what is the active form of folic acid? how's it formed?
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
how does THF function
it transfers single carbon atoms in several critical biochemical reactions
what are the carbon atoms THF uses in biochemical reactions?
which is most reduced/oxidized?
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
how to carbons attach to THF?
to nitrogen 5 or nitrogen 10 or both nitrogens 5 and 10
what is the structure of the most reduced form of C that THF can carry
what is intermediately reduced form of C that THF can carry
what is most oxidized form of C that THF can carry?
what is the fate of the intermediate N5,N10 methylene THF?
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
what is the fate of the N5-CH=N10 methenyl THF?
1) it can be used directly
2) it can be reduced to the N5-CH2-N10 methylene THF
IT CANNOT BE OXIDIZED TO N5-CH2-N10 THF
what are the major donors of C atoms to the folic acid pool?
amino acids: serine, glycine, histidine
what do serine and glycine do re: THF production
each donates a carbon atom to produce N5-CH2-N10 THF, methylene THF
this product can be used or reduced or oxidized
what does histidine do re: THF production
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
what is the relationship between serine and glycine re: THF
how many serines are neede dto produce 2 N5-CH2-N10 THF
what biochemical reaction is N5CH3THF used in
aka methyl B12
it regenerates methionine from homocysteine
thus when we regenerate methionine, the methyl group could've come from serine, glycine, or histidine
what biochemical reaction is N5-HC=N10 methenyl THF used in
provides 2 of the C atoms in the synthesis of purine rings
what biochemical rxn is N5-CH2-N10 methylene THF used for?
synthesize dTMP from dUMP
this is part of pyrimidine synthesis pathway
how does 1 C unit get transferred from N5, N10 methylene FH4 to dUMP to form dTMP?
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
which is the active form - THF or DHF?
what cancer drugs inhibit the dUMP -> dTMP reaction?
how do they work?
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
what can you do for a patient you suspect has a folate deficiency?
give them an injection of histidine
assay for formamino glutamic acid in their blood
what is pernicious anemia?
what causes it?
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
what is most reduced form of carbon carried in fotale pool?
most oxidized form?
most reduced: methyl, CH3
most oxidized: methenyl; equivalent to formyl, forminino
serine can eventually donate 2 of its C atoms to folate pool
is glycine an essential aa?
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.
each form of C carried in folate pool can participate in produciton of an important biochemical. name the reacitons
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
why are methotrexate and 5-fluorouracil used as anti-cancer drugs?
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.
why does folat edeficiency result in anemia and growth failure?
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.