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Amino Acid Catabolism

by Kevin Ahern, PhD
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    00:02 Now as we consider amino acid catabolism, I’ve talked about some of the individual reactions, but I want to now describe them in the entirety of the picture of all the amino acids.

    00:12 Amino acid catabolism is broken down to three categories.

    00:15 The first category being amino acids that are glucogenic.

    00:18 These are amino acids that are broken down into glycolysis or gluconeogenesis intermediates.

    00:24 And the glucogenic amino acids are shown in the figure on the right labeled in green.

    00:29 The ketogenic amino acids are those that are broken down to form acetyl-CoA.

    00:34 And they’re shown in the figure on the right on the red.

    00:37 There are still amino acids that appear as intermediates in both pathways and they’re known as ketogenic and gluconeogenic.

    00:43 And those amino acids are shown in the pathway in blue.

    00:49 Now the pathways that are shown on the right schematically depict on the left, the glycolysis pathway starting with glucose and ending with pyruvate.

    00:58 And the circle on the right depicts the citric acid cycle and the intermediates of the citric acids cycle.

    01:05 So we can see how the breakdown of these amino acids feed into these individual pathways.

    01:12 The glucogenic amino acids, you can see here, alanine, I won’t describe them all.

    01:16 You can see the list of amino acids that are present.

    01:19 The ketogenic amino acids are not nearly as a bond, but there’s only two that primarily produce the acetyl-CoA, lysine and leucine.

    01:27 And here are the amino acids that are involved in both.

    01:30 Yeah, it’s complicated. It looks complicated.

    01:32 But there’s actually some simplification to it.

    01:35 And that is that there are only six molecules that are involved in the catabolism of amino acids They’re shown here.

    01:42 Pyruvate, acetyl-CoA, oxaloacetat, alpha-ketoglutarate, succinyl-CoA and fumarate.

    01:49 All of the animo acids can be broken down to produce those six intermediates.

    01:56 It’s because of this, and you see the four of these individual molecules up here in citric acid cycle, it’s because of this that we describe the citric acid cycle is anaploratic.

    02:06 It can use intermediates from elsewhere to break down.

    02:09 It can also be a source of intermediates for making some of these amino acids.

    02:14 So amino acid catabolism is pretty important as we can see.

    02:18 It has to be able to deal with the means.

    02:20 The cell has to be able to shuffle, make the proper amount of amino acids and not produce too much ammonia if it’s going to function properly.

    02:29 Most of the pathways ultimately shuffle amines through transamination.

    02:34 But the addition in the movement of ammonia is also an important consideration.

    02:39 That’s why glutamate transamination is important because it can transfer or accept amines and it can also handle ammonia.

    02:48 Glutamate is therefore essential for the transport of all of these nitrogen containing compounds away from sensitive tissue and delivering it to the liver.

    02:56 And we’ve seen how the glucose alanine cycle helps the brain, for example, to take away that excess amine without losing glutamate.

    03:05 In the liver, this is where the ammonia is used for urea synthesis and for excretion.

    03:10 The liver handles a lot of things in our body.

    03:13 The brain as I’ve said is sensitive to ammonia levels and so they have to be very careful to balance things appropriately there.

    03:20 Glutamate is a neurotransmitter as I said and that’s why the glucose alanine pathway is important because alanine provides a way of getting the amine out of the brain or the ammonia out of the brain.

    03:31 Now as you might imagine, we’ve got some complicated pathway that we’ve talked about and I’ve talked about some of the diseases that are involved.

    03:37 I want to review some of those and bring up some of the others that were involved in amino acid catabolism.

    03:43 Alcaptonuria, we’ve seen, is involved in the catabolism of phenylalanine and tyrosine.

    03:49 Mathylmalonic academia, which I haven’t describe, arises from defects in the catabolism of methionine, threonine, isoleucine and valine.

    03:57 Mapel syrup disease arose from the breakdown problems associated with the branched-chain amino acids.

    04:04 Homocystinuria arises from the problems with breakdown of methionine.

    04:10 Tryrosinemia, of course, from the breakdown products of tyrosine.

    04:14 And argininemia arises from problems associated with the breakdown of arginine.

    04:18 And I’ll say more about that in the lecture of the urea cycle.

    04:21 Hyper-methioninemia arises and its name is suggests from the deficiencies in breakdown of methionine.

    04:27 Hyperlysinemia from deficiencies in the breakdown of lysine.

    04:31 Glycine encephalopathy from the deficiencies in breakdown of glycine.

    04:35 Propionic academia from the deficiencies to the breakdown of these four enzymes.

    04:39 And finally, hyperprolinemia from deficiencies in the breakdown of proline.

    04:44 Now one of the things happens to amino acids that we also have to consider chemically is that after they get built into proteins, many of them are chemically modified.

    04:53 That chemical modification actually enhances the ability of a protein to function or may have important things relating to the signaling involved in proteins or protein-protein interactions.

    05:05 So the most commonly chemically modified amino acids are shown on the screen here.

    05:10 I’ve got two examples in terms of structure.

    05:12 And you can imagine using your knowledge of chemistry what the others actually look like.

    05:18 Hydroxylysine is one.

    05:19 It’s very commonly modified that we’ve seen.

    05:22 And phosphotryrosine is important in the signaling process.

    05:25 Serine is a target in the cell for glycosylation and phosphorylation.

    05:30 This arises from the side chain of serine that has a hydroxyl group, and that’s where a phosphate can be attached by a kinase or a sugar can be attached in the process of glycosylation.

    05:41 Threonine has the same side chain as serine does, a hydroxyl group.

    05:45 And like serine, it’s a target for phosphorylation and glycosylation.

    05:50 Now lysine is really a complicated amino acid in terms of modification.

    05:56 The most common modifications of it involved methylation and acetylation and hydroxylation.

    06:01 I’ve listed below a variety of other kinds of modifications that can happen to serine.

    06:06 I won’t list all of them here.

    06:07 But suffice it to say that lysine is the most commonly covalently modified enzyme found in proteins.

    06:14 Methionine is modified in prokaryotes by the addition of a formal group as I’ve described earlier in the lectures on amino acid metabolism.

    06:22 And that happens only in prokaryotes and only for the first amino acid going into proteins and prokaryotes.

    06:28 Arginine can be acetylated and the acetylation of amino acids like arginine and also lysine is important because they have positive charges.

    06:37 And the addition of an acetyl group changes that positive charge to a neutral charge.

    06:42 If we’re talking about histones which is where this modification occurs, histones are very positively charged proteins that interact with DNA, which is negatively charged.

    06:53 So if we take that arginine or lysine and convert it from a positive charge to a zero charge, we change the attraction that occurs between the histone and the DNA.

    07:05 And we’ll see more about that in our discussion of the control gene expression of DNA.

    07:13 Proline is a target for hydroxylation.

    07:15 Of course, that happens in collagen as I’ve described in the collagen lecture.

    07:19 Cysteine as I’ve mentioned earlier is a very important amino acid, because within proteins, it can react with a second cysteine.

    07:26 So two cysteines can get together and make a disulfide bond.

    07:30 I’ve mentioned that earlier in the lectures on amino acid metabolism.

    07:34 And that disulfide bond that forms between two cysteines is a very important structural feature to help stabilize proteins.

    07:41 Histidine is not very much modified.

    07:44 But occasionally, histidine is phosphorylated.

    07:47 That’s something we’ve learned only in recent years.

    07:50 The significance of that phosphorylation is not completely clear at this time.

    07:54 Glycine is a target for myristoylation.

    07:57 That involves the attachment of a myristic acid to a glycine.

    08:02 Glutamic acid is a target for carboxylation, and I’ve talked about that earlier as related to the importance of carboxylation, the blood clotting process.

    08:12 And finally, asparagine is a target for glycosylation just like the serine and threonine, meaning that this amino acid side chain is linked to sugar residues.

    08:24 In this set of lectures, I’ve described the metabolic pathways involved of the 20 amino acids, plus the one rare amino acid, selenocysteine.

    08:34 There’s a lot of diversity here.

    08:35 There’s a lot of different reactions that are occurring in a lot of different controls.

    08:39 This is necessary as we’ve described to balance the individual amino acids.

    08:44 We’ve also seen how those amino acids are chemically modified and how the breakdown of those amino acids can lead to numerous genetic diseases.


    About the Lecture

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

    • Amino Acid Catabolism
    • Amino Acids Modified After Incorporation Into a Protein

    Included Quiz Questions

    1. Glucogenic amino acids include those broken down into pyruvate and oxaloacetate.
    2. Ketogenic amino acids break down to ketone bodies, such as acetone.
    3. Most amino acids are ketogenic.
    4. All of the answers are true.
    5. None of the answers are true.
    1. Transamination is the most common mechanism
    2. Brain levels of ammonia are kept high.
    3. Glutamine accepts ammonia from many sources.
    4. All of the answers are true.
    5. None of the answers are true.
    1. ...involves phosphorylation of serine and threonine.
    2. ...involves carboxylation of arginine.
    3. ...results in glycosylation of proline.
    4. All of the answers are true.
    5. None of the answers are true.

    Author of lecture Amino Acid Catabolism

     Kevin Ahern, PhD

    Kevin Ahern, PhD


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