SYLLABUS 16: Methylation Reactions in Biological Systems: Vitamin B12 Metabolism

Question 1

is met essential or nonessential aa?

Answer 1

essential

Question 2

why is Met important

Answer 2

1. protein synthesis
2. it’s the major biological methylating agent used in synthesis of imp biochemicals

Question 3

what is SAM

Answer 3

s-adenosyl methionine, the methylating agent produced in the methionine adenosyl transferase rxn

Met + ATP -> SAM + 3Pi

Question 4

what are some important compounds whose synthesis requires a methyl group from SAM?

Answer 4

epinephrine

creative

methylated nucleotides

phosphatidylcholine

melatonin

Question 5

what happens is SAM gives up its methyl group

Answer 5

converted to S-adenosyl homocysteine, SAH, which is hydrolyzed to adenoside + homocysteine

Question 6

how can homocysteine become methionine again?

what inhibits this rxn?

Answer 6

homocysteine can accept a methyl group from the vitamin cofactors N⁵-methyl tetrahydrofolate and B12, cobalamine

high levels of methionine inhibit this reaction b/c do not need to generate methionine

Question 7

what is the active methyl cycle

Answer 7

Methyl-tetrahydrofolate carries methyl group on N-5 of its structure

N5-CH3-THF transfers the methyl group to vitamin B12, cobalamine => methyl cobalamine

methyl cobalamine transfers the methyl to homocysteine => methionine

Question 8

what is the purpose of the transulfuration pathway

Answer 8

to convert methionine -> homocysteine -> cysteine in conditions of sufficient methionine available, when do not need more methionine

Question 9

describe the transulfuration pathway

Answer 9

1. homocysteine reacts w/ serine, forms cystathionine, via cytathionine synthase, a PLP requiring enzyme
   * is feedback inhibited by high levels of cysteine*
2. cystathionine is hydrolzed by cystahionase, also requires PLP, to cysteine + a-ketobutyrate

a-ketobutyrate -> propionyl coA -> succinyl CoA -> TCA cycle

Question 10

is serine an essential amino acide?

is cysteine

Answer 10

serine: no
cysteine: semi; can be biosynthesized from serine + met

Question 11

when is propionyl CoA produced during aa metabolism?

Answer 11

from a-KB, which came from cystathione, which came from serine + methionine

from valine, isoleucine, and threonin e

Question 12

what’s the fate of succinyl CoA?

Answer 12

1. can be further oxidized in TCA cycle for energy
2. can be metabolized to OAA and eventually to gulcose during gluconeogenesis
3. can produce heme

Question 13

what does propionyl CoA -> succinyl CoA require?

Answer 13

vitamin B12

Question 14

what mammalian reactions require vitamin B12?

Answer 14

1. conversion of homocysteine -> methionine
2. conversion of propionyl CoA -> succinyl CoA

Question 15

how is transulfuration pathway involved in oxidative stress mitigation?

Answer 15

it produces cysteine, which is 1 of 3 amino acids that make up GSH

so large amounts of cysteine are needed for GSH, not just for protein synthesis

thus transulfuration pathway is important as protectant against oxidative stress via cystein formation

Question 16

fates of homocysteine?

Answer 16

1) go back, become methionine
2) become cysteine

Question 17

what is homocystinuria

Answer 17

metabolic inborn error of metabolism in which large amounts of homocysteine accumulate

Question 18

what does homocystinuria cause

Answer 18

neurological disorders/severe mental retardation

failure to grow and thrive

skeletal abnormalities

cardiovascular abnormalities

dislocation lens sockets

arthritis

Question 19

what causes homocystinuria

Answer 19

deficiency of vitamins B6, B12, or THF

deficiencies in cystathione synthase or homocysteine methyl transferase

Question 20

how do you treat someone w/ homocystinuria?

Answer 20

if it’s a vitamin cofactor that’s missing, then give them supplement of the cofactor- B6, B12

if the enzyme is deficient, though, must limit methionine in the diet, which is possible b/c it’s an essential aa, and add cysteine to the diet since methione is limited and cystine comes from methionine

Question 21

what are part of the toxic effects on the body of high levels of homocysteine?

Answer 21

1. elevated oxidative stress
2. smooth muscle cell proliferation

Question 22

vitamin B12 structure?

Answer 22

1 corrin ring of 4 pyrrole rings linked together, + central cobalt atom

at “rest”, 6th substituent: OH

Question 23

what is the 6th substituent of Cobalt of Vitamin B12

1. at rest
2. as a drug
3. during digestion
4. as a vitamin cofactor?

Answer 23

1. at rest, = OH
2. as a drug, = CN
3. during digestion, = CN -> OH
4. vitamin cofactor, = methyl, methylcobalamine, or deoxyadenosine, deoxyadenosyl cobalamine

Question 24

how is deoxyadenosyl B12 formed

Answer 24

Cobalamine, B12 + ATP -> deoxyadenosyl B12 + 3pi

catalyzed by deoxyadenosyl transferase

Question 25

in what reaction is deoxyadenosyl-B12 a cofactor

Answer 25

methylmalonyl mutase reaction when methylmalonyl CoA -> succinyl CoA

Question 26

what’s connection between methylmalonyl CoA and B12?

Answer 26

propionyl CoA is made from the breakdown of 4 amino acids, isoleucine, valine, Met, Threonine, and odd-chain fatty acids

Propionyl Co A -> D methyl-malonyl Co A -> L-methylmalonyl Co A -> succinyl Co A

L-methylamlonyl CoA -> succinyl CoA requires using the cofactor form of Vitamin B12, Deoxyadenosyl to do a rearrangement reaction with a free radical intermediate which the mutase uses

without it get disease

Question 27

what causes methylmalonyl aciduria?

Answer 27

if the rearrangement reaction w/ free radical from L-methylmalonyl CoA -> succinyl CoA doesn’t occur

due to:

1. absence of the mutase
2. deficiency in vitamin B12
3. defiency of the transferase that makes B12

Question 28

what does methylmalonyl aciduria result in?

Answer 28

severe mental retardation

death

GI irritation

Question 29

treatment of methylmalonyl CoA?

Answer 29

restrict Met, Val, The, Isoleucine in diet

Question 30

what causes pernicious anemia?

Answer 30

deficiency in vitamin B12

Question 31

what are some critical biochem components which are produced from methylation rxns?

Answer 31

epinephrine, creatine, methylated nucleotides, phosphatidylcholine, and melatonin.

Question 32

what regulates whether homocysteine is converted to Met or Cysteine? is cysteine an essential amino acid?

Answer 32

whether homocysteine is converted to methionine or cysteine is controlled by high
levels of methionine, which inhibit homocysteine from becoming methionine

cysteine is thus essential when met is high

Question 33

what cofactors are needed for homocystein methyl transferase (methionine synthase) reaction?

Answer 33

N5-methyl-THF and B12

Question 34

how can you treat homocystenuria?

Answer 34

by either supplementing the patient’s diet with B6 or
B12 if it’s the cofactor that’s missing, or if it’s the enzyme that’s missing, then limit
methionine in the patient’s diet, and add in cysteine, since limiting methionine means
less cysteine will be produced

Question 35

what ultimately happens to the carbon skeleton of homocysteine (met) after transsulfuration?

Answer 35

Through transsulfuration, the carbon skeleton of homocysteine is converted to aketobutyrate,
which is decarboxylated to propionyl CoA, which is metabolized to
succinyl CoA, which can be oxidized in the TCA cycle for energy, or metabolized to OAA
and eventually glucose via gluconeogenesis, or can produce heme.

Question 36

why is vitamin B12 necessary? what specific compounds require B12 for ultimate conversion to succinyl CoA?

Answer 36

Vitamin B12 is necessary because it’s a crucial cofactor in many metabolic reactions –
including conversion of homocysteine to methionine and conversion of propionyl CoA to
succinyl CoA. L-methylmalonyl CoA requires Vitamin B12 as a cofactor to do a
rearrangement reaction involving B12 as a free radical intermediate to become Succinyl
CoA.

Question 37

what causes methylmalonyl aciduria and how is it treated?

Answer 37

Methylmalonyl aciduria is caused by a B12 deficiency, absence of the mutase, or
deficiency of the transferase that makes B12, any of which leads to inability to convert
L-methylmalonyl CoA to Succinyl CoA. This is treatable by restricting Met, Val, The, and
Isoleucine in the diet, since these are all precursors to Propionyl CoA.

Question 38

what causes pernicious anemia? how is it treated?

Answer 38

stomach parietal cells make Intrinsic Factor which is needed to bind and absorb dietary B12 in the ileum

pernicious anemia happens when you get autoimmune destruction of these parietal cells

so the pathway to absorbing B12 gets disrupted

treat by giving IV B12 or forms that can be absorbed w/o IF