| Chem 432 |
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Spring 2002 |
| Lecture Notes:: 23 January |
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Let's Begin by reviewing the amino acid carbon skeleton catabolism we covered last semester.
The catabolic breakdown of most of the amino acids is summarized in the Main Routes of Amino Acid Catabolism diagram in your packet. A couple of overview comments. First, quite a number of aa catabolic pathways have irreversible steps, as symbolized by the heavy arrows in the diagram. These amino acids will be essential (that is must be provided by the diet). Generally we find that amino acids are essential in mammals (cannot be synthesized) if they are only needed to make protein. Non-essential aa's, such as serine, are biosynthesized by us because they have important roles in intermediary metabolism, not because they are needed to make protein. Second, amino acids can be categorized as being glucogenic (can be used in Gluconeogenesis) or ketogenic (cannot be used in Gluconeogenesis). Most aa's can be at least partially used in glucose synthesis. But ilu is only partially glucogenic (note some goes directly to acetyl-CoA), while leu and lys are fully ketogenic.
We will begin by looking at the catabolism of amino acids by groups: 3-C (feed into pyruvate), 4-C (feed into oxalacetate), and 5-C (feed into glutamate).
3-C aa's: Ser and ala are converted in single step processes to pyruvate. Cys is converted after first oxidizing and removing sulfur as sulfate.
4-C aa's: Asn is hydrolyzed in one step to aspartate, which in turn is transaminated in one step to oxalacetate. Threonine feeds into the TCA cycle through succinyl-CoA instead of oxalacetate. Thr is first deaminated via a dehydratase as seen earlier, then decarboxylated by Pyruvate DH Complex to give propionyl-CoA, which is then transformed via a series of steps to give succinyl-CoA.
Propionyl-CoA metabolism: propionyl-CoA is an intermediate in the catabolism of a number of amino acids, as well as in the breakdown of odd-chain fatty acids. Propionyl-CoA (3-C) enters the TCA Cycle at succinyl-CoA (4-C), thus another carbon must be added to bring it into mainstream metabolism. A biotin-dependent carboxylase adds carbon dioxide at the cost of one ATP to give D-methylmalonyl-CoA. D-methylmalonyl-CoA is then racemized to L-methylmalonyl-CoA. Methylmalonyl-CoA is a branched-chain, whereas succinyl-CoA is straight-chain: the carboxyl group and a hydrogen must be exchanged. This exchange requires C-C bond-breaking and making, a process apparently involving a Co-C bond intermediate. The cobalamin cofactor derived from Vit B12 is used in catalyzing this reaction {overhead}.
5-C aa's: Five aa's feed into glutamate which in turns feeds into the TCA cycle at 2-oxo-glutarate. His is first deaminated, then the ring is opened and the formamino group is then donated to the one-carbon pool (see later). Two of these reactions are irreversible so his is essential. Proline is first oxidized and then hydrolyzed to open the ring and give glutamaldehyde which is oxidized to give glutamate. Note that the glutamaldehyde tends to spontaneously refold to the ring, which can then be reduced to synthesize proline. It is thus not essential. Gln is hydrolyzed in one step to glutamate. Arginine is hydrolyzed to ornithine by arginase from the urea cycle. Ornithine is then transaminated to glutamaldehyde as seen with proline. Arginine is essential for infants because the arginase removes essentially all of the arg made in the urea cycle, and glutamaldehydes tendency to cyclize means it cannot be effectively synthesized from glutamate. (Bacteria use a blocking group to stop cyclization at this stage.)
Branched chain amino acids: val, leu, and ilu. The metabolism of each of these three amino acids begins with the same theme: transaminase; DH Complex; b-oxidation. Due to the irreversible nature of the DH Complex all three are essential. In the case of ilu this pattern leads to propionyl-CoA without modification. Val goes through the first two steps of b-oxidation after which its structure dictates different reactions to reach propionyl-CoA. With leu the b-oxidation is interrupted after the first step, at which point carbon dioxide is added to give 3-methyl-3-hydroxy-glutaryl-CoA the same HMG seen in ketone body synthesis. Its remaining catabolism is a reversal of ketone body synthesis.
Lysine: Note the unusual "transamination" of the epsilon amino group where lysine is first reduced using NADPH and condensed with 2-oxo-glutarate to give L-saccharopine. Saccharopine is then split and oxidized using NAD+ to give glutamate and "lysine aldehyde." The aldehyde is then oxidized again and the resulting 2-aminoadipate now follows the branched chain pattern: transaminase, DH Complex, and beta-oxidation to give acetoacetyl-CoA and finally two acetyl-CoA's.
Tyrosine and Phenyalanine: The last two amino acids on the diagram are broken into two parts: half feeds into the TCA cycle at fumerate (glucogenic), and the other half goes to acetoacetate (ketogenic). Phe is first hydroxylated using molecular oxygen and the cofactor tetrahydrobiopteran to give tyr. Tyrosine is thus only an essential aa if insufficient phe is present in the diet to synthesize it. Tyr is next transaminated followed by a couple of oxidations of the benzene ring using molecular oxygen and involving iron as a cofactor. These reactions open the ring, which is then hydrolyzed to give fumerate and acetoacetate.
The main sources of carbon for the pool are:
Serine turns out to be one of the most metabolically active amino acids. It has a very high turn-over rate: it is a major source of carbons in the one-carbon pool, and it is used in the synthesis of glycine. One of the various pathways for serine synthesis from glucose is shown below:
Serine can now be used to provide a methylene group to H4-folate. (Note that Serine hydoxymethyl transferase uses PLP to catalyze a C-C bond cleavage in this reaction.)
The glycine produced in the transferase reaction can now be used to provide a second methylene group via Glycine synthase. So how many of glucose's 6 carbons can be incorporated by this pathway? (Get two serines/glucose; one carbon + glycine from each serine, then a second carbon from glycine with the remaining carbon lost as carbon dioxide. Therefore 4/6 glucose carbons can go into the one-carbon pool.)
Tetrahydrofolate is the major carrier involved in single carbon transfers. As can be seen in the Main Folate Metabolic Pathways diagram, H4-folate can carry carbon in the various oxidation states required in a variety of metabolic reactions:
 Pathway Diagrams |
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Last modified 23 January 2002
