Amino Acid Catabolism

Transcription

1 Amino Acid atabolism 3-1 Lec #8 To date we have considered the catabolism of carbohydrates and lipids with the object of generating energy in the form of ATP. Both give rise to AcoA which is fed through the TA cycle for complete conversion to. Amino acids are treated in much the same fashion except that removal and excretion of the amino group provides an additional level of complexity. arbohydrates Lipids AcoA TA cycle Amino acids 3 Given that the first step in many of the degradative pathways is the removal of the amino group from the carbon skelton of the amino acid, we will first consider transamination and deamination reactions. 1. Transamination We will first consider a generic transamination which will illustrate the fact that and α-ketoglutarate are always involved in transaminations. 3 amino acid 3 Amino acid transaminase + pyridoxal phosphate + α-keto acid α-ketoglutarate 3 3 Alanine transaminase + pyridoxal phosphate alanine pyruvate α-ketoglutarate

2 3-3 3 Aspartate transaminase + pyridoxal phosphate + oxaloacetate aspartate α-ketoglutarate In these and all other transaminase reactions, and α-ketoglutarate are always involved and the enzyme is named for the other amino acid. Mechanism of transamination (involving pyridoxal phosphate) 1a alanine (= 3 ) a P 3 P 3 3 pyridoxal P 4a 3a 3 P 3 P.. 3 3

3 3-3 5a 6a 3 P 3 P 3 3 ' pyruvate 7b.. α-ketoglutarate (= - ) 7a.. 3 P 3 P 6b '.. 5b 3 ' 3 3 P 3 P 3 3

4 3b ' 4b ' P 3 P b ' 1b ' 3 P 3 P 3 3 pyridoxal P For alanine transaminase, = 3 (alanine to pyruvate) and ' = - (α-ketoglutarate to ).. Deamination xidative (An oxidation-reduction reaction accompanies the deamination process.) The most common deamination is that of catalyzed by dehydrogenase.

5 3-5 3 Glutamate dehydrogenase + 3 AD + + AD + + α-ketoglutarate The reaction is reversible, but the enzyme has a high K M for 3 resulting in the enzyme operating as a deaminase at low [ 3 ]. A close relative of dehydrogenase, although not quite as common, is alanine dehydrogenase. 3 Alanine dehydrogenase alanine AD + + AD pyruvate A less common oxidative deamination is catalyzed by the amino acid oxidases which use many, but not all, amino acids as substrates. 3 amino acid With dehydrogenase as the most common deaminase, the most common route for deamination of other amino acids involves an initial transamination to generate a, followed by deamination of the. AD + + amino acid (~13) Amino acid oxidase + α-ketoglutarate 3 α-ketoacid + 3 urea transamination deamination AcoA α-ketoacid AD +

6 on-oxidative deamination 3-6 on-oxidative deaminations, as the name implies, are not accompanied by an oxidation-reduction reaction. They tend to be more varied in the chemistry involved and we will consider just two at this juncture. 3 Aspartate -deaminase + 3 aspartate fumarate AcoA 3 Serine dehydratase serine pyruvate As a bit of a preview that illustrates how there is overlap among so many different pathways, threonine dehydratase carrries out a very similar reaction generating the 4 -ketobutyrate and we will come across this enzyme in the isoleucine biosynthetic pathway later. 3. Interconversion of and glutamine. 3 Glutamine synthetase 3 P 3 ATP + 3 ADP + Pi glutamine The carboxyl group is phosphorylated transiently during the reaction. Glutaminase 3

7 4. Urea ycle - nitrogen excretion 3-7 Lec #9 Before getting into a description of how various amino acids are catabolized (degraded), we will first consider how excess nitrogen generated from the removal of amino groups is excreted. This is a particularly important issue in higher organisms where the 3 is extremely toxic because reversal of dehydrogenase upsets energy metabolism by lowering AD levels and, as a result, ATP production. What has evolved in some organisms is the urea cycle which converts ammonia into urea. The advantages of urea lie in its non-toxicity and high water solubility which allow the accumulation of highly concentrated solutions ( 7 to 8 M) of urea. The series of reactions also makes up part of the biosynthetic pathway for arginine in most organisms, but only those organisms that have adopted the final enzyme arginase are capable of producing urea. 3 urea We will first look at the details of the individual reactions and then we will look at how the reactions are coordinated into a cycle ATP arbamoyl phosphate synthetase ( 3 ) P 3 + ADP + Pi carbamoyl phosphate + P 3 rnithine transcarbamolyase 3 carbamoyl phosphate Pi ornithine 3 citrulline

8 Argininosuccinate synthetase 4 3 citrulline aspartate ATP Pi IPPase AMP + PPi 3 argininosuccinate Argininosuccinate lyase + 3 argininosuccinate 5 3 arginine fumarate ** aspartate Arginase + 3 arginine 3 ornithine 1 from carbamoyl P (glu or any other AA) and 1 from aspartate (glu or any other AA)

9 With regards energy requirements, 4 ATP equivalents are required to produce one urea: ATPs are used by carbamoyl P synthetase and ATP equivalents (1 ATP to AMP + Pi) are used by argininosuccinate synthetase. 3-9 In other words, -excretion is an energy requiring process. verall view: utside mitochondria Inside mitochondria ornithine ornithine arginine UEA carbamoyl P ATP 3 fumarate argininosuccinate citrulline citrulline AD + AD ATP aspartate #1 glu -KG malate AD + AD AA # glu -KG -keto acid amino acid -keto acid amino acid Illustrating the importance of the urea cycle to humans is the fact that a deficiency in any one of the urea cycle enzymes results in hyperammonaemia or elevated ammonia levels. A complete deficiency (mutation in both genes) results in a coma and death at birth. A partial deficiency results in mental retardation which can be limited by placing the infant on a low amino acid diet. The effects of high ammonia seem to be related to their influence on dehydrogenase. ormally a deaminase, this enzyme can be reversed by high ammonia resulting in a drop in AD levels and in TA cycle intermediates ( -KG is reduced), which in turn reduces ATP production via oxidative phosphorylation, as well as an increase in the levels of a neurotransmitter. Brain and nerve cells do not develop normally.

10 5. Amino acid degradation pathways 3-10 So as a generality we can say for a majority of the amino acids the normal route for degradation follows the following scheme. amino acid -KG AD urea -keto acid AD + The question that follows is what happens to the -keto acids and the answer is that, for energy extraction, they are all ultimately converted into AcoA to be fed into the TA cycle. The pathways differ for each of the amino acids. AcoA This is illustrated in the following scheme. -KG glu gln his pro arg * * val ile met succoa B uneven chain fatty acids fumarate phe tyr asp * The multiplicity of entry points arises from fragmentation of the carbon skeletons. The amino acids marked by * have already been covered through existing reactions. Pathways for the degradation of all amino acids will not be covered, only those marked by A, B and. onsideration of these three groups will provide a good cross section of examples of degradative pathways. asp asn AA B * phe tyr leu lys trp A carbohydrates pyruvate ala cys gly ser thr * A thr ile leu trp AcoA acetoacetate fatty acids TA ycle

11 Example A gly ser pyr AcoA thr acetaldehyde acetate AcoA 3-11 * See below ethanol 3 acetaldehyde AD Alcohol dehydrogenase AD threonine Acetaldehyde dehydrogenase 3 3 AD + + AD + + Serine hydroxymethyl transferase (Threonine aldolase) 3 **See below F 4 - F 4 3 glycine Serine hydroxymethyl transferase (Threonine aldolase) serine acetate 3 3 Serine dehydratase ATP oas pyruvate see page -4 Pi AcetyloA synthetase IPPase AMP + PPi AD + + SoA AD 3 oas Pyruvate dehydrogenase acetyl oa (x) 3

12 3-1 * Ethanol metabolism, like threonine degradation, generates the very poisonous acetaldehyde. Because there is often considerable ethanol being metabolized at one time, a rapid and efficient system has evolved to remove it involving acetaldehyde dehydrogenase. The importance of the enzyme is illustrated by a common cause of alcohol intolerance being a deficiency in acetaldehyde dehydrogenase. This is particularly true among orientals who often exhibit facial flush from the release of neurotransmitters and feel ill as a result of the blood acetaldehyde concentration being some 15 to 0 times higher than in other races. As examples, low acetaldehyde dehydrogenase levels are evident in 44% of Japanese, 53% of Vietnamese and up to 45% hinese. ** Tetrahydrofolic acid or F 4 "active portion" 1 4 pteridine 3 p-aminobenzoic acid (PABA) (usually a polymer of 8 to 14) 3 serine Serine hydroxymethyl transferase (Threonine aldolase) + 3 glycine 5 -hydroxymethyl F 4 5, 10 -methylene F 4

13 3-13 AD + + AD + + AD + AD + 3 Methenyl F 4 reductase Methylene F 4 reductase 5, 10 -methenyl F 4 5, 10 -methylene F 4 5 -methyl F 4 purine biosynthesis purine, pyrimidines, amino acids amino acids The point here is that a 1-carbon unit can be transferred amoung several different oxidation states and pathways. As a reminder, folate metabolism is considerably more complex than this limited exposure might suggest. In addition, to the different oxidation states for the 1-carbon unit, the pteridine portion of the molecule can exist in three different oxidation states and the family of folates is enlarged even further by the variation in poly chain length, normally between 8 and folate dihydrofolate 3 4 tetrahydrofolate

14 Example B acetoacetate AcoA phe tyr fumarate AcoA 3-14 Lec # α-kg glu phenylalanine tetrahydro dihydro biopterin biopterin Phenylalanine hydroxylase*** tyrosine Tyrosine transaminase p-hydroxyphenyl pyruvate p-ydroxy phenylpyruvate dioxygenase Maleyl acetoacetate isomerase fumarylacetoacetate maleylacetoacetate omogentisate 1,-dioxygenase homogentisate Fumaryl acetoacetase + malate AA pyruvate Acetyl-oA 3 acetoacetate (see next page) fumarate TA cycle

15 3 acetoacetate succinyloa succinate SoA oas β-ketoacyloa β-ketoacyloa transferase thiolase 3 (fatty acid acetoacetyl-oa degradation page -5) oa-s 3 acetyl-oa ( X) 3-15 TA cycle ***The enzyme phenylalanine hydroxlylase (also phenylalanine monooxygenase) is missing in ~1 in 10,000 humans, a result of a genetic defect. The result is that phenylalanine cannot be metabolized to tyrosine for both degradation and tyrosine synthesis (if needed). This results in phenylalanine serving as a substrate for tyrosine transaminase and the formation of phenylpyruvate which accumulates in the blood and is excreted in the urine. Phenylpyruvate (or one of its metabolites) interferes with normal brain and nerve cell development leading to mental retardation in infants if it is not diagnosed and treated. The syndrome is known as phenylketonuria and the PKU test is typically carried out on all new born in the hospital. Elevated phenylpyruvate leads to the infant being placed on a phenylalanine-free diet for the first 5 to 10 years of life. 3 α-kg glu phenylalanine Tyrosine transaminase phenylpyruvate Another effect of a deficiency in phenylalanine hydroxylase is a lighter skin color caused by the elevated phenylalanine levels inhibiting the synthesis of melanin from tyrosine. Phe X Tyr Melanin inhibited by Phe

16 Example val Succinyl-oA valine -KG glu AD + AD + + Branched chain amino acid oas transaminase 3 3 -keto acyl -keto isovalerate dehydrogenase isobutyryl oa (same cofactors Same two enzymes are used for val, leu and ile. as pyr dehydrogenase) FAD If the -keto acyl dehydrogenase is missing as a result of a genetic defect, the -keto acids from val, leu and ile accumulate in the blood and urine impairing brain and nerve cell development. The unique smell of the -keto acids has given rise to the name of the disease - maple syrup urine disease. Acyl-oA dehydrogenase SoA 3 3 FAD oas SoA SoA 3 -hydroxyisobutyrate -ydroxy isobutyryl-oa hydrolase 3 -hydroxyisobutyryl-oa Enoyl-oA hydratase 3 methylacrylyl-oa AD + -ydroxy isobutyrate dehydrogenase AD + + AD + AD + + SoA SoA 3 methylmalonyl semialdehyde oas Methylmalonyl semialdehyde dehydrogenase 3 propionyl-oa 1. Propionyl-oA carboxylase. Methylmalonyl-oA epimerase 3. Methylmalonyl-oA mutase succinyl-oa (from fatty acid degradation page -7 and -8)

17 To complete valine degradation to : 3-17 succ-oa succ fum mal AA pyr acetyl-oa (4) 1 (AA) TA ycle 3 4 Summary of amino acid catabolism 1. Transamination and deamination. Urea cycle 3. Amino acid degradation focusing on three examples A - Thr, Ser, Gly B - Phe, Tyr - Val but also Glu, Gln, Asp, Ala 4. verall picture for catabolism arbohydrates Fatty acids Acetyl-oA TA ycle GTP ADP + Pi Amino acids ( ) urea cycle -868 kj/mol ATP AD AD + FAD FAD ATP + Electron Transport hain next +868 kj/mol hν