Cysteine Metabolism

by Kevin Ahern, PhD

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    00:02 Now, homocystinuria, as I said, is pretty severe disease.

    00:05 It's a genetic disease caused by deficiency of this enzyme known as cystathionine beta-synthase.

    00:11 There are many problems that can arise in homocystinuria.

    00:15 There are musculoskeletal anomalies that occur, intellectual disabilities.

    00:20 And one of the reasons we have these intellectual disabilities is because of the problems with the nerve tissue in the brain.

    00:26 Seizures can result.

    00:28 There are numerous eye anomalies that can arise.

    00:31 Vascular disease because again, we have a lot of homocysteine that's occurring because it's not being converted into cysteine in the process.

    00:38 We see cystinuria which is an unrelated genetic disease that has many symptoms that are similar.

    00:45 We have the inability of the kidneys to reabsorb cysteine.

    00:48 So when this happens, then we see problems associated with the kidneys including high levels of cysteine, high levels of lysine, ornithine and arginine that appear in the urine.

    00:59 And because we have these things accumulating, we develop kidney stones that also occur.

    01:03 This is a not disease that you would wish on anyone.

    01:07 There are at least three other ways of making cysteine and I illustrate these here.

    01:11 One of these comes from serine directly.

    01:14 In this process, acetylcholine gets -- It donates an acetyl group to serine to make O-acetyl L-serine.

    01:22 That is shown here.

    01:23 And the next step of the process, the sulfur that ends up being cysteine comes with the addition of a hydrogen sulfite.

    01:30 This splits out the acetate in the process leaving behind cysteine for making proteins.

    01:36 A second way of making cysteine actually comes from proteins itself.

    01:40 One of the things that cysteine can do on a protein is combine with another cysteine in a covalent bond to make this dicysteine as it were called L-cysteine.

    01:50 This involves the joining of the two cysteines together by a disulfide bond as shown in the middle of the molecule.

    01:56 When proteins are cleaved and they've had this bond occur in them, L-cysteine is the product of the breakdown of the protein.

    02:04 Making cysteine from L-cysteine is a trivial matter.

    02:07 It simply involves breaking that bond.

    02:09 And breaking that bond involves a reduction.

    02:11 The electrons in the process come from NADH as you can see here, the product of the reaction producing two cysteins.

    02:19 The last way of making cysteine starts with an oxidized form of cysteine known as cysteic acid.

    02:25 Cysteine is fairly, readily oxidized and so it's not uncommon that cells will have L-cysteic acid within them.

    02:33 To make cysteine from that, simply involves a reduction and that reduction involves again, hydrogen sulfite as we can see here.

    02:41 In the process, the sulfur is donated to the cysteic acid The sulfite which was the oxidized sulfur is released and cysteine is produced.

    02:50 Another member of the cysteine family is that of one of the rare amino acids I talked about earlier.

    02:56 This amino acid is known as selenocysteine.

    02:58 And it does not normally occur in proteins.

    03:00 It's very rarely appearing.

    03:02 And it's way of appearing in proteins is unusual and its synthesis is also a little unusual.

    03:09 Selenocysteine is sometimes called the 21st amino acid because it's not coded for genetic code.

    03:16 It uses a stop codon that's in the translational process to get into a protein, and I won't talk about that here.

    03:25 The way in which selenocysteine is made is actually made on a transfer RNA.

    03:31 Now, the other amino acid who has a modification that occurs on a transfer RNA is that of methionine, and I'll talk about that later in this set of lectures.

    03:40 When we look at this process, we see what the way in which selenocysteine is made.

    03:45 Selenocysteine synthesis starts with serine, which is why it's in the serine family.

    03:50 But serine itself is not directly modified until after the serine has been joined to a transfer RNA.

    03:57 You'll notice that this is a non-serine transfer RNA.

    04:01 So this is a special transfer RNA that has joined the serine in the process of making selenocysteine.

    04:08 So, here we have the product of that reaction.

    04:10 The serine has been linked to the special transfer RNA.

    04:14 There are two chemical reactions in which the serine is modified and the modification of the serine involves the alteration of the hydroxyl group to put a seleno group into that.

    04:25 Two enzymes are involved known as cell A and cell B.

    04:29 The product of that reaction is this selenocysteine linked to the transfer RNA.

    04:34 And the selenocysteine linked to the transfer RNA is then incorporated into proteins using this odd stop codon mechanism of getting it in.

    04:43 It's because of this that selenocysteine is so rarely found in proteins.

    About the Lecture

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

    • Cysteine Metabolism & Health
    • Selenocysteine Metabolism

    Included Quiz Questions

    1. It is closely linked to methionine metabolism.
    2. It requires hydrogen sulfide.
    3. It can occur from arginine.
    4. It cannot occur from serine.
    5. None of the answers are correct.
    1. All of the answers are true.
    2. It is made on a tRNA.
    3. It is incorporated into proteins in an unusual fashion.
    4. It does not have a codon in the genetic code.
    1. Sel A
    2. Sel D
    3. Sel C
    4. Sel B
    5. Sel E

    Author of lecture Cysteine Metabolism

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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