26 Questions
What is the primary source of one-carbon units for H4folate?
Conversion of serine to glycine
What is the preferred cofactor for methyl transfer in biological reactions?
S-adenosylmethionine
What is the role of methylcobalamin in the activated-methyl cycle?
Methyl donor in the formation of methionine
What is the consequence of a deficiency in methionine synthase or unavailability of methylcobalamin?
Folates become trapped as N5-methyl THF
What is the consequence of not regenerating THF from N5-methyl THF?
Megaloblastic anemia
What is the result of vitamin B12 deficiency on folate metabolism?
Folates are trapped as N5-methyl THF
What is the relationship between vitamin B12 deficiency and megaloblastic anemia?
Vitamin B12 deficiency alone will not cause megaloblastic anemia in the presence of sufficient folate
What is the consequence of the accumulation of homocysteine in the blood and urine?
Heart disease and hypertension
What percentage of heart disease cases are attributed to high levels of homocysteine?
10%
What is the consequence of a lack of vitamin B12 in the methyl malonyl CoA mutase reaction?
Accumulation of odd-carbon numbered fatty acids
Why can't folate alone treat the neurological symptoms of pernicious anemia?
The accumulation of odd-carbon numbered fatty acids in neural membranes cannot be reversed by folate
Which of the following tissues does not contain branched-chain aminotransferase?
Liver
What is the role of deoxyadenosylcobalamin in the conversion of methylmalonyl-CoA to succinyl-CoA?
It is necessary for the conversion of methylmalonyl-CoA to succinyl-CoA
What is the consequence of a defect in intestinal absorption of vitamin B12?
Pernicious anemia and neurological disorders
Why are branched-chain amino acids not degraded in the liver?
The liver lacks the enzyme branched-chain aminotransferase
What is the result of homogentisate oxidation in urine?
It turns black.
What is the first step in the catabolism of amino acids in most mammals?
Transfer of NH3 to α-ketoglutarate.
Which of the following amino acids can be synthesized from pyruvate?
Only alanine.
What is the source of essential amino acids in humans?
Dietary protein.
What is the product of the amidation of aspartate?
Asparagine.
What is the primary event that occurs in the pathway of phenylketonuria?
Transamination of phenylalanine to phenylpyruvate
What is the primary cause of severe mental retardation in individuals with phenylketonuria?
Accumulation of phenylalanine or its metabolites in the brain
What is the role of tetrahydrobiopterin in the phenylalanine hydroxylase reaction?
It acts as a cofactor to facilitate the reaction
What is the treatment for individuals with phenylketonuria caused by a defect in dihydrobiopterin reductase?
Providing supplements of L-dopa and 5-hydroxy-Trp
What is the primary effect of alkaptonuria, another heritable disease of phenylalanine metabolism?
Arthritis
What is the defective enzyme in alkaptonuria?
Homogentisate dioxygenase
Study Notes
Vitamin B12 Deficiency
- Vitamin B12 deficiency leads to an inability to degrade odd-numbered fatty acids, resulting in neurological disorders.
- In pernicious anemia, B12 deficiency leads to the accumulation of odd-carbon numbered fatty acids in neural membranes.
Branched-Chain Amino Acid Degradation
- Branched-chain amino acids (valine, leucine, and isoleucine) are not degraded in the liver.
- These amino acids are oxidized in muscle, adipose tissue, kidney, and brain, which contain branched-chain aminotransferase.
Phenylketonuria (PKU)
- PKU is caused by a lack of phenylalanine hydroxylase, leading to the accumulation of phenylalanine and its metabolites in the blood and tissues.
- This can cause severe mental retardation, but can be prevented by strict dietary control of phenylalanine intake.
Alkaptonuria
- Alkaptonuria is a heritable disease of phenylalanine metabolism, caused by a defect in homogentisate dioxygenase.
- This defect leads to the development of arthritis.
Folate and Vitamin B12 Interconnection
- Vitamin B12 is required for the methionine synthase reaction, which regenerates methionine from homocysteine.
- If vitamin B12 is not available, the folates become trapped as N5-methyl THF and cannot regenerate THF, leading to megaloblastic anemia.
Methionine Synthase Reaction
- The methionine synthase reaction requires vitamin B12 as a cofactor.
- This reaction is necessary for the regeneration of methionine and S-adenosylmethionine.
S-Adenosylmethionine (AdoMet)
- AdoMet is the preferred cofactor for methyl transfer in biological reactions.
- AdoMet is synthesized from ATP and methionine and is a potent methyl donor.
Regeneration of AdoMet
- The regeneration of AdoMet occurs through the activated-methyl cycle.
- This cycle requires vitamin B12 as a cofactor for the methionine synthase reaction.
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