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| This enzyme, along with the cofactor pyridoxal phosphate, catalyzes the hydrolysis of cystathionine to cysteine, ammonium ion (NH4+), and alpha-ketobutyrate. This reaction serves as a branch point for the pathways leading to the disposition of methoinine sulfur during the catabolism of that amino acid and the anapleurotic pathway that converts alpha-ketobutyrate to the TCA cycle intermediate succinyl CoA. (cf. fig. 39.12, p. 737). Deficiency of this enzyme or its cofactor, synthesized from Vitamin B6, is characterized by cystathionuria but no other clinical abnormalities; it is considered a benign disorder (cf. clinical note sidebar, p. 738). |
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| Product of the cystathionine synthase reaction in the methionine catabolism/cysteine synthesis pathway that combines serine and homocysteine. |
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| cystathionine synthase -- |
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| This enzyme catalzes the reaction homocysteine + serine --> Cystathionine + H20. Its absence leads to increased methionine and homocysteine levels and decreased cystathionine and cysteine levels (cysteine is later in the pathyway). This was seen in the case of Homor Sistine as discussed in the text. |
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| synthesized from the carbons and nitrogen of serine and the sulfur of homocysteine (derived from methionine) Pg. 737. |
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| Nine amino acids are essential due to the fact that the carbon skeleton cannot be synthesized which means these amino acids must be obtained from the diet. The essential amino acids are: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine. (Arginine only required for growth) |
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| glucogenic amino acids --> |
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| The catabolism of these amino acids leads to the formation of pyruvate or TCA cycle intermediates. These amino acids are important for anaplerotic reactions of the TCA cycle, as well as gluconeogenesis. All amino acids EXCEPT for lysine and leucine are glucogenic. |
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| With serine, eventually forms cysteine via the cystathionine synthase and cystathionase reactions (cf. fig. 39.12, p. 737). |
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| Derived from methionine metabolism and can be converted back into methionine by using both methyl-FH4 and vitamin B12 Pg. 751 (Ch. 40) |
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| a chemical process that introduces a hydroxyl group (-OH) into an organic compound. |
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| The catabolism of these amino acids leads to the formation of ketone bodies. Lysine and Leucine are purely ketogenic amino acids. Isoleucine, tryptophan, phenylalanine and tyrosine are ketogenic and also glucogenic. |
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| required for synthesis of SAM and is obtained from the diet or produced from homocysteine, which accepts a methyl group from vitamin B12. Can be regenerated from homocysteine by a reaction that requires both FH4 and vitamin B12. Pg. 742, Pg. 760 (Ch. 40) |
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| 5,10-methylene tetrahydrofolate |
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| phenylalanine hydroxylase |
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| Phenylketonuria is a rare condition where new borns have the inability to break down the amino acid phenylalanine. Phenylalanine plays a role in the bodys pigment production of melanin and thus babies with PKU have lighter skin, eyes and hair than their siblings. Other symptoms include seizures, delayed mental and social skills, skin rashes, tremors, hyperactivity and mental retardation. PKU is a treatable disease that involves a diet with low levels of phenylalanine. The most common form of PKU, called classic PKU, results from the absence of the phyenylalanine hydroxylase enzyme which catalyzes the breakdown of phylalanine to tyrosine. This reaction is: phylalanine + BH4 + O2 --> Tyrosine + BH2 + H20 |
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| an intermediate of the citric acid cycle, it is synthesized to ?-ketoglutarate by ?-ketoglutarate dehydrogenase, coenzyme A is added also during this time, succinyl Co A is synthesized from propionyl CoA. |
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| Tetrahydrofolate (FH4) --> |
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| a coenzyme that is used to transfer one-carbon groups at various oxidation states. Used in both amino acid degradation and biosynthesis Pg. 731 |
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| Tetrahydrobiopterin (BH4) --> |
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| a cofactor that is required for ring hydroxylation reactions. The reactions involve molecular oxygen and one atom of oxygen is incorporate into the product. BH4 is important for the synthesis of tyrosine and neurotransmitters. Pg. 731 & 734. |
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