SYLLABUS 17: Metabolism of 1C Fragments, Folic Acid Biochemistry

Question 1

what is the active form of folic acid? how’s it formed?

Answer 1

folic acid is derived from the diet or ingested as a vitamin

it’s reduced by dihydrofolate reducates to dihydrofolate, DHF or FH2

DHF is reduced by dihydrofolate reducatase to tetrahydrofolate, THF or FH4

THF or FH4 is the active cofactor form

Question 2

how does THF function

Answer 2

it transfers single carbon atoms in several critical biochemical reactions

Question 3

what are the carbon atoms THF uses in biochemical reactions?

which is most reduced/oxidized?

Answer 3

1) methyl group -CH3: the most reduced C
2) methylene group -CH2: intermediately reduced C
3) methenyl group =CH: most oxidized C
4) formyl group -HC=O which is equivalent to methenyl
5) formimino group HC=NH which is equivalent to methenyl

Question 4

how to carbons attach to THF?

Answer 4

to nitrogen 5 or nitrogen 10 or both nitrogens 5 and 10

Question 5

what is the structure of the most reduced form of C that THF can carry

Answer 5

N5-methyl-tetrahydrofolate

Question 6

what is intermediately reduced form of C that THF can carry

Answer 6

N5, N10-methylene-tetrahydrofolate

Question 7

what is most oxidized form of C that THF can carry?

Answer 7

Question 8

what is the fate of the intermediate N⁵,N¹⁰ methylene THF?

Answer 8

1) can be used directly
2) can be reduced to the N⁵CH3 THF by methylene THF reductase
3) can be oxidized to N⁵-HC=N¹⁰ methenyl THF

Question 9

what is the fate of the N⁵-CH=N¹⁰methenyl THF?

Answer 9

1) it can be used directly
2) it can be reduced to the N⁵-CH₂-N¹⁰methylene THF

IT CANNOT BE OXIDIZED TO N⁵-CH₂-N¹⁰ THF

Question 10

what are the major donors of C atoms to the folic acid pool?

Answer 10

amino acids: serine, glycine, histidine

Question 11

what do serine and glycine do re: THF production

Answer 11

each donates a carbon atom to produce N⁵-CH₂-N¹⁰ THF, methylene THF

this product can be used or reduced or oxidized

Question 12

what does histidine do re: THF production

Answer 12

breaks down to produce N-formiminoglutamic acid, which reacts w/ THF to form glutamic acid + N⁵-formimino THF

this is equivalent to methenyl THF

this product can be used or reduced

Question 13

what is the relationship between serine and glycine re: THF

Answer 13

glycine <-> serine

Question 14

how many serines are neede dto produce 2 N⁵-CH₂-N¹⁰ THF

Answer 14

1 serine

Question 15

what biochemical reaction is N⁵CH₃THF used in

Answer 15

aka methyl B12

it regenerates methionine from homocysteine

thus when we regenerate methionine, the methyl group could’ve come from serine, glycine, or histidine

Question 16

what biochemical reaction is N⁵-HC=N¹⁰ methenyl THF used in

Answer 16

provides 2 of the C atoms in the synthesis of purine rings

Question 17

what biochemical rxn is N⁵-CH₂-N¹⁰ methylene THF used for?

Answer 17

synthesize dTMP from dUMP

this is part of pyrimidine synthesis pathway

Question 18

how does 1 C unit get transferred from N⁵, N¹⁰ methylene FH₄ to dUMP to form dTMP?

Answer 18

1. thymidylate synthase uses N⁵, N¹⁰ methylene FH₄ and produces deoxythymidine monophsphate, dTMP

this produces FH₂, dihydrofolate (DHF)

2. dihydrofolate reductase uses NADPH to reduce DHF back to THF
3. THF picks up a C from serine and restarts the cycle

Question 19

which is the active form - THF or DHF?

Answer 19

THF

Question 20

what cancer drugs inhibit the dUMP -> dTMP reaction?

how do they work?

Answer 20

1. **5-Fluorouracil **inhibits **thymidilate synthase **which produces dTMP from dUMP
2. **Methotrexate (aka aminopterin, amethopterin) **inhibits **dihydrofolate reductase **which regenerates DHF from THF; this inhibits thymidine synthesis and **purine synthesis, **and works as an anti-inflammatory

Question 21

what can you do for a patient you suspect has a folate deficiency?

Answer 21

give them an injection of histidine

assay for formamino glutamic acid in their blood

Question 22

what is pernicious anemia?

what causes it?

Answer 22

when there’s a deficiency in glycoprotein, intrinsic factor, which is needed for absorption of B12

this causes disturbances of folate metabolism as N⁵-CH₃-THF accumulates cannot go back to N⁵-methyl-N¹⁰-THF, as this reaction requires B12

no IF = no B12 = folate metabolism is messed up, and most of C atoms in Folate pool accumulate as N⁵-CH₃-THF

this means you run out of N⁵-N¹⁰-methylene-FH₃ and cannot synthesize C atoms 2 and 8 of the purine ring

Question 23

what is most reduced form of carbon carried in fotale pool?

most oxidized form?

Answer 23

most reduced: methyl, CH3

most oxidized: methenyl; equivalent to formyl, forminino

Question 24

serine can eventually donate 2 of its C atoms to folate pool

how?

is glycine an essential aa?

Answer 24

Serine eventually donates 2 of its C atoms to the folate pool by adding C to THF to
regenerate N5, N10-Methylene FH4.

Glycine is a non-essential amino acid because it can
be produced from serine by loss of a C.

Question 25

each form of C carried in folate pool can participate in produciton of an important biochemical. name the reacitons

Answer 25

methyl carbon: regenerates methionine from homocysteine

methylene carbon: used to syntheisze dTMP from DUMP

methenyl carbon: used to provide 2 of the C atoms in purine ring synthesis

Question 26

why are methotrexate and 5-fluorouracil used as anti-cancer drugs?

Answer 26

Methotrexate is anti-tumor drug because it inhibits the dihydrofolate reductase
reaction, which takes DHF -> THF using NADPH, making THF active and allowing for
resynthesis of N5, N10-methylene FH4, which is important for making new dTMP from
dUMP, or DNA/RNA synthesis. Blocking this means blocking new nucleic acid synthesis.
5-fluorouracial is an anti-tumor drug because it blocks the thymidilate synthase
reaction, which synthesizes dTMP from dUMP.

Question 27

why does folat edeficiency result in anemia and growth failure?

Answer 27

Folate deficiency results in growth failure because folate metabolism is integral to the
synthesis of dUMP to dTMP which is important for growth and development. It also
causes pernicious anemia because when there’s a deficiency in Intrinsic Factor or
Glycoprotein, there are disturbances to folate metabolism since the methyl-THF
accumulates and cannot go back to the N5-methyl-N10-THF because this reaction
requires B12. No B12 means that most of the C atoms in the Folate pool accumulate as
N5-methyl-THF so you run out of N5-N10-methylene-FH3 and cannot synthesize C
atoms 2 and 8 of the purine ring.