Methionine, Threonine and Lysine Metabolism

by Kevin Ahern, PhD

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    Methionine metabolism is kind of complicated as we study it coming from aspartic acid. The breakdown of methionine actually overlaps with cysteine metabolism as we briefly seen. The complicated synthesis that comes from aspartate, however, I'll show here with a set of arrows. Now, the other set of lectures where I've talked about vitamin B12 metabolism, I actually showed those seven reactions. So I'm not going to show them again here. But suffice it to say that there are seven chemical steps that occur: phosphorylation, an oxidation that accompanies dephosphorylation, another oxidation that occurs, this process creates homoserine. Homoserine we remember was an intermediate in the synthesis of the cysteine from methionine. Succinylation involves the addition of a succinate molecule. The cysteine is replacing the succinate in the next step of the process. There's then loss of a pyruvate and an ammonion ion to produce the homocysteine and homocysteine was the molecule we remember whose concentration is a problem in the production of cysteine. And finally, methylation of the homocysteine by an N5-methylfolate creates methionine. This was the reaction that I described in the other lecture that requires vitamin B12. There are other ways of making methionine. Homocysteine, as we've seen, can be converted to methionine by the alternate pathway that I'm going to show you here. Now, this pathway is another way of getting a methyl group on to homocysteine. The difference between homocysteine and methionine is that methyl group. So glycine betaine is the source of methyl group here. It combines with homocysteine to make dimethylcysteine and methionine. So we can see that a methyl group has transferred from the glycine betaine on to the homocysteine to make methionine. The enzyme catalyzing this reaction is betaine, homocysteine, methyltransferase. And the reaction here occurs in the liver. We see...

    About the Lecture

    The lecture Methionine, Threonine and Lysine Metabolism by Kevin Ahern, PhD is from the course Amino Acid Metabolism. It contains the following chapters:

    • Methionine Metabolism
    • Threonine Metabolism

    Included Quiz Questions

    1. ...a sulfhydryl gets methylated.
    2. ...homocysteine is produced.
    3. ...cysteine acts a precursor.
    4. All of the answers are true.
    5. None of the answers are true.
    1. None of the answers are true.
    2. It is the first amino acid put into proteins of eukaryotic cells.
    3. It is made before being joined to a tRNA.
    4. It uses S-Adenosylmethionine (SAM) as the source of the formyl group.
    5. All of the answers are true.
    1. All of the answers are true.
    2. It overlaps with that of methionine.
    3. It involves a homoserine intermediate.
    4. It requires 2 ATP and 2 NADPH.
    5. None of the answers are true.
    1. All of the answers are true.
    2. It is synthesized in a complicated set of reactions.
    3. It is the most covalently modified amino acid.
    4. It accumulates in blood in hyperlysinemia.
    5. None of the answers are true.

    Author of lecture Methionine, Threonine and Lysine Metabolism

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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