental vitamin pills. Laboratory results indicated a low level of serum B12.
The initial laboratory profile, determined when Jean T. first presented to her physician with evidence of early alcohol-induced hepatitis, included a hematologic analysis that showed that Jean T. was anemic. Her hemoglobin was 11.0 g/dL (reference range, 12 to 16 g/dL for an adult woman). The erythrocyte (red blood cell) count was 3.6 million cells/mm3 (reference range, 4.0 to 5.2 million cells/mm3 for an adult woman). The average volume of her red blood cells (mean corpuscular volume [MCV]) was 108 fL (reference range = 80 to 100 fL; 1 fL = 10–12 mL), and the hematology laboratory reported a striking variation in the size and shape of the red blood cells in a smear of her peripheral blood (see Chapter 42). The nuclei of the circulating granulocytic leukocytes had increased nuclear segmentation (polysegmented neutrophils). Because these findings are suggestive of a macrocytic anemia (in which blood cells are larger than normal), measurements of serum folate and vitamin B12 (cobalamin) levels were ordered.
I. Tetrahydrofolate (FH4)
A. Structure and Forms of FH4
Folates exist in many chemical forms. The coenzyme form that functions in accepting one-carbon groups is tetrahydrofolate polyglutamate (Fig. 38.2), generally referred to as just tetrahydrofolate or FH4. It has three major structural components, a bicyclic pteridine ring, para-aminobenzoic acid, and a polyglutamate tail consisting of several glutamate residues joined in amide linkage. The one-carbon group that is accepted by the coenzyme and then transferred to another compound is bound to N5, to N10, or both.
Different forms of folate may differ in the oxidation state of the one-carbon group, in the number of glutamate residues attached, or in the degree of oxidation of the pteridine ring. When the term “folate” or “folic acid” is applied to a specific chemical form, it is the most oxidized form of the pteridine ring (see Fig. 38.2). Folate is reduced to dihydrofolate and then to tetrahydrofolate by dihydrofolate reductase present in cells. Reduction is the favored direction of the reaction; therefore, most of the folate present in the body is present as the reduced coenzyme form, FH4.
B. The Vitamin Folate
Folates are synthesized in bacteria and higher plants and ingested in green leafy vegetables, fruits, and legumes in the diet. The vitamin was named for its presence in green, leafy vegetables (foliage). Most of the dietary folate derived from natural food sources is present in the reduced coenzyme form. However, vitamin supplements and fortified foods contain principally the oxidized form of the pteridine ring.
As dietary folates pass into the proximal third of the small intestine, folate conjugases in the brush border of the lumen cleave off glutamate residues to produce the monoglutamate form of folate, which is then absorbed (see Fig. 38.2, upper structure, when n = 1). Within the intestinal cells, folate is converted principally to N5-methyl-FH4, which enters the portal vein and goes to the liver. Smaller amounts of other forms of folate also follow this route.
The liver, which stores half of the body’s folate, takes up much of the folate from the portal circulation; uptake may be through active transport or receptor-mediated endocytosis. Within the liver, FH4 is reconjugated to the polyglutamate form before being used in reactions. A small amount of the folate is partially degraded, and the components enter the urine. A relatively large portion of the folate enters the bile and is subsequently reabsorbed (very similar to the fate of bile salts in the enterohepatic circulation).
N5-Methyl-FH4, the major form of folate in the blood, is loosely bound to plasma proteins, particularly serum albumin.
One-carbon groups transferred by FH4 are attached either to N5 or N10, or they form a bridge between N5 and N10. The collection of one-carbon groups attached to FH4 is known as the one-carbon pool. While they are attached to FH4, these one-carbon units can be oxidized and reduced (Fig. 38.3). Thus, reactions that require a carbon in a particular oxidation state may use carbon from the one-carbon pool that was donated in a different oxidation state.
The individual steps for the reduction of the one-carbon group are shown in Figure 38.3. 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 tends to accumulate in the cell.
D. Sources of One-Carbon Groups Carried by FH4
Carbon sources for the one-carbon pool include serine, glycine, formaldehyde, histidine, and formate (Fig. 38.4). These donors transfer the carbons to folate in different oxidation states. Serine is the major carbon source of one-carbon groups in the human. Its hydroxymethyl group is transferred to FH4 in a reversible reaction, catalyzed by the enzyme serine hydroxymethyltransferase. This reaction produces glycine and N5,N10-methylene-FH4. Because serine can be synthesized from 3-phosphoglycerate, an intermediate of glycolysis, dietary carbohydrate can serve as a source of carbon for the one-carbon pool. The glycine that is produced may be further degraded by the donation of a carbon to folate. Additional donors that form N5,N10-methylene-FH4 are listed in Table 38.1.
|FORM OF ONE-CARBON DONOR PRODUCEDb
|Purine (C2 and C8)
|N5-formimino-FH4 is converted to N5,N10-methenyl-FH4
|Methionine and dimethylglycine
|Glycine (there are many others; see Fig. 38.9B)
dUMP, deoxyuridine monophosphate; dTMP, deoxythymidine monophosphate; FH4, tetrahydrofolate; SAM, S-adenosylmethionine.
aThe major source of carbon is serine.
bThe carbon unit attached to FH4 can be oxidized and reduced (see Fig. 38.3). At the methyl level, reoxidation does not occur.
Histidine and formate provide examples of compounds that donate carbon in different oxidation levels (see Fig. 38.4). Degradation of histidine produces formiminoglutamate (FIGLU), which reacts with FH4 to donate a carbon and nitrogen (generating N5-formimino-FH4), thereby releasing glutamate. Formate, produced from tryptophan oxidation, can react with FH4 and generate N10-formyl-FH4, the most oxidized folate derivative.
E. Recipients of One-Carbon Groups
The one-carbon groups on FH4 may be oxidized or reduced (see Fig. 38.3) and then transferred to other compounds (see Fig. 38.4 and Table 38.1). 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.
Because the conversion of serine to glycine is readily reversible, glycine can be converted to serine by drawing carbon from the one-carbon pool.
The nucleotide deoxythymidine monophosphate (dTMP) is produced from deoxyuridine monophosphate (dUMP) by a reaction in which dUMP is methylated to form dTMP (Fig. 38.5). The source of carbon is N5,N10-methylene-FH4. Two hydrogen atoms from FH4 are used to reduce the donated carbon to the methyl level. Consequently, dihydrofolate (FH2) is produced. Reduction of FH2 by NADPH in a reaction catalyzed by dihydrofolate reductase (DHFR) regenerates FH4. This is the only reaction involving FH4 in which the folate group is oxidized as the one-carbon group is donated to the recipient. Recall that DHFR is also required to reduce the oxidized form of the vitamin, which is obtained from the diet (see Fig. 38.2). Thus, DHFR is essential for regenerating FH4 both in the tissues and from the diet. These reactions contribute to the effect of folate deficiency on DNA synthesis because dTMP is required only for the synthesis of DNA.