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17 Cards in this Set

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  • Back
Describe Methionine Metabolism.
*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
What are the cofactor requirements of methionine metabolism?
B6 (PLP)
Where does cysteine's S come from? and where does cysteine's carbons come from in methionine synthesis?

Homocysteine can be toxic. What are two ways to get rid of it?
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.
What is cysteine used to make?
What is cystinuria How can you treat it?
*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
How does B12, B6 and folate control homocysteine levels?
*B12 with methyl from FH4-C1 pool remethylates homocysteine to reform methionine
*PLP which needs B6 reacts with homocysteine and serine to form @-ketobutyrate
What are the roles of SAM?
**greatest use in human body is to make creatine**
makes two neurotransmitters - epinephrine, melatonin
Single Carbons come from what aa?
What are products of the single carbon pool (use the methyl and return the FH4)?

Cannot make DNA without it!!
How is FH4 formed?
Folate + NADPH + dihydrofolate reductase --> Dihydrofolate (FH2) + NADPH --> FH4

*Folate form Foliage
*important in making RNA and DNA
*need dihydrofolate reductase to make FH4
What happens when you have a folate deficiency?
*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)
What two reactions require B12?
Homocysteine methylase
Methylmalonyl CoA mustase
What toxic substrates are formed when there is a B12 deficiency? What do they cause?
Homocysteine - causes heart disease
Methylmalonate - causes demyelination
What is the main reason for B12 deficiency?
The intrinsic factor that allows the body to absorb and transport B12 does not work as well as we age.
What is the cause of homocystinuria? Symptoms? Therapy?
*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
Why must you make sure that a folate deficiency is not really a B12 deficiency?
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.