生物化学：Chapter9 Metabolism of Amino Acid

1、Chapter 9Metabolism of Amino Acid,Section Physiological function and nutritional value of protein,Physiological functions of protein can not be replaced by carbohydrates or lipids 2. Nitrogen balance 3. Nutritional value of proteins: quantity + quality, Essential amino acids (8 kinds): * Can not be

2、synthesized by human body * Must be supplied in the diet Non-essential amino acids（12 kinds）,Nutritional value of proteins is related to the ratio of essential amino acids and non-essential amino acids,Made by simple reactions,Made by complex routes (in bacteria Protects cells from the accumulation

3、of abnormal proteins; Numerous physiological processes are dependent on timely degradative and synthetic reactions: eukaryotic cell cycle control and antigen presentation.,1-8,Protein degradation pathway in lysosomes occurs in lysosomes ATP independent Non-specific protein degradation for extracellu

4、lar proteins membrane proteins long half-life intracellular proteins,Pathway of tissue proteins degradation,Ubiquitinproteasome degradation pathway occurs in cytosol ATP dependent Specific protein degradation for abnomal proteins short life proteins,Pathway of tissue proteins degradation,* Ubiquitin

5、: 8.5KD small protein of 76 aas * 泛素活化酶（ubiquitin activating enzyme, E1) * 泛素结合酶（ubiquitin coupling enzyme, E2) * 泛素-蛋白连接酶(ubiquitin-protein ligase, E3) * 蛋白酶体（proteasome)- huge complex,Ubiquitination,Active Pr,ubiquitination,Proteasome：huge complex Proteasome exist in cell nuclear and cytoplasm, de

6、grade abnormal and short-life proteins.,26s protein complex,20S core particle (CP) 19S regulatory particles (RP):18 subunits,6 subunits have ATPase activity.,2 loop: 7 subunits/loop 2 loop: 7 subunits/loop,Amino acid metabolic pool,Dietary protein,Synthesis of Nonessential amino acids,Tissue protein

7、,Tissue protein,Sources and fates of amino acids,Other pathways: Nitrogen-containing Compounds, one-carbon units, etc.,. Amino acid metabolic pool,excretion from urine ammonia urea in liver *deamination non-essential aa -ketoacid CO2 +H2O + ATP Glc or lipids amine non-essential aa *decarboxylation C

8、O2,Fates of amino acids,ammonia,R C COOH O,. Deamination of amino acids,Amino acids R C COOH NH2,The first step in the catabolism of aas is removing of their -amino group.,Four types of deamination: 1. Tansamination 2. L-Glu oxidative deamination 3. Associated deamination* -transamination associated

9、 with oxidative deamination -transamination associated with purine nucleotide cycle 4. Amino acid oxidase deamination,A reaction catalyzed by transaminase in which an amino group is transferred from one molecule to another.,1. Transamination,Coenzyme: Pyridoxyl phosphate (PLP) (VitB6),(Transaminase)

10、,Distribute widely; Reversible reaction: degradation of amino acids amino acids synthesis Can not remove free ammonia from molecular; Most of amino acids can take part in transamination; There are different aminotransferases with specificity for different amino acids.,The most important aminotransfe

11、rases are to catalyze amino transfer between L-Glu and -ketoacid:,Alanine aminotransferases (ALT) / glutamate-pyruvate transaminase (GPT),Aspartate aminotransferase (AST) / glutamate-oxaloacetate transaminase (GOT),oxaloacetate,glutamate,- ketoglutarate,AST,COOH HC CH2 COOH,NH2,Asp,COOH C=O CH2 COOH

12、,The activities of ALT and AST in different normal adult tissues,Diagnosis value of serum aminotransferases,Two important enzymes in the clinical diagnosis of human disease are serum GOT and GPT- abundant in heart and liver (intracellular enzymes). They are released as part of the cell injury that o

13、ccurs in myocardial infarction, infectious hepatitis, or other damage to either organ. Assays of these enzyme activities in blood serum can be used both in diagnosis and in monitoring the progress of a patient during treatment.,their coenzyme,Pyridoxal phosphate-derived from vitamin B6, is the coenz

14、yme of transaminase serving as transporter of amino group in the reaction.,The mechanism of transamination,The functional part of pyridoxal phosphate is an aldehyde functional group attached to a pyridine ring.,active in most tissue except skeletal and cardiac muscles; allosteric enzyme (ATP/GTP inh

15、ibits, ADP/GDP activates).,L-Glutamate dehydrogenase,2. L-Glu oxidative deamination,Reversible,glutamate,a-ketoglutarate,amino acid,a-keto acid,H2O+NAD+,NH3+NADH+H+,glutamate dehydrogenase,Transaminase,3. Transamination associated with oxidative deamination (Associated deamination in most tissues),D

16、egradation of amino acids,Synthesis of nonessential amino acids,glutamate,a-ketoglutarate,amino acid,a-keto acid,H2O+NAD +,NH3+NADH+H+,glutamate dehydrogenase,Transaminase,4. Transamination associated with purine nucleotide cycle in skeleton muscles and other tissues,(In kidney and liver cells),5. O

17、xidative deamination of AAs,NH3,O R-CH-COOH,deamination,-ketoacid,Be oxidized to produce CO2+H2O+ATP via citric acid cycle and oxidative phosphorylation. Be converted into non-essential amino acids by amination. Be converted into glucose, lipids or ketone bodies.,. Fates of the carbon atoms of amino

18、 acids after deamination:,glucogenic amino acids,ketogenic amino acids,ketogenic and glucogenic amino acids,glucose,Ketone bodies,Section Metabolism of Ammonia,Ammonia is extreme toxic compound to central nervous system. The major way for detoxification of ammonia is to be converted into water-solub

19、le, nontoxic urea in the liver, which is mainly passed via the bloodstream to the kidneys and excreted in the urine.,1. Deamination of amino acids and amine. 2. Large quantity of ammonia is produced in intestine. (1) from putrefaction of dietary protein (2) from hydrolysis of urea: blood urea diffus

20、e into intestine, degraded by bacterial urease. The ammonia is re-absorbed into the blood liver. NH4+ (ammonium ion) NH3 + H+ (ammonia) So alkaline soapsuds can NOT be used in clinical clyster for victim with hyperammonemia. 3. Reabsorbed from kidney in which ammonia is produced by hydrolysis of glu

21、tamine by glutaminase,. Sources of ammonia in the body:,glutamine glutamate + NH3,Gln,Gln,Glu,NH3,NH3,H+,NH4+,Glu,NH3,NH3,NH3,glutaminase,Blood,cell,urine,Acidification of urine,. Transportation of ammonia The ammonia need to be transported from the generated tissue to the liver in a nontoxic form.

22、1. Alanine-Glucose cycle: To transport ammonia in the nontoxic form of alanine from muscle to liver. To regulate blood glucose indirectly and supply available glucose for muscle.,COOH CH NH2 + NH3 (CH2) 2 COOH,Glu,CONH2 CH NH2 (CH2) 2 COOH,Gln,glutaminase,Gln synthetase,In brain or muscle,In liver o

23、r kidney,In liver, NH3 urea In kidney, NH3 + H+ NH4+ is excreted in the urine.,2. Transportation of ammonia with glutamine: Glutamine is a major transport form of ammonia.,ATP,ADP+Pi,正常细胞可合成天冬酰胺； 白血病细胞不能或很少能合成天冬酰胺，必须依靠血液从其他组织器官运输而来； 因此，临床上可用天冬酰胺酶治疗白血病。,Asp+Gln,Asn+Glu,Transportation of ammonia,Glu G

24、lutamine NH3 NH4+ urine,Kidney,reabsorbed,Most tissues,Liver,Muscle,Glutamate,Glutamate,Urea,Amino acid,NH4+,Glutamine synthetase,NH4+,NH4+,Glutaminase,Glutamine,Glutamine,Glu,KG,Glu,KG,pyruvate,Alanine,pyruvate,Alanine,Glucose-alamine cycle,Glucose,Glucose,. Metabolic fates of ammonia,Synthesis of

25、urea in liver Synthesis of nonessential amino acids and nucleotides Excretion from kidney as NH4+,. Synthesis of Urea,Liver is the major organ for urea synthesis Ornithine cycle (or urea cycle) Krebs and Henseleit 1932,Formed from 2 NH3 and CO2,The significance is to convert ammonia into urea that c

26、an be excreted from kidney and to detoxify ammonia,Process of urea cycle: Formation of Carbamoyl phosphate Formation of citrulline Formation of arginine Synthesis of urea The first two steps occur in Mit. Other steps occur in cytoplasm,urea cycle,Occurs in the matrix of mitochondria. Enzyme: carbamo

27、yl phosphate synthetase(CPS- I) CPS- I is a regulatory enzyme (N-acetylglutamate, AGA as a positive modulator). CPS-is distinct from the cytosolic CPS- form, which has a separate function in pyrimidine biosynthesis.,Formation of Carbamoyl phosphate,Carbamoyl phosphate synthetase I,ornithine transcar

28、bamoylase,TCA cycle,Summary 1. Material: 2NH3 (free NH3, Asp), CO2 2. Process: first in MIT then in cytosol 3. Energy: The synthesis of one molecule of urea requires four high-energy phosphate groups.,2NH3 + CO2 + 3ATP + H2O urea+2ADP+ AMP+4Pi,Arginine,(AGA),Regulation of CPS- I,The urea synthesis c

29、onverts toxic ammonia to nontoxic urea. When liver function is damaged, urea synthesis will be blocked, then the blood ammonia levels increase. Hyperammonemia and hepatic coma and death will occur.,Hyperammonemia and ammonia toxicosis,Hyperammonemia and ammonia toxicosis,Liver function Urea synthesi

30、s Blood ammonia Ammonia in brain TCA in brain Energy for brain Brain function ,Glutamate glutamine,NH3,NH3,NADH,ATP,In brain,Mechanism:,. Decarboxylation of amino acids,RCH2NH2 + CO2,R-CH-NH2 COOH,amine,Amino acid,Section Metabolism of Individual Amino Acids,decarboxylase,pyridoxal phosphate (PLP),H

31、ave important physiological functions,Amine oxidase,RCHO RCOOH,Glu -aminobutyric acid (GABA) : inhibitory neurotransmitter Cys taurine: a component of conjugated bile acid His histamine: a mediator of inflammatory and allergic reaction Trp 5-hydroxytryptamine (5-HT) inhibitory neurotransmitter and v

32、asoconstrictor Polyamine: ornithineputrescinespermidinespermine,Synthesis of histamine,It is secreted by mast cells as a result of allergic and inflammatory reactions. Histamine is a powerful vasodilator,serotonin （5-HT）,Serotonin also called 5-hydroxytryptamine. act as inhibitory neurotransmitter a

33、nd vasoconstrictor Regulation of sleep, temperature and blood pressure.,polyamines,Promoting the proliferation of cells,SAM,. Metabolism of one-carbon units,* One-carbon units are groups containing one-carbon atom produced by some amino acid metabolism. * Types: methyl(-CH3) methylene (-CH2-) methen

34、yl(-CH=) formyl (-CHO) formimino(-CH=NH) * Carrier(coenzyme): tetrahydrofolate (FH4) * Serve as donors of one-carbon units in the biosynthesis of purine and pyrimidine.,Role of folic acid in amino acid metabolism,One carbon unit can not exist freely, FH4 serves as carrier to transfer the one-carbon

35、units.,(DHFR).,Dihydrofolate,(FH4),Folic acid is a kind of water-soluble vitamin.,Formation of one carbon unit,Ser、 Gly，His，Trp,Ser hydroxymethyTransferase,Glycine lyase,formiminoglutamic acid,N10 CHO FH4 N5、N10 = CH FH4 N5 CH = NH FH4 N5、N10 CH2 FH4 N5 CH3 FH4,NADH+H NAD,NADPH+H NADP,NH3,Interconve

36、rtion of one carbon units,The different forms of FH4 are interconvertible. The formation of N5-methyl - FH4 is inreversible reaction.,Significances of one-carbon unit metabolism,As the precursors for purine and pyrimidine synthesis Links amino acid metabolism with nucleotide metabolism. Deficiency o

37、f folic acid or block of one-carbon unit metabolism can influence DNA and RNA synthesis. The victim may get megaloblastic anemia.,巨幼红细胞性贫血,1. Metabolism of methionine,(SAM),. Metabolism of sulfur-containing amino acids,* active methyl group, which can be transferred to a variety of acceptor molecule

38、s (50 sorts), Transmethylation,SAM,S-Adenosyl-homocysteine,R A variety of methyl transferases R-CH3,- Ethanolaminecholine - Capping of mRNA - Methylation of DNA Methyl modification of enzymes Synthesis of creatine, epinephrine, carnitine Methylation in biotransformation,S-Adenosylmethionine (SAM) is

39、 a powerful methylating involve in:,FH4,N5-CH3 FH4,Met,Methyl transferase/ methionine synthase (Vit B12),SAM,hydrolase,methyltransferase,homocysteine,S-adenosyl homocysteine, Methionine cycle,Met,SAM,SAH,Homo-Cys,FH4,N5,N10- CH2-FH4,N5-CH3 FH4,R-CH3 R,Gly Ser,ATP PPi +Pi,Met cycle,One-carbon metabol

40、ism,The importance of Met cycle is to renew Met from one-carbon units for its reuse. The reaction for generation of N5-CH3-FH4 is virtually irreversible. Methionine synthase is the only mammalian enzyme known to act on N5-CH3-FH4. Deficiency of Vit B12 may reduce the enzyme activity, lead to accumul

41、ation of N5-CH3-FH4, and decrease functional FH4 level, which is insufficient for synthesis of nucleotides. The victim with the deficiency of Vit B12 may get megaloblastic anemia.,Significance of Met cycle, Creatine synthesis,Creatine phospate is a high-energy compound that can reversibly donate a p

42、hosphate group to ADP to form ATP.,CPK Isozymes: MM type BB type MB type,In liver,glucose,Met is glucogenic amino acid,2. Metabolism of cysteine and cystine, Interconversion of cysteine and cystine 2 SH -S-S- Metabolism of sulfate ATP + SO42- 3-PO3H2 - AMP SO3- (PAPS) is called active sulfate, which

43、 serves as sulfate donor in the sulfurylation reaction (biotransformation in liver, aminoglucose).,-2H,+2H,Phe Phe hydroxylase Tyr Dopa,Epinephrine,Phenylpyruvate,Norepinephrine,melanin,Dopamine (in brain),Dopa,Acetoacetate + fumarate,ketone bodies glucose,. Metabolism of aromatic amino acids,1. Met

44、abolism of phenylalanine and tyrosine (Phe and Tyr),CO2,SAM,肾上腺髓质,transamination,tyrosinase,* Tyr hydroxylase,catecholamines,Tyrosine gives rise to a family of catecholamines that includes: dopamine, norepinephrine, epinephrine.,Phe,albinism,Albinism disease is due to a inborn deficiency in tyrosinase, so melanin can not be synthesized. Melanin is a pigment that occurs in many tissues particularly in the eye, hair, and skin cornea. The patients have light color in tissues and they are photosensitive.,Deficiency in Phe hydroxylase leads to PKU. Since pat