1、胺基酸的代謝:生合成(biosynthesis of amino acidbiosynthesis of amino acid)固氮作用固氮作用:大氣中的氮在酵素催化下還原成氨(NH3)的作用，只有少數的原核微生物(prokaryotic microorganism)具有固氮作用的能力。氮循環(nitrogen cycle)轉胺反應轉胺反應(transamination reaction)(transamination reaction):將-amino acid的-amino group轉給-keto acid。transamination是一個可逆反應，是胺基是胺基酸合成與分解時所必需的反
2、應。酸合成與分解時所必需的反應。氮的利用1.植物無法進行固氮作用，需由土壤吸取NH4+然後用來合成胺基酸與核酸。2.動物利用胺基酸上的胺基合成胺基酸與核酸。氮的循環氮的循環(nitrogen cycle)(nitrogen cycle):各種生物體之間利用氮原子的循環。固氮作用固氮作用(nitrogen fixation)(nitrogen fixation)N2鍵能為225 kcal/mole是很不活潑的氣體。工業固氮法是Fritz Haber在1907-1909年所發明的，目前用在肥料的製造。反應式為N2+3H2 2NH3 在鐵的催化、5000C高溫和300 atm高壓力下作用。自然界只有
3、少數的原核生物具有固氮能力，如一些共生共生的根瘤菌(Rhizobium)。在豆科植物的根部有一些寄生在根部的根瘤菌能合成NH4+供植物利用，但豆科植物必需附出相當代價:(1)在根瘤內需維持非常低的O2(2)耗用植物約1/5的ATP。氮平衡氮平衡(nitrogen balance)(nitrogen balance):當吃進的氮源(主要是胺基酸)與排出的氮源相等時稱氮平衡。這是健康成人的情況。Positive nitrogen balancePositive nitrogen balance:吃入的氮源較排出的氮源多。這是生長中的小孩、孕婦與康復中的病人的狀況。Negative nitrogen
4、 balanceNegative nitrogen balance:食物中匱乏蛋白質無法補充排出的氮源。Kwashiorkor:是一種惡性的營養缺乏症，肇因於長期蛋白的攝取不足。症狀包括:生長停滯，肝腫大、潰瘍、下痢、心臟、腎臟功能受損等。流行於非洲。TransportTransport:輸送胺基酸進入細胞是由膜上特異的輸送蛋白負責，至少分成兩類:1.需鈉離子的輸送(Na+-dependent amino acid transport)是一種second active transport system。2.不需鈉離子的輸送:如-glutamyl cycle。胺基的反應胺基的反應 1 1 轉胺作
5、用轉胺作用(transamination)(transamination):反應由aminotransferase催化胺基酸的合成大部分以glutamate充當amino group donor。有三個轉胺作用在代謝上具有重要的功能。1-ketoglutarate/glumate pair在胺基酸合成與分解反應 2 oxaloacetate/asparate pair 在urea cycle3 pyruvate/alanine pair 在alanine cycle胺基酸的生合成圖14.2 維生素B6Fig.14.2pyridoxal-5-phosphate(PLP):由pyridoxine(
6、Vitamin B6)組成是轉胺作用的coenzyme。需PLP的酵素中，PLP是接在Lysine residue上。PLP與胺基酸上的amino group結合形成中間物(intermediate)(如圖)，形成Schiff base圖14.3 轉胺作用之機制(上)圖14.3 轉胺作用之機制(下)2 2 胺離子胺離子(NH(NH4 4+)併入有機分子中併入有機分子中:分兩類1.對-keto acid還原加胺作用:Glutamate dehydrogenase催化-ketoglutarate加胺作用。此反應具可逆性，但真核生物中似乎傾向於NHNH4 4+的釋出。2.使Asparate與glut
7、amate轉成asparagine與glutamine。在腦中存有豐富的glutamine synthetase(因腦對NHNH4 4+的毒性特別敏感)。The biosynthetic pathways to glutamate and glutamine1.Glutamine synthetase catalyzes the reaction of glutamine and NH4+to yield glutamine.Glutamate+ATP -glutamyl phosphate+ADP-glutamyl phosphate+NH4+glutamine+Pi+H+Sum:Gluta
8、mate+NH4+ATP Glutamine+ADP+Pi+H+Glutamine synthetase is found in all organisms.3.L-glutamate dehydrogenase-Ketoglutarate+NH4+NADPH L-Glutamate+NADP+H2O In eukaryotic cells,L-glutamate dehydrogenase is located in the mitochondrial matrix.The reaction equilibrium favors reactants,and the Km for NH4+(a
9、bout 1 mM)is so high that the reaction probably makes only a modest contribution to NH4+assimilation into amino acids and other metabolitesGlutamine synthetase is a primary regulatory point in nitrogen metabolism In E.coli,glutamine synthetase has 12 identical subunits of Mr 50,000 and is regulated
10、both allosterically and by covalent modification.Alanine,glycine,and at least six end products of glutamine metabolism are allosteric inhibitors of the enzyme.Each inhibitor alone produces only partial inhibition,but the effects of multiple inhibitors are move than additive,and all eight together vi
11、rtually shut down the enzyme.Allosteric regulation of glutamine synthetaseX-ray crystal structure of glutamine synthetaseRegulation of glutamine synthetase by covalent modificationAdenylylation of Tyr397 of glutamine synthetase increases sensitivity to the allosteric inhibitors,and activity decrease
12、s as more subunits are adenylylated.Both adenylylation and deadenylylation are promoted by adenylyltransferase(AT).The activity of AT is modulated by binding to a regulatory protein called PII,and the activity of PII,in turn,is regulated by covalent modification(uridylylation),again at a Tyr residue
13、.The AT complex with PII-UMP stimulates deadenylylation,where the same complex with PII stimulated adenylylation of glutamine synthetase.Both uridylylation and deuridylylation of PII are brought about by a single enzyme,uridylyltransferase(UT).Uridylylation is inhibited by binding of glutamine and P
14、i to UT and is stimulated by binding of-ketoglutarate and ATP to PII.The uridylylated PII also mediates the activation of transcription of the gene encoding glutamine synthetase,thus increasing the cellular concentration of the enzyme;the deuridylylated PII brings about a decrease in transcription o
15、f the same gene.An adenylylated Tyr residue of glutamine synthetaseCascade leading to adenylylation of glutamine synthetaseAsparagine的合成，需 Asparagine synthase胺基酸的合成胺基酸的合成(synthesis of the(synthesis of the amino acid)amino acid)在動物中所有的在動物中所有的NAANAA是由是由glycerate-3-glycerate-3-phosphatephosphate、pyruva
16、tepyruvate、-ketoglutarate-ketoglutarate或或oxaloacetateoxaloacetate合成而來。合成而來。另外tyrosine是由phenylalanine合成而來。Biosynthesis of amino acids All amino acids are derived from intermediates in glycolysis,the citric acid cycle,or the pentose phosphate pathway.Nitrogen enters these pathways by way of glutamate
17、and glutamine.Whereas most bacteria and plants can synthesize all 20 amino acids,mammals can synthesize only about half of them.These are the nonessential amino acids,not needed in the diet.The remainder,the essential amino acids,must be obtained from food.Overview of amino acid biosynthesis The car
18、bon skeleton precursors derived from three sources:glycolysis(pink),the citric acid cycle(blue),and the pentose phosphate pathway(purple).Serine,glycine,and cysteine are derived from phosphoglycerateBiosynthesis of serine from 3-phosphoglycerate and of glycine from serine in all organismsIn the live
19、r of vertebrates,glycine can be madeby another route:the reaction catalyzed byGlycine synthase(also called glycine cleavage enzyme):CO2+NH4+N5,N10-methylene tetrahydrofolate+NADH+H+Glycine+tetrahydrofolate+NAD+Biosynthesis of cysteine from homocysteine and serine in mammalsThree nonessential and six
20、 essential amino acids are synthesized from oxaloacetate and pyruvateChorismate is a key intermediate in the synthesis of tryptophan,phenylalanine,and tyrosineConcerted InhibitionSix products derived from glutamineserve as negative feedback modulators of the enzyme,and the overall effects of these a
21、nd othermodulators are more than additive.Such regulation is called Concerted inhibition.Fig.14.4胺基酸的生合成途徑有一個共同特性:它們的碳骨架是由glycolysis、pentose phosphate pathway與citric acid cycle的中間產物而來，可分為6個族群。1.Glutamate family:1.Glutamate family:以-ketoglutarate為先驅物，合成glutamate、glutamine、proline與arginine。2.Serine fa
22、mily:2.Serine family:以glycerate-3-phosphate為先驅物，合成serine、glycine與cysteine這一族的成員在生合成(anabolism)中扮演重要的角色。Glycine是合成嘌呤(purine)、porphyrin(紫質)、glutathione的先驅物(precursor)。Serine是合成ethanolamine與sphingosine的先驅物。cysteine在硫代謝中具重要的角色。3.Asparate3.Asparate家族家族:以oxaloacetate為先驅物，合成aspartate、asparagine、lysine、meth
23、ionine與threonine。Fig.14.74.pyruvate家族:以pyruvate為先驅物，包含alanine、valine、leucine與isoleucinealanine是由pyruvate進行轉胺作用合成的glutamate+pyruvate alanine+-ketoglutarate催化此反應的酵素為alanine transaminase(又稱glutamic pyruvic transaminase:GPTGPT)5.Aromatic家族:是由phosphoenolpyruvate與erythrose-4-phosphate起始合成，包含phenylalanine、
24、tyrosine與tryptophan。而tyrosine是phenylalanine合成而得。6.Histidine:在健康的成人是屬於NAA，是由phosphoribosylpyrophosphate(PRPP)，ATP與glutamine所合成。莽草酸途徑莽草酸途徑（shikimic acid pathway又叫做分支酸途徑分支酸途徑，Chorismate pathway）是一個存在於細菌、真菌、藻類以及寄生生物和植物中的代謝途徑，用於芳香族胺基酸（苯丙氨酸、酪氨酸和色氨酸）的生物合成。這個代謝途徑在動物中不存在)chorismate:可合成phenylalanine、tyrosine與
25、tryptophan。若加入terpenoids則可合成tocopherol或ubiquinone。嘉磷塞嘉磷塞（Glyphosate）商品名稱為年年春年年春(Roundup)、好過春好過春、治治草春草春、日產春日產春、好伯春好伯春、草甘膦草甘膦等，是一種廣效型的有機磷除草劑有機磷除草劑。嘉磷塞主要是阻礙芳香胺基酸的生物合成，即苯丙胺酸、色胺酸及酪胺酸通過莽草酸途徑的合成。它對EPSPS合酶（5-烯醇丙酮酸莽草酸-3-磷酸合酶,5-enolpyruvylshikimate-3-phosphate synthase）有抑制作用。Fig.14.12單碳的代謝單碳的代謝(one-carbon(one
26、-carbon metabolism)metabolism)在生合成途徑(biosynthetic pathway)中，最重要單碳的攜帶者(one carbon carrier)包含(1)葉酸(folic acid)(2)S-Adenosylmethionine。另外Vit B12也有此功能。葉酸葉酸(Folic acid)(Folic acid)Folic acidFolic acid:分子包含一個pteridine nucleus與para-aminobenzoic acid，然後接在glutamic acid上。四氫葉酸(tetrahydrofolic acid:THF)是生物活性的形式
27、。能攜帶methyl、methylene、methenyl與formyl group，結合位置在pteridine ring的N5與N10的位置。Folic acid可經由dihydrofolate reductase的催化還原成THF，NADPH是輔因子。Fig.14.13Table 14.2Fig.14.14Table 14.3Fig.14.15S-adenosylmethionine(SAM)S-adenosylmethionine(SAM):單碳的代謝中，SAM是甲烷基的供應者(methyl group donor)。由ATP與methionine在SAM synthase催化下形成。
28、SAM失去一個-CH3後會形成S-Adenosylhomocysteine(SAH)，可經由N5-methyl THF補充。若methionine缺乏時也可利用choline的甲烷基加入homocysteine成methionine。Vitamine BVitamine B1212(cobalamin):是一個含鈷的分子，鈷離子上接一個氰胺(cyanide)，僅少數的微生物能合成。動物可從小腸的菌叢中獲得或從肝臟、雞蛋、蝦與肉類等食物中獲得。Fig.14.16Fig.14.17惡性貧血惡性貧血(pernicious anemia)(pernicious anemia)Vitamin B12缺乏
29、會導致嚴重的病症，症狀除了低紅血球數量，還包括虛弱與中樞神經受損。1926年三位美國醫生發現服用肝臟可冶療惡性貧血(只發生在人類)，才揭開Vit B12的神秘面紗。1956年Hohgkin女士解出其三度空間結構。造成惡性貧血的原因如下:1大部分是因為一個糖蛋白具Vit B12吸收因子(intrinsic factor)缺乏造成2胃腸病造成Vit B12吸收被抑制。3大量使用抗生素造成腸內益菌減少，Vit B12吸收減少。Glutathione(GSH):Glutathione(GSH):-Glutamylcysteinylglycine:Glutamylcysteinylglycine:GSH
30、的功能如下:1.幫助合成生物分子，如eicosanoids、DNA與RNA等。(把催化反應的酵素變成還原態)。2.抗氧化劑(Antioxidant):抗oxidative stress的分子，阻止細胞膜被破壞。3.在細胞間輸送胺基酸特別是cysteine與methionine。-GlutamylGlutamyl cycle cycle:存在一些組織中，具主動運輸(active transport)數個胺基的特性。4.形成GSH conjugation，以利一些物質排出。Fig.14.18Fig.14.19神經傳導物質神經傳導物質(neurotransmitters)(neurotransmit
31、ters):具有激發性(excitatory)與抑制性(inhibitory)效應:Excitatory neurotransmitters:如乙醯膽鹼(acetylcholine)或glutamate等，能促進被傳導的細胞膜去極化(depolarization)，能增加action potential。Inhibitory neurotransmitters:如glycine與GABA，會造成Repolarization即抑制action potential的形成。Table 14.4圖14.A 兒茶酚胺之生合成nitric oxide synthase許多神經傳導物質胺基酸或其衍生物(de
32、rivatives)後者又稱biogenic amines。兒茶酚胺兒茶酚胺(catecholamine)(catecholamine):包括多巴明(dopamine)、正腎上腺素(nonepinephrine)與腎上腺素(epinephrine)，是酪胺酸(tyrosine)的衍生物。在腦中是屬於激發性的神經傳導物質。N(nonepinephrine)與NE(epinephrine)皆具有調控代謝作用的功能。正腎上腺素正腎上腺素:是腎上腺髓質的嗜鉻細胞(chromaffin cells)分泌，能促進三酸甘油脂與肝糖的分解，在驚嚇、遇冷或運動與低血糖時分泌。腎上腺素腎上腺素:是腎上腺髓質的嗜鉻
33、細胞(chromaffin cells)分泌，在受壓力、創傷或激烈運動時，能促進三酸甘油脂與肝糖的分解。兩者皆是激素(hormone)巴金森氏症巴金森氏症(Parkinsons disease)(Parkinsons disease):以震顫麻痺(paralysis agitans)而聞名，是一種腦部受損造成移動失調，常出現在40歲以上的成人。症狀:為顫抖、肌肉僵硬、移動困難。只知在腦的黑質(substantia nigra)部位的一些神經細胞(neuron)無法產生多巴明(dopamine)可能是造成此疾病的原因。L-DOPA L-DOPA:dopamine是由L-DOPA合成而得，因為do
34、pamine無法通過blood-brain barrier(是一種障壁，用來阻止毒物進入腦內)，用來冶療巴金森氏症(Parkinsons disease)。組織胺組織胺(histamine)(histamine):是一個胺類，許多組織都可以產生，它是過敏(allergic)與發炎(inflammatory)反應的媒介物(mediator)，可剌激胃酸的產生等。由L-histidine失去二氧化碳(decarboxylation)而得。此反應是由histidine decarboxylase催化，PLP是其輔因子。抗組織胺的藥物常用來冶療過敏性鼻炎、蕁麻疹等。但會有嗜唾、疲勞副作用(影響到網狀系統)。一氧化氮一氧化氮(nitric oxide)(nitric oxide):是一個自由基，非常活潑的氣體，生命期很短，但扮演許多重要的生理角色。許多細胞都能分泌NO，生理功能包括促進血管的舒張、抑制血小板的凝集與破壞外來細胞(由巨噬細胞分泌)。在神經細胞NO是具有neurotranmitter的作用，可能與學習與記憶形成有關。NO的合成:由arginine轉成citrulline並釋出NO，此反應是由nitric oxide synthase所催化。NOS在神經細胞是受鈣離子活化，若在一般細胞則由細胞激素(cytokine)所活化。
