1、Amino Acid Oxidation and the Production of Urea,Catabolism of amino acids in mammals,Contents,Degradation of proteins Overview of catabolism of amino acids Deamination of amino acids Transportation of ammonia Metabolic fates of amino groups Nitrogen excretion and the urea cycle Decarboxylation of am

2、ino acids One carbon units,1. Degradation of proteins,Sources of amino acids undergoing oxidative degradation in animals Diet Released during protein turnover Cellular proteins during starvation or disease condition (such as diabetes ),The fraction of metabolic energy obtained from amino acids, whet

3、her they are derived form dietary or from tissue protein, varies greatly with the type of organism and with metabolic conditions.,Degradation of proteins Degradation of dietary proteins and absorption of amino acids Degradation of cellular proteins,Dietary protein is enzymatically degraded to amino

4、acids,Pepsin (胃蛋白酶) cleaves polypeptides into smaller peptides (N-terminal side of Tyr, Phe, Trp residues) in Stomach. Further degradation in intestine by Pancreatic enzymes.,Dietary protein,Gastrin（胃泌素）,HCl,Pepsin,Pepsinogen,42 amino acid residues are removed from the N-terminal end,Proteolytic enz

5、ymes (proteases) secreted by Pancreas,Trypsin 胰蛋白酶 (Lys, Arg) Chymotrypsin 糜蛋白酶 (Phe, Trp, Tyr) Elastase 彈性蛋白酶 (Val, Ala, Ser, Lys) Carboxypeptidase 羧基肽酶 Aminopeptidase 氨基肽酶,腸液中腸激酶的作用和小腸粘膜細胞的消化作用,腸激酶(Enterokinase) 對胰蛋白酶原的激活 胰腺最初分泌出來的各種蛋白酶和肽酶均以無活性的酶原形式存在，小腸分泌液促進了相關酶原的激活，從而激活蛋白質的消化途徑。 小腸粘膜細胞對蛋白質的消化作用

6、小腸粘膜細胞的刷狀緣及胞液中存在寡肽酶及二肽酶，徹底把寡肽水解成氨基酸,Trypsinogen,Enterokinase腸激酶,Trypsin,Chymotrypsin,Elastase,Carboxypeptidase A&B,Zymogens are converted into their active forms,Chymo-trypsinogen,Proelastase,Procarboxypeptidase A&B,Aminopeptidase exopeptidase dipeptidase endopeptidase,胰蛋白酶原,腸激酶,胰蛋白酶,糜蛋白酶原,糜蛋白酶,彈性蛋白

7、酶原,彈性蛋白酶,羧基肽酶原A及B,羧基肽酶A及B,Zymogens are converted into their active forms,外切酶氨肽酶,內切酶,限制性內切酶,外切酶羧肽酶,最終產物氨基酸,不同蛋白酶之間功能上的區別,1. 載體轉運：小腸，肌肉，腎小管細胞,載體蛋白,氨基酸,Na+,三聯體,氨基酸進入細胞,Absorption of Amino acids,2 -谷氨酰基循環：小腸，腎小管細胞，腦細胞,該循環分為兩個階段:,1. 谷胱甘肽對氨基酸的轉運 2. 谷胱甘肽的再合成 轉運一個氨基酸, 消耗 3分子ATP。 某些氨基酸, 如Pro, 不能通過此循環轉運。,Degr

8、adation of cellular proteins,There are two major intracellular devices in which damaged or unneeded proteins are broken down. lysosomes proteasomes,Lysosomes deal primarily with,Extracellular proteins, e.g., plasma proteins, that are taken into the cell, e.g., by endocytosis. Cell-surface membrane p

9、roteins that are used in receptor-mediated endocytosis.,Proteasomes deal primarily with endogenous proteins,Proteins synthesized within the cell such as transcription factors cyclins which must be destroyed to prepare for the next step in the cell cycle proteins encoded by viruses and other intracel

10、lular parasites proteins that are folded incorrectly,2. Overview of the catabolism of amino acids in mammals,Enzyme,NH3,-Keto acid,3. Deamination of amino acids,Transamination Oxidative deamination Transdeamination,Transamination （轉氨基作用）,在轉氨酶的催化下，某一氨基酸的-氨基轉移到另一種-酮酸的酮基上，生成相應的氨基酸，而原來的氨基酸則轉變成-酮酸，反應的實質是

11、氨基在-氨基酸和酮酸的轉移。 the process of transfering an amino group from one compound to another,Transamination,Pyridoxal phosphate(PLP) participates in the transfer of -amino groups to -keto acids,PLP is bound to the enzyme through a Schiff-base linkage to a Lys residue at the active site,Mechanism of transam

12、ination,Transaminase (轉氨酶) or aminotransferase (氨基轉移酶),作用：催化氨基酸和-酮酸間進行氨基 和酮基的互換。 體內存在著多種轉氨酶，催化不同AA與-酮酸的轉氨基作用，其中以催化L- 谷氨酸-酮酸轉氨基反應的轉氨酶 (谷丙轉氨酶GPT和谷草轉氨酶GOT) 最為重要。 輔酶: VB6的磷酸酯-磷酸吡哆醛 (Pyridoxal phosphate, PLP),作用是傳遞氨基。 生理意義：所催化的反應完全可逆，平衡常數近于1。既是氨基酸的分解代謝過程，也是體內某些非必需AA合成的重要途徑。,谷丙轉氨酶 (glutamate-pyruvate tran

13、saminase, GPT ) 谷草轉氨酶 (glutamate-oxaloacetate transaminase, GOT）,Glu,Pyruvate,-Ketoglutarate,GPT,Ala,GPT和GOT廣泛分布于各組織細胞內，在不同組織中含量不同,查肝功為什么要抽血化驗轉氨酶指數呢？,轉氨酶為細胞內酶，血清中活性很低，各組織器官中以心和肝的活性最高。當某種原因使細胞膜通過性增高，轉氨酶可大量釋放入血，導致血清中轉氨酶活性增高。 抽血化驗若轉氨酶比正常水平偏高則有可能肝組織受損破壞，肝細胞的轉氨酶進入血液。（結合乙肝抗原等指標進一步確定是什么原因引起的） 常作為疾病診斷、觀察療效和

14、預后的指標： 急性肝炎：S-GPT、S-GOT 心肌梗塞：S-GOT,Oxidative deamination (氧化脫氨基作用）,酶L-氨基酸氧化酶、D-氨基酸氧化酶,脫氫 亞氨基酸不穩定,H2O+H+,水化,NH4+,Glutamate undergoes oxidative deamination L-Glu氧化脫氨基作用,L-谷氨酸脫氫酶,L-谷氨酸脫氫酶的特點,催化L-谷氨酸氧化脫氨生成-酮戊二酸、NH3和NADH+H+ 。 以NAD+或NADP+為輔酶的不需氧脫氫酶。 線粒體。 分布于肝、腎、腦等組織，酶活性較強。 該酶是能使氨基酸直接脫去氨基的主要酶，其與轉氨酶協同作用是體內脫

15、氨基的主要方式。 為變構酶 GTP和ATP為變構抑制劑 GDP和ADP為變構激活劑,Transdeamination (聯合脫氨基作用),The combined action of an aminotransferase and glutamate dehydrogenase,氨基酸在轉氨酶作用下，將-氨基轉給-酮戊二酸分子生成-酮酸和谷氨酸，谷氨酸再經過L-谷氨酸脫氫酶的作用，脫去氨基產生游離氨并生成-酮戊二酸，這種轉氨酶和L-谷氨酸脫氫酶的協同作用稱聯合脫氨基作用。,聯合脫氨基作用轉氨基作用與氧化脫氨基作用的聯合,意義：體內氨基酸脫氨基的最重要方式 體內合成非必需氨基酸的主要途徑,谷氨酸

16、脫氫酶為中心的聯合脫氨,-KG,谷氨酸,轉氨酶,谷氨酸脫氫酶,谷氨酸脫氫酶為中心的聯合脫氨反應,谷氨酸,L-谷氨酸脫氫酶,-酮戊二酸,轉氨酶,NH4+,-氨基酸,NAD+H2O,-酮酸,NH3,2H,L-谷氨酸脫氫酶及谷-某轉氨酶的活性強、分布廣，是動物體內大部分氨基酸脫氨的方式,NADH+H+,谷氨酸脫氫酶為中心的聯合脫氨,概念：由轉氨酶和腺苷酸脫氨酶等多種酶聯合作用下脫去氨基產生游離氨的過程。 關鍵酶的分布特點：肌肉中L-谷氨酸脫氫酶活性低，而腺苷酸脫氨酶活性高。 意義：是心肌、骨骼肌和腦組織脫氨基的主要方式，實驗證明腦組織細胞中的氨有50%是由該循環產生的。,嘌呤核苷酸循環 (Purin

17、e nucleotide cycle)脫氨基作用,腺苷酸琥珀酸,草酰乙酸,嘌呤核苷酸循環聯合脫氨基,NH3,天冬氨酸,次黃苷酸,H2O,NH3,H2O,蘋果酸,谷-草轉氨酶,若外環境NH3大量進入細胞，或細胞內NH3大量積累-氨中毒,某些敏感器官（如神經、大腦）功能障礙。 表現：語言障礙、視力模糊、昏迷、死亡。,所以，氨的及時轉運和排泄非常必要,4. Transportation of ammonia,Glutamine transports ammonia in the bloodstream Alanine transports ammonia from muscles to the li

18、ver,Glutamine transports ammonia in the blood stream,H2O,NH3,谷氨酰胺,谷氨酸,glutaminase,谷氨酰胺的運氨作用,Gln是氨的一種轉運形式，它主要從腦、肌肉等組織向肝或腎運送氨。 Gln是大腦等組織解氨毒和運輸氨的重要形式。 Gln在肝中釋放 NH3 用于合成尿素（主）。 Gln在腎小管分解產生的NH3與H+結合成NH4+ ，中和固定酸。,Alanine transports ammonia from muscles to the liver,Glucose-alanine cycle 葡萄糖-丙氨酸循環,(主要是肌肉),各

19、組織細胞,脫氨,NH3,谷氨酸,-酮戊二酸,谷氨酸,丙酮酸,丙氨酸,谷氨酰胺,血液,肝臟,脫氨，轉化為排泄形式,氨基氮轉運的一般途徑,5. Metabolic fates of amino groups,Amino group catabolism,水生生物直接擴散脫氨（NH3）,哺乳、兩棲動物排尿素,直接排氨，毒性大，不消耗能量； 轉化后排氨，形式越復雜，越安全，但越耗能。,體內水循環迅速，NH3濃度低，擴散流失快，毒性小。,體內水循環較慢，NH3濃度較高，需要消耗能量使其轉化為較簡單，低毒的尿素形式。,氨基氮的排泄 各種生物根據安全、價廉的原則排氨,鳥類、爬蟲排尿酸,不溶于水，毒性很小，合

20、成需要更多的能量。,為什么這類生物如此排氨？,水循環太慢，保留水分同時不中毒得付出高能量代價。,高等植物，以谷氨酰胺或天冬酰胺形式儲存氨，不排氨。,6. Nitrogen excretion and the urea cycle,Urea Biosynthesis (尿素的生成),實驗： 動物切除肝臟，輸入氨基酸后，血氨濃度升高； 動物保留肝臟、切除腎臟，輸入氨基酸后，血中尿素濃度升高； 動物肝臟、腎臟同時切除，輸入氨基酸后，血中尿素含量較低，但血氨濃度升高； 結論：肝臟是合成尿素的主要器官,Urea cycle or ornithine cycle (鳥氨酸循環) was discovere

21、d in 1932 by Hans Krebs and Kurt Henseleit,Urea cycle and reactions that feed amino groups into the cycle,Urea is produced from Ammonia in five enzymatic steps,First, a carbamoyl phosphate (氨基甲酰磷酸)，an activated carbamoyl group donor, is produced in the mitochondrial matrix with the catalysis of carb

22、amoyl phosphate synthetase I (氨基甲酰磷酸合成酶 I, CPS-).,Production of carbamoyl phosphate (氨基甲酰磷酸的合成),氨基甲酰磷酸的合成需要兩個活化步驟,消耗2分子的ATP,氨基甲酰磷酸 (Carbamol phosphate),CPS-,AGA,Production of carbamoyl phosphate,(N-acetyl glutamatic acid, AGA),CPS-I is allosterically activated by AGA,氨基甲酰磷酸合成酶 I 的特點,細胞定位：肝細胞線粒體 催化底物

23、：NH3+CO2+2ATP+H2O 產物：氨基甲酰磷酸(氨基甲酰供體) 作用：氨基甲酰供體，為進一步合成尿素而解氨毒奠定基礎 調節：N-乙酰谷氨酸（AGA）為變構激活劑 意義：其活性可作為肝細胞分化程度的指標,The urea cycle begins in the mitochondrial matrix when a carbamoyl group is transferred to ornithine (鳥氨酸) to form citrulline (瓜氨酸), in a reaction catalyzed by ornithine transcarbamoylase(鳥氨酸氨基甲酰

24、轉移酶).,Formation of citrulline(瓜氨酸的合成),(Carbamol phosphate),(Ornithine),(Citrulline),Formation of citrulline,The citrulline passes mitochondrion to the cytosol. Formation of argininosuccinate by an ATP-dependent condensation reaction between the amino group of Asp and the ureido (carbonyl 羰基 ) group

25、of citrulline, catalyzed by argininosuccinate synthetase (精氨酸代琥珀酸合酶). The argininosuccinate(精氨酸代琥珀酸) is then cleaved to form Arg and fumarate(延胡索酸), in a reaction catalyzed by argininosuccinate lyase.,Formation of arginine (精氨酸的合成),Formation of arginine,(Citrulline),(Asp),(Argininosuccinate),(Argini

26、nosuccinate),(Arginine),(Fumarate),arginase cleaves Arg to form urea and ornithine. Ornithine is transported into the mitochondrion to initiate another round of the urea cycle.,Arginine is cleaved to yield urea (精氨酸水解生成尿素),Formation of urea,(Arginine),(Urea),(Ornithine),Ornithine cycle,尿素循環,部位肝臟細胞,氨

27、基酸,（外來的或自身的）,-酮戊二酸 （轉氨作用）,谷氨酸,谷氨酸,酮戊二酸,NH4+,CO2,2ADP+Pi+H+,2ATP,Pi,鳥氨酸,瓜氨酸,氨甲酰磷酸,Pi,瓜氨酸,轉氨基氨,精氨琥珀酸,ATP,AMP+PPi,延胡索酸,鳥氨酸,精氨酸,H2O,尿素,消耗4個高能鍵,天冬氨酸,Urea cycle and reactions that feed amino groups into the cycle,合成尿素是體內氨的主要去路（尿素是AA代謝 的主要終產物） 尿素分子中的2個氮原子，1個來自氨，另一個則來自天冬氨酸；C來自CO2 反應部位：肝細胞的線粒體和胞液 合成1分子尿素需要消耗

28、4個高能磷酸鍵(不考慮脫氫反應） 意義：解氨毒-把有毒的NH3轉變成無毒的尿素 重要的酶：精氨酸代琥珀酸合成酶（限速酶, 氨基甲酰磷酸合成酶I（CPS-I）,鳥氨酸循環的小結,Links between the urea cycle and citric acid cycle,Hyperammonemia(高氨血癥),血 氨,氨基酸,脫氨,腸道,吸收,腎小管,分泌,合成,尿素,合成,合成氨基酸等 含氮化合物,銨鹽,生成,排出,合成,谷氨酰胺,高氨血癥：肝功能嚴重損傷時，尿素合成發生障礙， 血氨濃度升高，稱為高氨血癥 高氨血癥引起肝性腦病的生化機理： 肝功能嚴重受損尿素合成障礙高血氨癥 氨進入腦

29、組織 合成Glu、Gln酸性(直接傷腦) -酮戊二酸TAC循環 腦組織ATP生成大腦功能紊亂肝性腦病,肝昏迷與氨中毒,生成 non-essential amino acids 轉變成 Carbohydrate及Lipids 生糖氨基酸(glucogenic amino acids)、生酮氨基酸(ketogenic amino acids) 、生糖兼生酮氨基酸(glucogenic and ketogenic amino acids) 氧化供能：進入三羧酸循環徹底氧化分解,6. Fate of carbon skeletons (- keto acids)氨基酸碳骨架的代謝,The carbon

30、 skeletons of the amino acids converted into seven major metabolic intermediates,Acetoacetyl-CoA Acetyl-CoA Pyruvate a-ketoglutarate, Succinyl-CoA, Fumarate, Oxaloacetate,Summary of the points of entry of the standard amino acids into the citric acid cycle,碳骨架的氧化（肝臟中）,乙酰乙酰CoA,苯丙氨酸 酪氨酸 亮氨酸 賴氨酸 色氨酸,丙氨

31、酸 蘇氨酸 甘氨酸 絲氨酸 半胱氨酸,丙酮酸,精氨酸 組氨酸 谷氨酰胺 脯氨酸,谷氨酸,異亮氨酸 甲硫氨酸 纈氨酸,苯丙氨酸 酪氨酸,天冬酰胺 谷氨酰胺,Some amino acids are converted to intermediates of citric acid cycle by simple removal of the amino groups,Ala to pyruvate Asp to oxaloacetate (Asn is converted to Asp catalyzed by asparaginase ,門冬酰胺酶 ) Glu to a-ketoglutara

32、te (Gln is converted to Glu catalyzed by glutaminase,谷氨酰胺酶 ),Some amino acids are converted to intermediates of citric acid cycle through complicated pathways,Ten amino acids are degraded to acetyl-CoA Five of the ten are degraded to acetyl-CoA via pyruvate (Trp, Ala, Ser, Gly, Cys, and Thr in some

33、organisms). Portions of the carbon skeletons of six amino acids Trp, Phe, Try, Lys, Leu,and Ile are converted directly to acetyl-CoA and/or to acetoacetyl-CoA.,Trp, Ala, Ser, Gly, and Cys and Thr are converted to pyruvate before being converted to acetyl-CoA.,Gly has two pathways,Bacteria,Animals,Tr

34、p, Phe, Tyr, Lys, Leu, and Ile are also converted to acetyl-CoA,Tryptophan as precursor,Phe and Tyr catabolism and diseases,酪氨酸血癥,尿黑酸,Phe and Tyr are also degraded to fumarate,尿黑酸癥,Alternative pathways for catabolism of Phe in phenylketonuria,Five amino acids are converted to a-ketoglutarate,Four am

35、ino acids are converted to Succinyl-CoA,Branched-chain amino acids are not degraded in the liver (extrahepatic tissues: muscle, adipose, kidney, and brain),Asp and Asn are degraded to oxaloacetate,Summary of the glucogenic and ketogenic amino acids,在氨基酸脫羧酶的催化，體內部分氨基酸可進行脫羧基作用生成相應的胺。 催化酶：氨基酶脫羧酶（輔酶為磷酸吡

36、哆醛，PLP） 意義：生成的胺類物質常具有重要的生理功用或藥理作用 *胺氧化酶能將胺類物質氧化成醛類或酸類物質，從而避免胺類在體內蓄積。,7. Decarboxylation of amino acids (氨基酸的脫羧基作用),8. One carbon units (OCU),概念：含一個碳原子的基團 種類：甲基 (methyl)、甲烯基 (methylene、甲炔基(methenyl)、甲酰基 (formyl)及亞氨甲基 (formimino) 一碳單位主要來源于絲氨酸、甘氨酸、組氨酸及色氨酸的代謝。,Several enzyme cofactors play important roles in one-carbon transfer reactions,Some enzyme cofactors important in one-car