Lecture 1 (1A) - Folate and Vitamin B12 Flashcards
1
Q
Coenzymes and cofactors
A
- additions to protein function
- essential for function of enzyme
2
Q
Coenzyme
A
a small molecule essential for the activity of some enzymes
- not protein, usually vitamins
- a cofactor molecule that helps an enzyme catalyze a particular reaction by binding with it
3
Q
Cofactors especially in
A
electophilic catalysis
- catalysis by a Lewis acid (any chemical species that abstracts an electron pair from the reactant)
- cofactors altered in reaction = need regeneration
4
Q
Coenzyme examples
A
- biotin –> carboxylation (carrying carbon around)
- flavin coenzymes –> oxidation/reduction (membrane-bound processes)
- nicotinamide coenzymes –> oxidation/reduction
- pyridoxial phosphate –> amino group transfer
- cobalamin (B12) coenzymes –> alkylation
- tetrahydrafolate –> C1 (one-carbon) unit transfer (active form of folic acid
- we can’t synthesize it but in green veg and synthesized in bacteria)
5
Q
Folic acid
A
- abundant in green vegetables
- synthesized by bacteria
- from diet in animals (we can’t synthesize it)
6
Q
Folic acid structure
A
2-amino-4-oxo-6-methylpterin + ρ-aminobenzoic acid (ρ-ABA) + glutamates
• pteridine ring is very important
7
Q
The active form of folic acid
A
tetrahydrofolate (THF)
8
Q
THF carries C1 on
A
N5 or on N10 or both
• transfers C1 units in different oxidation states
9
Q
3 oxidation states of THF
A
- methanol
- formaldehyde
- formate
10
Q
Increasing states of oxidation
A
- methyl
- methylene
- formyl, formimino, methenyl
11
Q
Differing THF derivatives
A
- N5 - Methyl - THF
- N5,N10 - Methylene - THF
- N5 - Formyl - THF
- N10 - Formyl - THF
- N5 - Formimino - THF
- N5,N10 - Methenyl - THF
- THF derivatives interchange
- in equilibrium
12
Q
C1 entry into C1 unit pool
A
- 4 routes
- main entry routes = serine, glycine
- less so = histididne • even less = formate
- all use THF for entry
13
Q
THF derivatives interchange, ATP…
A
if ATP is generally needed for a reaction, it will most likely be one way with no back reaction because so much energy was required to drive it in the first place
14
Q
Entry reactions
• enzyme = serine hydroxymethyl transferase
A
serine + THF
==>
glycine + N5,N10-Methylene-THF
• transferring hydroxymethyl from serine to another molecule
15
Q
Entry reactions
• enzyme = glycine synthase (working in reverse)
A
glycine + NAD+ + THF
==>
CO2 + NH4+ + NADH + N5,N10-Methylene-THF
16
Q
Entry reactions
• enzyme = glutamate formimino transferase
A
histidine + THF
==>
glutamate + NH4+ + N5-formimino-THF
17
Q
Entry reactions
• enzyme = N10-formyl-THF synthetase
A
formate + ATP + THF
==>
ADP + Pi + N10-formyl-THF
18
Q
Enzymes ending in -etase
A
usually associated with an APT event
19
Q
Coenzyme: biotin
A
carboxylation
20
Q
Conezyme: flavins
A
oxidation/reduction
(membrane bound processes)
21
Q
Conenzyme: nicotinamides
A
oxidation/reduction
(soluble proteins - flavins)
22
Q
Coenzyme: pyridoxal phosphate
A
amino group transfer
23
Q
Coenzyme: cobalamin (B12)
A
alkylation
24
Q
Coenzyme: THF
A
C1 (one-carbon) unit transfer
25
THF - active form of
folic acid
• we don't make it but in green vegetables and sythesized by bacteria
26
Uses of the C1 pool
utilization of carbon in all different states
27
Uses of the C1 pool:
N5,N10-Methylene-THF
dUMP --\> dTMP
28
Uses of the C1 pool:
N10-Formyl-THF
synthesis of purines
29
Uses of the C1 pool:
N5-Methyl-THF
* homocysteine --\> methionine (not a productive reaction)
* poor at transferring methyl group
* converted to SAM (hangs on to anything, not very selective, because is high energy structure so transfers methyl to any acceptor)
30
S-Adenosylmethionine (SAM)
* more reactive (than N5-Methyl-THF that makes it)
* readily passes on methyl group
* in the process eventually converted back to methionine
Homocysteine + N5-Methyl-THF
--\>
Methionine + THF
= only way back into C1 pool
• creates the activated methyl cycle
31
Uses of the C1 pool:
Sulphonamides
* antibiotics
* structural unit similar to p-Aminobenzoic acid (p-ABA)
* bacteria synthesize folic acid
* sulphonamides inhibit p-ABA inclusion
* can cause certain types of anemia
32
Uses of the C1 pool
Vitamin B12
33
Vitamin B12 structure
* 1964 D Hodgkin
* contains cobalt
* co coordinated to tetrapyrrole ring system - corrin ring similar to haem
* ring group forms equatorial ligands
34
Vitamin B12 structure - Axial ligands
5th
heterocyclic base (sugar phosphate ring)
5,6-Dimethylbenzimaidazole (DMB)
• through sugar and phosphate to the equatorial ring
35
Vitamin B12 structure - Axial ligands
6th
a cyano group
* isolated --\> CN
* tissue --\> H2O or OH-
36
Vitamin B12 coenzyme forms
* adenine - 5'-adenosylcobalamin
* methyl - methylcobalamin
37
Uptake/storage of Vitamin B12
* made by bacteria
* animals - derived from diet (meat, because we can't make it ourselves)
* stored in liver
* use about 3migrograms daily
* store about 3-5 years supply in liver (huge abundance to use but we have huge difficulty getting it into our bodies)
38
Uses of Vitamin B12 coenzymes 3 uses
* ribonucleotides --\> deoxy form
* intramolecular rearrangements
* methylation
39
Uses of Vitamin B12 coenzymes
Methylations
• THF regeneration in methionine synthesis
homocysteine + N5-Methyl-THF
--\>
methionine + THF
* enzyme: homocysteine methyl transferase aka methionine synthase
* this enzyme is B12 dependent
40
Vitamin B12 diseases
* pernicious anemia (Vitamin B12 deficiency)
* megaloblastic anemia (folate deficiency)
41
Vitamin B12 diseases
absorption
* body absorbs vitamin B12 poorly
* absorption promoted by intrinsic factor (IF)
* glycoprotein secreted by gut = can take B12 from gut into body, physical transport
* when IF low --\> pernicious anemia
42
Vitamin B12 diseases pernicious anemia
* when intrinsic factor (IF) low
* autoimmune disease, may attack IF and knock it out of the gut so B12 can't be absorbed
* deficiency somewhere in B12 metabolism
* very slow onset because we use such a small amount daily
* begins with megaloblastic anemia (MA)
43
Vitamin B12 diseases megaloblastic anemia
* identical in appearance to folate deficiency initially
* coenzyme B12 goes to a low concentration
* lowers methionine synthase activity
(homocysteine + N5-Methyl-THF --\> methionine + THF)
* N5-Methyl-THF levels go up
* imbalance occurs in C1 pool which diminishes all other THF-based reactions
* folate involved in making RBC (hence anemia) - given to pregnant women so no anemia in babies
44
Treatment of megaloblastic anemia
* folate deficiency from low in diet --\> eat more in diet
* drug-induced --\> alter drug regime
45
Treatment of pernicious anemia
• Vitamin B12 deficiency from low IF
--\> more in diet as supplement
--\> sometimes actual B12 injections needed
46
B12 and propionyl CoA general
propionyl CoA --\> (rearrrangement by B12)
Succinyl CoA --\>
citric acid cycle
47
B12 and propionyl CoA specific
propionyl CoA --\>
(carboxylation - carboxylic acid added)
(ATP hydrolysis - ATP --\> ADP + Pi)
D-methylmalonyl CoA --\>
(D isomer racemized to L isomer)
L-methylmalonyl CoA --\>
(intramolecular rearrangement/isomerization)
succinyl CoA --\>
citric acid cycle
• catalyzed by methylmalonyl CoA mutase with a derivative of B12 (cobalamin) as its coenzyme)
48
Methionine --\> SAM (+ Pi + PPi)
* ATP gives adenosyl to sulfur atom of methionine
* synthesis of SAM is unusual because the triphosphate group of ATP is split into pyrophosphate and orthophosphate
49
SAM --\>
• transfers methyl group to acceptor --\>
forms S-Adenosylhomocysteine
• S-adenosylhomocysteine hydrolyzed to homocysteine and adenine SAM --\>
(transfers methyl group to acceptor) S-adenosylhomocysteine--\>
(hydrolysis, H2O in, adenosine out) homocysteine
50
Methionine can be regenerated by
the transfer of a methyl group to homocysteine from N5-Methyl-THF
* catalyzed by methionine synthase AKA homocysteine methyltransferase
* coenzyme that mediates the transfer of a methyl group is methylcobalamin - derived from vitamin B12
51
Methionine from homocysteine
homocysteine + N5-Methyl-THF
--\> (transfer of methyl group to homocysteine from N5-Methyl-THF)
Methionine + THF
52
Activated methyl cycle
• the methyl group of methionine is activated by the formation of SAM
SAM --\>
(Active methyl CH3 out)
S-adenosylhomocysteine --\>
(H2O in)
Homocysteine --\>
(methyl CH3 in) Methionine --\>
(ATP in) back to SAM
53
The methyl of the activated methyl cycle
methyl enters in the conversion of homocysteine into methionine, then made highly reactive by addition of adenosyl group - which makes the sulfur atoms (+) charged and the methyl groups are more electrophilic
• the high transfer potential of the S-methyl group enables it to be transferred to a wide variety of acceptors)
54
Tetrahydrofolate = cofactor 3 components
* pteridine ring
* p-aminobenzoate
* 1 or more glutamate residues
55
Most reduced oxidation state
* oxidation state = methanol
* CH3
* name = methyl
56
Intermediately reduced oxidation state
* oxidation state = formaldehyde
* CH2
* name = methylene
57
Most oxidized oxidation state
* oxidation state = formic acid
* CHO = formyl
* CHNH = formimino
* CH=(double bond) = methenyl
58
Activated methyl cycle
