Lecture 25

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Questions and Answers

What is the primary source of one-carbon units for H4folate?

Conversion of serine to glycine (correct)

Synthesis of ATP and methionine

Conversion of methionine to serine

Regeneration of N5-methyl THF

What is the preferred cofactor for methyl transfer in biological reactions?

N5-methyl THF

S-adenosylmethionine (correct)

N5,N10-methylene H4folate

Methylcobalamin

What is the role of methylcobalamin in the activated-methyl cycle?

Forming methionine synthase

Regenerating S-adenosylmethionine

Donating methyl groups in biosynthetic reactions

Methyl donor in the formation of methionine (correct)

What is the consequence of a deficiency in methionine synthase or unavailability of methylcobalamin?

Folates become trapped as N5-methyl THF Signup and view all the answers

What is the consequence of not regenerating THF from N5-methyl THF?

Megaloblastic anemia Signup and view all the answers

What is the result of vitamin B12 deficiency on folate metabolism?

Folates are trapped as N5-methyl THF Signup and view all the answers

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 Signup and view all the answers

What is the consequence of the accumulation of homocysteine in the blood and urine?

Heart disease and hypertension Signup and view all the answers

What percentage of heart disease cases are attributed to high levels of homocysteine?

10% Signup and view all the answers

What is the consequence of a lack of vitamin B12 in the methyl malonyl CoA mutase reaction?

Accumulation of odd-carbon numbered fatty acids Signup and view all the answers

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 Signup and view all the answers

Which of the following tissues does not contain branched-chain aminotransferase?

Liver Signup and view all the answers

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 Signup and view all the answers

What is the consequence of a defect in intestinal absorption of vitamin B12?

Pernicious anemia and neurological disorders Signup and view all the answers

Why are branched-chain amino acids not degraded in the liver?

The liver lacks the enzyme branched-chain aminotransferase Signup and view all the answers

What is the result of homogentisate oxidation in urine?

It turns black. Signup and view all the answers

What is the first step in the catabolism of amino acids in most mammals?

Transfer of NH3 to α-ketoglutarate. Signup and view all the answers

Which of the following amino acids can be synthesized from pyruvate?

Only alanine. Signup and view all the answers

What is the source of essential amino acids in humans?

Dietary protein. Signup and view all the answers

What is the product of the amidation of aspartate?

Asparagine. Signup and view all the answers

What is the primary event that occurs in the pathway of phenylketonuria?

Transamination of phenylalanine to phenylpyruvate Signup and view all the answers

What is the primary cause of severe mental retardation in individuals with phenylketonuria?

Accumulation of phenylalanine or its metabolites in the brain Signup and view all the answers

What is the role of tetrahydrobiopterin in the phenylalanine hydroxylase reaction?

It acts as a cofactor to facilitate the reaction Signup and view all the answers

What is the treatment for individuals with phenylketonuria caused by a defect in dihydrobiopterin reductase?

Providing supplements of L-dopa and 5-hydroxy-Trp Signup and view all the answers

What is the primary effect of alkaptonuria, another heritable disease of phenylalanine metabolism?

Arthritis Signup and view all the answers

What is the defective enzyme in alkaptonuria?

Homogentisate dioxygenase Signup and view all the answers

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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