Amino Acids Metabolism Chapter 9 Amino Acids Metabolism The biochemistry and molecule biology department of Henan univercity

Main Contents §1.Nutritional Function of Proteins §2. Digestion, Absorption and Putrefaction §3. General Metabolism of Amino Acid Dynamics of amino acid metabolism Deamination ammonia Metabolism Metabolism of -keto acid §4. Metabolism of Specific Amino Acid Decarboxylation Metabolism of one carbon unit Metabolism of sulfur-containing AAs Metabolism of aromatic AAs Metabolism of branched-chain AAs

§1. Nutritional Function of Proteins

The chemical composition of human body 16-20% lipid protein carbohydrate RNA inorganic DNA other water 50-60%

…… 皮肤 Collagen Muscle original protein hemoglobin immune globulin Fibrinogen antithrombin 细胞外蛋白质，它是由3条肽链拧成螺旋形的纤维状蛋白质，胶原蛋白是人体内含量最丰富的蛋白质，全身总蛋白质的30%以上。人体皮肤成分中，有70%是由胶原蛋白所组成。真皮层的胶原蛋白（下图黄色部分）被氧化、断裂后，对表皮的支撑作用就消失了，因此造成不均一的塌陷，这样皱纹就产生了。 运动供能 从最低等的细菌鞭毛运动到高等动物的肌肉收缩都是通过蛋白质实现的。肌肉的松弛与收缩主要是由以肌球蛋白为主要成分的粗丝以及以肌动蛋白为主要成分的细丝相互滑动来完成的。运输功能  在生命活动过程中，许多小分子及离子的运输是由各种专一的蛋白质来完成的。 机械支持和保护功能  高等动物的具有机械支持功能的组织如骨、结缔组织以及具有覆盖保护功能的毛发、皮肤、指甲等组织主要是由胶原、角蛋白、弹性蛋白等组成。 免疫和防御功能  生物体为了维持自身的生存，拥有多种类型的防御手段， 其中不少是靠蛋白质来执行的 。 例如抗体即是一类高度专一的蛋白质 ，  它能识别和结合侵入生物体的外来物质，如异体蛋白质、病毒和细菌等，取消其有害作用。 调节功能  在维持生物体正常的生命活动中，代谢机能的调节，生长发育和分化的控制，生殖机能的调节以及物种的延续等各种过程中，多肽和蛋白质激素起着极为重要的作用。此外，尚有接受和传递调节信息的蛋白质，如各种激素的受体蛋白等。 激素、供能   发展 蛋白质作为生命活动中起重要作用的生物大分子，与一切揭开生命奥秘的重大研究课题都有密切的关系。蛋白质是人类和其他动物的主要食物成分，高蛋白膳食是人民生活水平提高的重要标志之一。许多纯的蛋白质制剂也是有效的药物，例如胰岛素、人丙种球蛋白和一些酶制剂等。在临床检验方面，测定有关酶的活力和某些蛋白质的变化可以作为一些疾病临床诊断的指标，例如乳酸脱氢酶同工酶的鉴定可以用作心肌梗塞的指标，甲胎蛋白的升高可以作为早期肝癌病变的指标等。在工业生产上，某些蛋白质是食品工业及轻工业的重要原料，如羊毛和蚕丝都是蛋白质，皮革是经过处理的胶原蛋白。在制革、制药、缫丝等工业部门应用各种酶制剂后 ，可以提高生产效率和产品质量    。蛋白质在农业、畜牧业、水产养殖业方面的重要性，也是显而易见的。 在人体内不同组织器官的细胞内通过遗传基因的指令合成的不下十万余种蛋白质是生命的万能素材，蛋白质不仅构成我们身体，也支配我们的生命现象，包括影响我们对问题的思考，对过去的回忆及对未来的预想等精神活动。可以说，生命的存在与意义，感知与认识，思维与情感，物质与精神等内涵都将随之蛋白质的解明而可能逐渐被揭示。 enzyme Hormone、 receptor  ……

1.The significance of proteins Keep the cells and tissues growing, renewing and mending 2. Take part in some kinds of important physiological activities 3. Oxidation and supply energy

2、 Nitrogen balance Measuring the amount of intake and excretion of total nitrogen can help us to know the general situation of protein metabolism. 1) Normal nitrogen balance ：intake N = losses N 2) Positive nitrogen balance：intake N > losses N 3) Negative nitrogen balance ：intake N < losses N

Physiological requirements of proteins Lowest requirement: 30~50g/day Recommend requirement: 80g/day (65kg man)

3、Nutritional Value of Proteins 1、 Essential amino acids: Amino acids that cannot be synthesized by the body and must be obtained from the diet. Arginine and Histidine are essential only during periods of rapid growth (infancy and recovery illness)

4、complementary action of proteins Two or more plant proteins are consumed together which complement each other in essential amino acid content.

Digestion, Absorption and Putrefaction of Proteins Section 2 Digestion, Absorption and Putrefaction of Proteins

1、protein digestion （1） protein digestion in the stomach： pepsinogen gastric acid 、pepsin pepsin +  polypeptide

（2） protein digestion in the small intestine 1. Proteolytic enzymes of pancreatic juice trypsin: Arg, Lys (C) chymotrypsin: Tyr, Trp, Phe, Met, Leu (C) elastase: Ala, Gly, Ser (C) endopeptidase Exopeptidase：carboxypeptidase aminopeptidase

enterokinase trypsinogen trypsin chymotrypsinogen chymotrypsin proelastase elastase procarboxypeptidase carboxypeptidase

2、Amino acid absorption absorption site ：Mainly in the small intestine Absorption form ：Amino acid 、oligopeptide 、dipeptide absorption mechanism ：Active absorption process of energy dissipation

types a. Amino acid absorption carrier Neutral amino acids carrier Basic amino acid carrier Acidic amino acid carrier  imino acid and glycine carrier types

b.γ-glutamyl cycle amino acids transport L-glutathione synthesis

γ-Pancreatic acyl enzyme γ-glutamine Amino acids  intracellular extracellular γ-pancreatic Acid cyclization transferase amino acid 5-Oxygen proline Cys-Gly γ-Pancreatic acyl enzyme cysteine glycine peptidase Glu 5 - oxygen proline enzyme ATP ADP+Pi GSH γ-Glutamine cysteine γ-Glutamine cysteine synthase ADP+Pi ATP glutathione synthetase ATP ADP+Pi

c. The absorption of peptide Dipeptide carrier or tripeptide carrier Active transport 、energy consumption

3、 Putrefaction of proteins Concept: Some undigested proteins and no absorbed products are anaerobic decomposed by the bacteria in intestine. The products are toxic to body except few vitamin and fatty acid.

A. amines

false neurotransmitter phenylethylamine phenylethanolamine  tyramine β-hydroxy tyramine

NH3 NH4+ B. Production of ammonia (NH3) ammonia Metabolism on unabsorbed amino acids ammonia Urea hydrolyzed by urease NH3 NH4+

C. Production of Other pernicious compound Tyr → phenol Trp → indole Cys → hydrogen sulfide (H2S)

General Metabolism of Amino Acids Section 3 General Metabolism of Amino Acids Dynamics of amino acid metabolism Deamination ammonia Metabolism Metabolism of -keto acid

1、 Degradation of protein in cells ① Lysosomal pathway Extracellular proteins, membrane-associated proteins and long-lived proteins ATP-independent process Cathepsins ② Ubiquitin-mediated protein degradation Abnormal proteins, damaged proteins and short- lived proteins ATP and ubiquitin Proteasome

ubiquitin 1. ubiquitination 2. proteasome

19S RP: 18 subunits, 6 subunits have ATP synthase activity proteasome 26S proteasome 20S CP 19S RP: 18 subunits, 6 subunits have ATP synthase activity 2αloops ：7αsubunits 2βloops ：7βsubunits

Ubiquitin-mediated protein degradation 1. The E1 enzyme activates the ubiquitin molecule. This reaction requires energy in the form of ATP. 2. The ubiquitin molecule is transferred to a different enzyme, E2. 3. The E3 enzyme can recognise the protein target which is to be destroyed. The E2-ubiquitin complex binds so near to the protein target that the actual ubiquitin label can be transferred from E2 to the target. 4. The E3 enzyme now releases the ubiquitin-labelled protein. 5. This last step is repeated until the protein has a short chain of ubiquitin molecules attached to itself. 6. This ubiquitin chain is recognised in the opening of the proteasome. The ubiquitin label is disconnected and the protein is admitted and chopped into small pieces.

3. Dynamics of amino acid metabolism

4、 Deamination of AAs Four types: transamination oxidative deamination union deamination non-oxidative deamination

A. Transamination 1、 Transamination is the process by which an amino group, usually from glutamate, is transferred to an α-keto acid, with formation of the corresponding amino acid plus α-ketoglutarate. reversible Aminotransferases utilize a coenzyme - pyridoxal phosphate - which is derived from vitamin B6. Lys and Pro cannot be transaminated

1. glutamic pyruvic transaminase, GPT alanine transaminase, ALT

2. glutamic oxaloacetate transaminase, GOT aspartate transaminase, AST

Pyridoxamine : R= -CH2NH2 Pyridoxol : R= -CH2OH Vitamin B6 Pyridoxamine : R= -CH2NH2 Pyridoxol : R= -CH2OH Pyridoxal : R= -CHO Amino acid Pyridoxal-P α-Keto acid Pyridoxamine-P Glutamate α-Ketoglutarate transaminase

The mechanism of transamination reaction

All transferases contain pyridoxal phosphate (PLP), the coenzyme form of vitamin B6, as a prosthetic group. PLP is bound to the active site of the enzyme by electrostatic interactions and by an Schiff base (aldimine) bound with a Lys side chain of the apoprotein. PLP participates directly in the reaction

B.L-glutamic acid Oxidative deamination NH3 α-ketoglutarate NAD(P)+ NAD(P)H+H+ H2O In the mitochondrial matrix（liver、 brain 、kidney） coenzyme ：NAD+ or NADP+ inhibitor ：GTP、ATP activator ：GDP、ADP

C. Union deamination The α- amino group of most amino acids is transferred to α- ketoglutarate to form an α- keto acid and glutamate by transaminase. Glutamate is then oxidatively deaminated to yield ammonia and α- ketoglutarate by glutamate dehydrogenase. 首先，氨基酸与－酮戊二酸在转氨酶催化下，生成相应的α－酮酸和谷氨酸 然后谷氨酸又在L－谷氨酸脱氢酶作用下，脱去氨基又转变为α－酮戊二酸，并释放出NH3 这种联合脱氨基作用是可逆的过程，因此，这一过程又是体内合成氨基酸的主要途径。 但因必需氨基酸相应的α－酮酸在体内不能合成，故失去合成必需氨基酸的能力。

(3) Purine nucleotide cycle (in muscle)

Combined action of an Aminotransferase and purine nucleotide cycle in muscle

D. L-amino acid oxidase α- amino acids O2+FMNH2 L-amino acid oxidase α-keto acid NH4+H2O2

D. Metabolism of -keto acid a. Formation of non- essential AAs b. Formation of glucose or lipids types amino acids Glucogenic AAs others Glucogenic and ketogenic AAs Ile, Phe, Tyr, Trp, Thr Ketogenic AAs Leu, Lys c. Provide energy

Amino acids of converted into ketone bodies or fatty acids are termed ketogenic amino acids. Amino acids of converted into glucose are termed glucogenic amino acids. Amino acids of converted into both glucose and ketone bodies are termed glucogenic and ketogenic amino acids.

Fig.

catabolites of amino acid -Ketoglutarate Succinyl CoA Intermediates of TAC Fumarate Oxaloacetate PEP Glucose Pyruvate Acetyl CoA Fatty acid Acetoacetyl CoA Ketone bodies

Section 4 Metabolism of Ammonia

Source and outlet of ammonia Transportation of NH3 Formation of urea Hyperammonemia and ammonia poisoning

A、 Source and outlet of ammonia (NH3) 1. Sources: ① Deamination of AAs--main source Catabolism of other nitrogen containing compounds RCH2NH2 RCHO + NH3 Amine oxidase ② Putrefaction in the intestine Degradation of urea in the intestine ③ Kidney secretion (Gln)

2. Outlets: Formation of urea Formation of Gln Excrete in urine Synthesis of AA

二、 Transportation of NH3 Alanine-glucose cycle Transportation of ammonia by Gln

1. Alanine-glucose cycle

2. Transportation of ammonia by Gln

(mitochondria and cytosol) Site: liver (mitochondria and cytosol) Dog (blood test) Ammonia urea Cut the liver Cut the kidney Cut the liver 、 kidney

Mice liver biopsy＋ammonium salt ammonium salt ↓、 urea ↑ ⊕ Ornithine 、 Citrulline 、 Arginine NH3 Liver NH2 CO NH2 isotope labelling ： 15NH4Cl、NaH14CO3 H215N 14C 15NH2 O

Urea synthesis theory of orinithine cycle orinithine cycle or urea cycle or Krebs-Henseleit cycle This cycle was the first metabolic cycle discovered by(Hans Krebs and Kurt Henseleit in 1932).

(CH2)3 (CH2)3 (CH2)3 NH2 NH2 CO NH NH2 C NH NH The urea cycle NH3 NH2 H2N-CH COOH NH2 CO NH Ornithine Urea H2O (CH2)3 H2N-CH COOH Arginase NH2 C NH NH Citrulline H2O (CH2)3 H2N-CH COOH NH3 H2O Arginine

N-acetyl glutamate (AGA) Process ----- ornithine cycle 1、 Formation of Carbamoyl Phosphate In mitochondria CO2 + NH3 + H2O + 2ATP carbamoyl phosphate synthetaseⅠ (CPS-Ⅰ) (N-acetyl glutamate ，Mg2+) C O H2N O ~ PO32- + 2ADP + Pi Carbamoyl Phosphate N-acetyl glutamate (AGA) N-acetyl glutamate is an activator of the reaction.

Carbamoyl phosphate synthetase Ⅰ (CPSⅠ) is an allosteric enzyme and is absolutely dependent up on N-acetylglutamic acid (AGA) for its activity. consume twoATP AGA

2、Formation of citrulline (in mitochondria) OCT: ornithine carbamoyl transferase

Argininosuccinate Synthetase 3、Formation of Arginine The reactions occur in the cytosol NH CH COOH 2 C O (CH ) 3 Argininosuccinate Synthetase ATP AMP+PPi H2O Mg2+ + Citrulline Aspartate Argininosuccinate

argininosuccinate lyase Fumaric acid Argininosuccinate arginine

4. Formation of urea (in cytosol)

ornithine cycle urea CO2 + NH3 + H2O Pi α-keto acid Corydalis tuber 2ADP+Pi CO2 + NH3 + H2O carbamoyl phosphate 2ATP AGA ornithine cycle mitochondria Pi ornithine citrulline Argininosuccinate citrulline asparagus fern ATP AMP + PPi amino acid Argininosuccinate Malic acin α-ketoglutarate Glu α-keto acid ornithine urea ornithine Corydalis tuber cytosol

C. Summary of urea synthesis One nitrogen of urea molecule comes from ammonia, another nitrogen comes from Asp liver (mitochondria and cytosol) Synthesis of a urea will consume 4 ~P. Rate limiting enzyme: ASS

D.The regulation of Urea synthesis A high-protein diet 1. Dietary protein Low protein diet 2. CPS-Ⅰregulation：AGA ornithine activator 3. enzyme regulation ：

Relative activity of Urea synthetase system 4.5 163.0 1.0 3.3 149.0 enzyme Relative activity Carbamoyl phosphate synthetase Ⅰ ornithine carbamoyl transferase argininosuccinate synthetase argininosuccinate lyase arginase

hyperammonemia and ammonia ammonia poisoning

(in brain) α-ketoglutarate ↓ The possible mechanism of ammonia poisoning α-ketoglutarate Glutamic acid glutamine NH3 (in brain) α-ketoglutarate ↓ TAC ↓ ATP↓

§5. Metabolism of Individual Amino Acids

Metabolism of Specific Amino Acids Section 5 Metabolism of Specific Amino Acids Decarboxylation of amino acids Metabolism of one carbon unit Metabolism of sulfur-containing AAs Metabolism of aromatic AAs Metabolism of branched-chain AAs

§ 5.1 Decarboxylation of amino acids

1.γ-aminobutyric acid (GABA) coenzyme :Vit B6

2.Histamine

3. Trp→5-hydroxytryptamine (5-HT)  function ： Inhibitory neurotransmitter serotonin

aminopropyl adenosine 4.polyamines

Polyamines and SAM adenosine putrescine spermidine aminopropyl spermine

§ 5.2 Metabolism of one carbon unit 1.One carbon unit (Definition) One carbon units (or groups) are one carbon-containing groups produced in catabolism of some amino acids.

kinds -CH3 methyl methylene -CH2- methenyl -CH= formyl -CHO formimino -CH=NH One carbon units are carried by FH4

F FH2 FH4 NADPH+H+ NADP+ Tetrahydrofolic acid (FH4) The N5 and N10 of FH4 participate in the transfer of one carbon units. F FH2 FH4 dihydrofolate reductase NADPH+H+ NADP+

One-carbonunits are usually combined to the N5, N10 of FH4 N5—CH3—FH4 N5、N10—CH2—FH4 N5、N10=CH—FH4 N10—CHO—FH4 N5—CH=NH—FH4

2. One carbon unit exchange N10—CHO—FH4 purine (C2) H+ H2O NH3 N5, N10=CH—FH4 N5—CH=NH—FH4 purine (C8) NADPH+H+ NADP+ N5, N10—CH2—FH4 thymine (-CH3) NADH+H+ NAD+ methylation (-CH3) N5—CH3—FH4

3. Formation of one carbon unit (1) Ser→N5,N10-CH2-FH4

(2) Gly→N5,N10-CH2-FH4

(3) His →N5-CH=NHFH4

(4) Trp→N10-CHOFH4

4. Significance of one carbon unit Substance for synthesis of nucleic acid. SAM－ methylation

PABA+Poison organism +Glu （TMP） PABA+Poison organism +Glu Folic acid × FH4 methotrexate Folic acid synthetase sulfanilamide FH2 reducing ferment

§ 5.3 Metabolism of sulfur-containing AAs cysteine cystine Methionine

1. cysteine and cystine -2H +2H CH2SH CHNH2 COOH CH2 S 2

SAM: active methionine Methionine and one carbon unit Adenosyl transferase PPi+Pi + Methionine ATP SAM SAM: active methionine Adenyl active methyl group

S-adenosyl-L-homocysteine transmethylase RH RH—CH3 adenosyl SAM S-adenosyl-L-homocysteine  HCY(homocysteine) adenine

Methionine metabolism

methionine cycle (VitB12) ATP PPi+Pi adenosine H2O -CH3 Methionine SAM S-adenosyl-L-homocysteine homocysteine (VitB12) H2O adenosine RH ATP -CH3 PPi+Pi SAM

Significance (1) SAM is the direct donor of methyl in body. Methylation can synthesize many important materials such as: choline, creatine, etc. (2) N5-CH3FH4 is the indirect donor of methyl in the body. (3) The free folic acid or VitB12 decrease will cause the decrease of DNA, which will lead to anemia.

Formation of creatine creatine 、creatine phosphate 、creatinine Arg NH 2 H N C N CH 3 SAM Gly CH 2 COOH Formation of creatine

amidinotransferase  + L-arginine glycine SAM CH3 ATP H2O

4. Cysteine Taurine

Formation of PAPS

PAPS is the active sulfate group for addition to biomolecules

γ-Glutamine cysteine synthase 6. GSH ATP ADP+Pi glutamic acid + cysteine γ-Glutamine cysteine γ-Glutamine cysteine synthase ADP+Pi glycine +ATP GSH glutathione synthetase + 2H – 2H 2GSH GSSG

§ 5. 4 Metabolism of aromatic amino acids PHE Tyr Try

1. Phe, Tyr, Trp

Phe hydroxylase ↓→phenyl pyruvate in the body ↑ → phenylketonuria(PKU) → toxicity of central nervous system →developmental block of intelligence of children Treatment: control the input of Phe

Melanin Tryosinase Highly colored Melanin polymeric (Black polymer) dopa Tryosinase Highly colored polymeric intermediates Melanin (Black polymer) Melanin formed in skin (melanocytes), eyes, and hair In skin, protects against sunlight Albinism: genetic deficiency of tyrosinase Dopamine quinone

The hydroxy benzene pyruvic acid Phenylalanine transamination phenyl pyruvate ↑ phenylacetate Phenylketonuria hydroxylase (-) Tyrosine tyrosine hydroxylase Dopa decarboxylase DOPA dopamine tyrosinase (-) Catecholamine  norepinephrine DOPA  Epinephrine transamination DOPA quinone Albinism Indole-5,6 quinones Polymerize Melanin ↓ The hydroxy benzene pyruvic acid Homogentisate oxidase  Fumaric acid and acetoacetic acid homogentisic acid ↑ (-) Alkaptonuria

2.Metabolism of Tryptophan 5-Hydroxytryptamine One carbon unit Pyruvate and Acetoacetyl CoA Nicotinamide Melatonin

Tryptophan 5-HT – CO2 5-HT hydroxylation、 decarboxylase 5-HT HO-

§ 5.5 Metabolism of branched-chain AAs Leu Ile Val