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17 Cards in this Set
- Front
- Back
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Describe Methionine Metabolism.
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*methionine and ATP form SAM which donates a methyl group and forms homocysteine
*homocysteine is reconverted to methionine by acccepting a methyl group from the FH4-C1 pool via B12 *homocysteine can also react with serine for form cystathionine, the cleavage produces cysteine, NH4+ and @-ketobutyrate, which is converted to proprionyl CoA *which is carboxylated in biotin requiring reaction to form methylmalonyl CoA, which is rearranged to form succinyl CoA requiring B12 |
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What are the cofactor requirements of methionine metabolism?
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Folate
B12 B6 (PLP) |
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Where does cysteine's S come from? and where does cysteine's carbons come from in methionine synthesis?
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methionine
serine |
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Homocysteine can be toxic. What are two ways to get rid of it?
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1. remethylate ti form methionine (requires B12 carrying a methyl from FH4)
2. can conjugate w/serine to form cysteine (need PLP that requires B6) If system is PLP poor, which is needed in methionine pathway, tend to make homocysteine. |
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What is cysteine used to make?
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Glutathione
Taurine |
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What is cystinuria How can you treat it?
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*defective membrane transport systems for aa by the kidney and thus increased excretion in the urine
*transport of cysteine is defective *cysteine oxidized to cystine, which may crystallize forming kidney stones *dilute urine, low MET means low meat, D-penicillamine, sones to bust stones |
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How does B12, B6 and folate control homocysteine levels?
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*B12 with methyl from FH4-C1 pool remethylates homocysteine to reform methionine
*PLP which needs B6 reacts with homocysteine and serine to form @-ketobutyrate |
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What are the roles of SAM?
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**greatest use in human body is to make creatine**
makes two neurotransmitters - epinephrine, melatonin |
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Single Carbons come from what aa?
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serine
glycine histidine formaldehyde formate tryptophan |
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What are products of the single carbon pool (use the methyl and return the FH4)?
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sTMP
serine purines B12-C1 Cannot make DNA without it!! |
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How is FH4 formed?
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Folate + NADPH + dihydrofolate reductase --> Dihydrofolate (FH2) + NADPH --> FH4
*Folate form Foliage *important in making RNA and DNA *need dihydrofolate reductase to make FH4 |
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What happens when you have a folate deficiency?
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*megaloblastic anemia (large rbc precursor, low hematocrit level) results from failure of precursors to make final division
*low folate in first weeks of prgnancy can cause neural tube defect (spina bifida) |
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**Test**
What two reactions require B12? |
Homocysteine methylase
Methylmalonyl CoA mustase |
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What toxic substrates are formed when there is a B12 deficiency? What do they cause?
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Homocysteine - causes heart disease
Methylmalonate - causes demyelination |
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What is the main reason for B12 deficiency?
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The intrinsic factor that allows the body to absorb and transport B12 does not work as well as we age.
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What is the cause of homocystinuria? Symptoms? Therapy?
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*cystathionine synthase is defective
*homocysteine does not react w/serine to form cysteine *homocysteine accumulates and oxidized to homocystine and excreted in the urine cardiovascular disease, thromboembolisms (25% die by 30), deformity of chest, lens dislocation pyridoxine megatherapy, loer methionine intake, increase cysteine intake, take folic acid |
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Why must you make sure that a folate deficiency is not really a B12 deficiency?
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Folate carries single carbons of differing oxidation states that typically happen in reversible reactions, however methyl-FH4 is not a reversible reaction. Methyl-FH4 is required to methylate Vit B12. So if there is B12 deficiency folate gets stuck in methyl-F4 form and eventually all folate is trapped in this form. The body shows a folate deficiency when there is actually a B12 deficiency.
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