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上海交通大学 上海交大 生物化学 课件 chapter 24fatty


CHAPTER 24 METABOLISM

FATTY ACID

FAs, containing a long hydrocarbon chain and a terminal carboxylate group, have 4 major physiologic roles e.g CH3(CH2)14COOH
as building blocks of phospholipids磷脂 and 磷脂 glycolipids糖脂 these amphipathic (两亲 molecules 糖脂, 两亲) 糖脂 两亲 are important components of biological membranes many proteins are modified by the covalent attachment of FAs, which targets them to membrane locations膜定位 膜定位 FA are fuel molecules, stored as triacylglycerols (TAG三脂酰甘油酯 (neutral fats甘油与脂肪酸形成 三脂酰甘油酯) 三脂酰甘油酯 甘油与脂肪酸形成 的不带电的酯or 的不带电的酯 triglycerides) FA derivatives serve as hormones and intracellular messengers 激素和胞内信使 如前列腺素 激素和胞内信使(如前列腺素)

TAG are highly concentrated energy stores
TAGs are reduced and anhydrous (无水的 无水的) 无水的 The yield from the complete oxidation of FA is about 9 kcal/g, in contrast with about 4 kcal/g for carbohydrates and protein ( x 2)
The reason for this is that FAs are much more highly reduced (C16H32O2 : C6H12O6) TAGs are very nonpolar and stored in a nearly anhydous无水的 form ( x 3) (1克干糖原结合 克水 克干糖原结合2克水 无水的 克干糖原结合 克水) a gram of nearly anhydous fat stores more than 6 times ( x 6) as much energy as a gram of hydrated glycogen

this is why TAGs rather than glycogen were selected in evolution as major energy reservoir In mammals, the major site of accumulation of TAG is the cytoplasm of adipose cells (fat cells) which are specialized for
synthesis and storage of TAG their mobilization into fuel molecules that are transported to other tissues by blood TAG通常含有 通常含有14~24C之间的偶 通常含有 之间的偶 数碳链脂肪酸, 数碳链脂肪酸,链可以是饱和 或不饱和的( 或不饱和的(含一个或几个双 键)。

TAGs are hydrolized by cAMP-regulated lipases脂酶

initial step in the utilization of fat as energy source: TAG+ 3H2O→glycerol+ 3 FAs +3H+ Hormones, such as epinephrine, norepinephrine (去甲肾上腺素 去甲肾上腺素), 去甲肾上腺素 glucagon and adrenocorticotropic hormone (促肾上腺皮质 促肾上腺皮质) 促肾上腺皮质 induce lipolysis: : hormones→adenylate cyclase腺苷酸环化酶 腺苷酸环化酶→cAMP→protein 腺苷酸环化酶 kinase A →lipase cAMP is a second messenger in activation of lipolysis in fat cells, as it is in activation of glycogen breakdown

In contrast, insulin inhibits lipolysis胰岛素抑制脂解 胰岛素抑制脂解 Glycerol is phosphorylated and oxidized to DHAP (and then isomerized to GAl-3-P) which is on both glycolytic and GNG pathways 脂肪组织中储存的脂肪转变成其它组织所需的能量须经历:1. TAG降解 脂肪组织中储存的脂肪转变成其它组织所需的能量须经历: 降解
为脂肪酸和甘油,从脂肪组织释放并运送到需能组织。 2. 脂肪酸被激活运送进入上述 为脂肪酸和甘油,从脂肪组织释放并运送到需能组织。 氧化) 脂肪酸逐步降解为乙酰CoA,然后进入柠 组织细胞的线粒体中进行降解 (β氧化 。 3. 脂肪酸逐步降解为乙酰 氧化 , 檬酸循环。 檬酸循环。

FAs are degraded by the sequential removal of two-carbon units
Franz Knoop made a critical contribution to elucidation of mechanism of FA oxidation
When a FA with an even (odd) number C was fed直链脂肪酸的 碳原子与苯环连接 直链脂肪酸的ω碳原子与苯环连接 直链脂肪酸的 碳原子与苯环连接, phenylacetic acid苯乙酸 (benzoic acid苯甲酸 苯甲酸) 苯乙酸 苯甲酸 was produced

FA are degraded by oxidation at the βcarbon The use of a synthetic label (the phenyl group) and radioisotope label to elucidate reaction mechanisms is a landmark in biochemistry合成性标记物用于生物化学 合成性标记物用于生物化学 领域早于放射性同位素几十年,它们用于 领域早于放射性同位素几十年 它们用于 阐明反应机制是生物化学发展史上的里 程碑

FAs are linked to coenzyme A before they are oxidized
β-oxidation of FAs occurs in mito ATP drives the formation of a thioester linkage between carboxyl group of FA and sulfhydryl group of CoA, making FAs activated脂肪酸羧基和 脂肪酸羧基和CoA巯基之 脂肪酸羧基和 巯基之 驱动)形成硫酯键 脂酰CoA),使脂肪酸活化 间(由ATP驱动 形成硫酯键 脂酰 由 驱动 形成硫酯键(脂酰 使脂肪酸活化 This activation reaction occurs on outer mito membrane catalyzed by acyl CoA synthetase: R- COO-+ ATP+ HS-CoA →(R-CO-AMP)脂酰腺苷酸中 脂酰腺苷酸中 以两个高能键为代价) 间物 →R-CO-S-CoA+ AMP+ PPi(以两个高能键为代价 以两个高能键为代价 The hydrolysis of PPi drives the overall reaction irreversible

a repeated theme: many biosynthetic reactions are made irreversible by hydrolysis of inorganic pyrophosphate焦磷酸水解 焦磷酸水解 Another recurring motif is: acyl adenylates脂酰腺苷酸中间物 脂酰腺苷酸中间物 (acyl-AMP) are frequently formed when carboxyl groups are activated in biochemical reactions被激活羧基经常与腺苷酸形成 被激活羧基经常与腺苷酸形成 酰基腺苷酸中间物 e.g. in AA activation for Pro synthesis: AA+ATP≒AA-AMP(aminoacyl-AMP氨酰 氨酰AMP)+PPi ≒ 氨酰 AA-AMP + tRNA ≒AA-tRNA+AMP(氨基酸然后与 (氨基酸然后与tRNA3’端 端 AMP中核糖的 ’羟基相连 。 中核糖的3’羟基相连)。 中核糖的

Carnitine 肉碱 肉碱carries long-chain activated FAs into the mito matrix
A special transport mechanism is needed to make long-chain acyl CoA molecules traverse inner mito membrane It is by conjugating (綴合 long-chain FAs to carnitine, a zwitterionic (兼性 綴合) 兼性) 綴合 兼性 compound formed from Lys来源于 来源于Lys的兼性化合物 来源于 的兼性化合物 acyl group is transferred from S atom of CoA to -OH of carnitine to form acylcarnitine This reaction is catalyzed by carnitine acyl transferaseⅠ肉碱酰基转移酶Ⅰ Ⅰ肉碱酰基转移酶Ⅰ it is bound to outer mito membrane 线粒体外膜 側) 线粒体外膜(側

Acyl carnitine is shuttled across inner mito membrane by a translocase 移位酶 acyl group is transferred back to CoA on matrix side of membrane This reaction is catalyzed by carnitine acyl transferaseⅡ肉碱 Ⅱ
酰基转移酶Ⅱ 酰基转移酶Ⅱ

Finally, carnitine is returned to cytosolic side by translocase, in exchange for an incoming acyl carnitine It is not required for permeation of medium-chain acyl CoAs into mito matrix C4—C14(典型地 或C10)脂酰 典型地C8或 脂酰CoA进入线粒体基 典型地 脂酰 进入线粒体基 质不需肉碱 The defect in the transferase or translocase (or a deficiency of carnitine) will impair the oxidation of long-chain FAs
demonstrating that impaired flow of a metabolite from one compartment of a cell to another can cause disease

Acetyl CoA, NADH, and FADH2 are generated in each round of FA oxidation
saturated acyl CoA is degraded by a recurring sequence of 4 reactions
oxidation by FAD: Acyl CoA+ E-FAD→trans-△2-enoyl CoA烯 △ 烯 脂酰+E-FADH2 脂酰 Hydration水合 Trans-△2 -enoyl CoA+ H2O→L-3水合: 水合 △ hydroxyacyl羟脂酰 羟脂酰CoA 羟脂酰 oxidation by NAD+: L-3-hydroxyacyl CoA+NAD+→3-ketoacyl 酮脂酰CoA+ NADH+ H+ 酮脂酰 thiolysis 硫解 CoA: 3-ketoacyl CoA (n carbons)+ HS硫解by CoA→acetyl CoA+ acyl CoA (n-2 carbons)

脂酰CoA(n)→“烯”→ “羟”→ “酮”→脂酰 脂酰 烯 羟 酮 脂酰CoA(n-2)+乙 乙 脂酰 酰CoA

1 1

2

相似

fatty acyl chain is shortened by 2 carbon atoms as a result of these reactions, and FADH2, NADH, and acetyl CoA are generated The former 3 reactions of 4 steps of β-oxidation pathway in each round of FA degradation closely resemble the last steps in the citric acid cycle: (β氧化与 氧化与TCA循环中的对应几步 十分 循环中的对应几步)十分 氧化与 循环中的对应几步
Acyl CoA→enoyl CoA→hydroxyacyl CoA→ketoacyl CoA Succ琥珀酸→fum延胡索酸→malate苹果酸→OAA

The preceeding reactions have oxidized -CH2- at C-3 to -C=O β碳原子 由亚甲基碳)被氧化 为羰基碳) 碳原子(由亚甲基碳 被氧化(为羰基碳 碳原子 由亚甲基碳 被氧化 为羰基碳 The long-chain, medium-chain, and short-chain acyl CoA DH catalyze FAs containing 12-18, 4-14, and 4和6 carbons, respectively 和 By contrast, β-ketothiolase β-酮硫解酶 hydroxyacyl CoA DH, and enoyl CoA hydratase 酮硫解酶, 酮硫解酶 短链脂酰 脂酰CoA脱氢酶氧化不同 长度碳原子的 底物 而 脱氢酶氧化不同 长度碳原子的)底物 have broad specificity 长、中、短链脂酰 脱氢酶氧化不同(长度碳原子的 底物, 水合酶)、 脱氢酶)、 酶4(烯脂酰 (烯脂酰CoA水合酶)、 (羟脂酰 水合酶)、5(羟脂酰CoA脱氢酶)、 (硫解酶)则普遍适用 长、 脱氢酶)、6(硫解酶)则普遍适用(长 短链底物通用) 中、短链底物通用

The complete oxidation of palmitate 棕榈酸 yields 106 ATP
palmitoyl CoA+ 7FAD+ 7NAD++ 7H2O→ 8 acetyl CoA+ 7FADH2+7NADH+ 7H+ So, the yield of ATP is: 10x8(acetyl CoA) +1.5x7(FADH2) + 2.5x7(NADH)-2 - =106 the complete oxidation of a molecule of paltimate yields 106 ATP

“In aerobic metabolism, about 0.8 equivalents of CO2 are produced per O2 consumed”(P164)需(有) 需有 氧代谢中,每吸入 分子氧气呼出0.8 分子CO2 。 每吸入1分子氧气呼出 氧代谢中 每吸入 分子氧气呼出 分子
脂代谢Fatty acid: 8(acetyl CoA) +7(FADH2) +7(NADH) =8 x (3 NADH + FADH2 ) 脂代谢 +7(FADH2) +7(NADH) =31 NADH+ 15 FADH2 In oxidative phosphorylation: 4H+ + 4e + O2 =2H2O (每一个还原型电子载体传递一对电子,46个电子载体 每一个还原型电子载体传递一对电子, 个电子载体 每一个还原型电子载体传递一对电子 (92个电子 共需23个氧分子作为电子受体。每个软脂酸(即8个acetyl CoA) 个电子)共需 个氧分子作为电子受体。每个软脂酸( 个 ) 个电子 共需 个氧分子作为电子受体 完全氧化生成16个CO2, CO2 :O2 =16:23≈0.7 完全氧化生成 个 : 糖代谢: 糖代谢: Glucose: C6H12O6 +6O2 =6CO2+6H2O 循环( 糖酵解: 呼出2个 循环 糖酵解:2NADH;丙酮酸脱氢:2NADH (呼出 个CO2 );TCA循环(呼出 ;丙酮酸脱氢: 4个CO2 ) :6个NADH 和2个FADH2 ;共产生 个还原型电子载体 个电 共产生12个还原型电子载体 个还原型电子载体(24个电 个 个 个 即需6个氧分子作为电子受体 子),即需 个氧分子作为电子受体 共呼出 个CO2 即需 个氧分子作为电子受体; 共呼出6个 CO2 :O2 =1 以及其它(如蛋白等)有机物质的氧化(量较小) 以及其它(如蛋白等)有机物质的氧化(量较小) 综上所述: CO2 :O2 ≈0.8 综上所述:

An isomerase and a reductase are required for the oxidation of unsaturated FA
Take palmitoleate棕榈油酸 (C16 Cis-△9) as an example 棕榈油酸 △
it is C16 unsaturated FA with one double bond After activated and transported across the inner mito membrane in the same way as palmitate palmitoleoyl CoA undergoes 3 cycles of degradation (β-oxidation) However, the cis-△3-enoyl CoA formed in the third round is not a substrate △ for the presence of cis-△3 prevents formation of trans-△2 △ △

This impasse is resolved by a new reaction that shifts the position and configuration of the cis-△3 double bond △
An isomerase converts this double bond into a trans-△2 double bond, which △ 若干轮β-氧化后 奇数双键都成为cis-△ 氧化后,奇数双键都成为 is a regular substrate in FA β-oxidation 若干轮 氧化后 奇数双键都成为 △3; (每一个奇数双键少收获一个 每一个奇数双键少收获一个FADH2) 每一个奇数双键少收获一个
异构酶将 △ 转变成trans-△2 —β-氧化的正常底物 奇数双键的普遍解决方式 氧化的正常底物(奇数双键的普遍解决方式 异构酶将cis-△3 转变成 △ 氧化的正常底物 奇数双键的普遍解决方式)

Take linoleate亚油酸 (C18 cis-△9, cis -△12) as another 亚油酸 △ △ example
The cis-△3 formed after 3 rounds ofβ-oxidation is converted into △ trans-△2 by the isomerase mentioned above △ Its cis-△12 poses a new problem: a cis-△4 is formed after 4 rounds △ △ ofβ-oxidation Dehydrogenation of this species by acyl CoA DH yields a 2,4-dienoyl intermediate, which is not a substrate for the next enzyme in βoxidation

This impasse is circumvented by 2,4-dienoyl CoA reductase, an enzyme that uses NADPH to reduce cis-△2, cis-△4 to △ △ cis-△3 将两个顺式双键还原成一个 △
The isomerase then converts cis-△3 to trans-△2 若干轮 氧化后 偶数 氧化后,偶数 △ △ 若干轮β-氧化后 双键都成为cis-△ 用 生成trans-△2, 这样形成了 双键都成为 △4;用acyl CoA DH 生成 △ 这样形成了2,4-二 二 再用2,4-二烯脂酰 二烯脂酰CoA还原酶 还原酶(NADPH作为还原力 将 作为还原力)将 烯脂酰中间物;再用 烯脂酰中间物 再用 二烯脂酰 还原酶 作为还原力 trans-△2, cis-△4 还原为 △3,然后异构酶再将 △3 转变成 然后异构酶再将cis-△ 转变成trans△ △ 还原为cis-△ 然后异构酶再将 此为偶数双键的普遍解决方式) △2 (此为偶数双键的普遍解决方式 (每一个偶数双键需消耗一个 此为偶数双键的普遍解决方式 每一个偶数双键需消耗一个 NADPH)

Only two extra enzymes are needed for the oxidation of any polyunsaturated FA
Odd-numbered double bonds are handled by isomerase一个奇数双 一个奇数双 键少收获一个FADH2, 键少收获一个 相当于1.5个 (相当于 个ATP) ) even-numbered ones by reductase and isomerase一个偶数双 一个偶数双 键需消耗一个NADPH, 键需消耗一个 , 相当于少收获2.5个 (相当于少收获 个 ATP) )

Odd-chain FAs yield propionyl丙 丙 酰 CoA in the final thiolysis step
The only difference between odd-chain FAs and even-chain FAs inβ-oxidation
propionyl CoA and acetyl CoA are produced in the final round of degradation

The activated three-carbon unit in propionyl CoA enters TCA cycle after it is converted into succinyl CoA(P641)三碳的 ( )三碳的propionyl CoA羧化生成四碳的 羧化生成四碳的succinyl CoA进入柠檬 羧化生成四碳的 进入柠檬 酸循环

Ketone bodies酮体 are formed from acetyl 酮体 CoA when fat breakdown predominates
“fats burn in the flame of carbohydrates”脂肪在糖的 脂肪在糖的 火焰中燃烧(脂肪在糖正常氧化的基础上氧化 脂肪在糖正常氧化的基础上氧化) 火焰中燃烧 脂肪在糖正常氧化的基础上氧化
acetyl CoA formed in FAβ-oxidation enters TCA only if fat and carbohydrate degradation are appropriately balanced because entry of acetyl CoA into TCA depends on the availability of OAA the concentration of OAA is lowered if carbohydrate is unavailable or improperly utilized OAA is normally formed from Pyr, the product of glycolysis 糖 酵解的产物Pyr用来 添补 用来“添补 其它中间物)使得 酵解的产物 用来 添补”OAA(和TCA其它中间物 使得 和 其它中间物 使得TCA 循环持续进行

In fasting or diabetes糖尿病 OAA is consumed to form Glc by 糖尿病, 糖尿病 GNG and is unavailable for condensation with acetyl CoA Under these conditions, acetyl CoA is diverted to formation of acetoacetate and D-3-hydroxybutyrate羟基丁酸 羟基丁酸 Acetoacetate乙酰乙酸 D-3-hydroxybutyrate and acetone丙酮 are 乙酰乙酸, 乙酰乙酸 丙酮 called as ketone bodies 酮体

Acetoacetate is a major fuel in some tissues
Acetoacetate and 3-hydroxybutyrate (its major site of production is liver) are normal fuels of respiration and quantitatively important as sources of energy heart muscle心肌 and renal cortex 心肌 肾皮质 use acetoacetate in preference to Glc Glc is major fuel for brain and red blood cells脑和红细胞 in well脑和红细胞 nourished people on a balanced diet However, brain adapts to acetoacetate during starvation and diabetes Acetoacetate→acetoacetyl CoA (by CoA transferase)→2 acetyl CoA (entering into TCA)

liver can supply acetoacetate to other organs because it lacks this particular CoA transferase (pay attention to glucokinase and G-6-Pase in liver!)肝脏是制造并输出酮体的主要场所。肝脏 肝脏是制造并输出酮体的主要场所。 肝脏是制造并输出酮体的主要场所 缺乏CoA transferase,即不能利用酮体作为自身 缺乏 , 燃料,这是肝脏利它性的又一体现。 燃料,这是肝脏利它性的又一体现。 Acetoacetate can be regarded as a water soluble, transportable form of acetyl units乙酰乙酸是乙 乙酰乙酸是乙 酰基的水溶性的可转运形式 High levels of acetoacetate in blood signify abundance of acetyl units and lead to decrease in rate of lipolysis in adipose tissue

Animal cannot convert FAs into Glc
animals are unable to convert FAs into Glc acetyl CoA cannot be converted into Pyr or OAA in animals动物不能把 动物不能把acetyl CoA转变成 转变成OAA或其它 或其它TCA循环的中间物 因通 循环的中间物(因通 转变成 或其它 循环的中间物
过每轮循环,它的两个碳原子以二氧化碳形式释出 不能视为acetyl CoA经 过每轮循环 它的两个碳原子以二氧化碳形式释出,OAA不能视为 它的两个碳原子以二氧化碳形式释出 不能视为 经 TCA氧化的产物 即不能将它转变为丙酮酸进而转变成糖 氧化的产物),即不能将它转变为丙酮酸进而转变成糖 氧化的产物

two C atoms of acetyl CoA enter TCA but two C atoms leave it OAA is regenerated but it is not formed de novo when the acetyl unit of acetyl CoA is oxidized by TCA Plants are able to convert acetyl CoA into OAA (because of two additional enzymes(异柠檬酸裂合酶和苹果酸合酶 植物通 异柠檬酸裂合酶和苹果酸合酶) 异柠檬酸裂合酶和苹果酸合酶 过乙醛酸循环能做到(2个 过乙醛酸循环能做到 个acetyl CoA 转化成一个琥珀酸)

FAs are synthesized and degraded by different pathways
Synthetic and degradative pathways are almost always distinct FA synthesis is not simply a reversal of degradative pathway

It consists of a new set of reactions and has some important features
site场所 cytosol—mito. 场所: 场所 immediate link中间物载体 ACP酰基载体蛋白 中间物载体: 酰基载体蛋白—CoA(both – 中间物载体 酰基载体蛋白 ( SH) enzyme integration形成酶复合体与否 multienzyme 形成酶复合体与否: 形成酶复合体与否 complex(fatty acid synthase)—seperated activated donor of two-carbon units活性二碳单位供体 活性二碳单位供体: 活性二碳单位供体 丙二酰ACP—acetyl CoA (released in malonyl-ACP丙二酰 丙二酰 degradation) reductant and oxidants还原剂和氧化剂 NADPH—NAD+, 还原剂和氧化剂: 还原剂和氧化剂 FAD 方向synthesis: from methyl to carboxyl end合成从甲基到羧基 方向 合成从甲基到羧基 端 degradation: from carboxyl to methyl end降解从羧基到甲基端 降解从羧基到甲基端 elongation by FA synthase complex stops upon formation of palmitate (C16) C16以上 脂肪酸 的合成和去饱和反应是由内 以上(脂肪酸 以上 脂肪酸)的合成和去饱和反应是由内 质网上的酶系催化的;氧化酶则可降解 氧化酶则可降解16碳以上的脂肪酸 质网上的酶系催化的 氧化酶则可降解 碳以上的脂肪酸

The formation of malonyl丙二酰 CoA is the committed step in FA synthesis -
acetyl CoA+ ATP +HCO3 →malonyl CoA+ ADP+ Pi+ H+ This irreversible reaction is committed step in FA synthesis

Acetyl CoA carboxylase contains a biotin prosthetic group
its carboxyl group is covalently attached toε-amino group of a Lys residue just as in Pyr carboxylase羧化酶都以生物素为辅基 都在酶的 羧化酶都以生物素为辅基, 羧化酶都以生物素为辅基 Lys 残基的 氨基与生物素的羧基之间形成酰胺键 从而形成 残基的ε-氨基与生物素的羧基之间形成酰胺键, 氨基与生物素的羧基之间形成酰胺键 长长的,可伸缩的分子线 可伸缩的分子线” “长长的 可伸缩的分子线” The carboxylation steps of acetyl CoA (to malonyl CoA) and Pyr (to OAA) are also similar: Biotin-E+ ATP+HCO3- →CO2~biotin-E+ ADP+ Pi CO2~biotin-E+ acetyl CoA→ malonyl CoA+ biotin-E It exemplifies a ping-pong reaction mechanism乒乓反应机理: 乒乓反应机理: 乒乓反应机理 one or more products are released before all substrates are bound This enzyme from E.Coli consists of 3 subunits
biotin carboxyl carrier protein生物素羧基载体蛋白 生物素羧基载体蛋白 biotin carboxylase生物素羧化酶 生物素羧化酶 Transcarboxylase转羧基酶 转羧基酶

length and flexibility of the link between biotin and its carrier protein enable activated carboxyl group to move from one active site to another in enzyme complex, as in Pyr carboxylase

Intermediates in FA synthesis are attached to an acyl carrier protein— ACP
Intermediates in FA synthesis in E.Coli are linked to sulfhydryl terminus巯基端 of phosphopantetheine磷酸 巯基端 磷酸 泛酰巯基乙胺 group of ACP
In degradation of FA, this unit is a part of CoA in synthesis it is attached to a Ser residue of ACP, a single polypeptide chain of 77 residues ACP can be regarded as a giant prosthetic group, a “macro CoA” CoA: 磷酸泛酰巯基乙胺 磷酸泛酰巯基乙胺+AMP衍生物 衍生物 ACP: 磷酸泛酰巯基乙胺 磷酸泛酰巯基乙胺+77个残基的肽链 “巨CoA”) 个残基的肽链(“ 个残基的肽链

The elongation cycle in FA synthesis
The enzyme system that catalyzes the synthesis of saturated longchain FAs from acetyl CoA, malonyl CoA, and NADPH is called FA synthase
constituent enzymes of bac. FA synthase are dissociated分散 分散 exist as multienzyme system in higher organisms多酶体系 两者的区别 脂肪酸 多酶体系 两者的区别:
合酶的整体性(高等生物以多酶复合体系形式存在 细菌中分离 合酶的整体性 高等生物以多酶复合体系形式存在,细菌中分离 高等生物以多酶复合体系形式存在 细菌中分离)

FA synthetic reactions are similar in both organisms相同点 合成反应内容相似 相同点:合成反应内容相似

FAs with even (odd) number of C atoms are synthesized starting with malonyl-ACP丙二酰 丙二酰 and acetyl-ACP乙酰 乙酰3+2(propionyl-ACP丙酰 丙酰3+3) 乙酰 丙酰
malonyl-ACP is formed from malonyl CoA by malonyl transacylase MT转丙二酰酶 转丙二酰酶 acetyl -ACP (propionyl-ACP) is formed from acetyl CoA (propionyl-CoA) by acetyl transacylase AT转乙 转乙 酰酶

Just as the two “preparing” reactions in FA βoxidation, there are 3 “preparing” reactions in FA synthesis:三步 准备 反应 个羧化两个转移 四步 正式 三步“准备 反应(1个羧化两个转移 四步“正式 准备”反应 个羧化两个转移),四步 正式”
反应

acetyl CoA→malonyl CoA
acetyl CoA→acetyl-ACP malonyl CoA→malonyl-ACP

The 4 “formal” reactions are: condensation, reduction, dehydration and reduction. (oxidation, hydration, oxidation, thiolysis in FAβoxidation)缩合 还原 脱水 再还原 对比 氧化 水合 再氧化 缩合,还原 脱水,再还原 对比: 氧化,水合 再氧化, 还原,脱水 再还原(对比 水合,再氧化
硫介) 硫介

condensation (by acyl malonyl-ACP condensing enzyme CE)
2+3-1→4, not direct 2+2→4 - why? Equilibrium for the synthesis of acetoacetyl-ACP乙酰乙酰 乙酰乙酰ACP 乙酰乙酰 from 2 acetyl-ACP is highly unfavorable In contrast, equilibrium is favorable if malonyl-ACP is a reactant because its decarboxylation contributes a substantial decrease in free energy脱羧驱动反应向正向进行(而3碳底物在前一步骤被羧化形成则 脱羧驱动反应向正向进行( 碳底物在前一步骤被羧化形成则 脱羧驱动反应向正向进行 为代价) 以ATP为代价) 为代价 In effect, the condensation reaction is driven by ATP, though ATP does not directly participate in condensation reaction. ATP is used to carboxylate acetyl CoA to malonyl CoA. The energy thus stored in malonyl CoA is released in decarboxylation accompanying the formation of acetoacetyl-ACP 乙酰乙酰 乙酰乙酰ACP Although HCO3- is required for FA synthesis, its C atom does not appear in the product( HCO3- “先上去,再下来”,它的碳原子没有 先上去, ( 先上去 再下来” 它的碳原子没有 渗入所合成的脂肪酸产物中) 渗入所合成的脂肪酸产物中 all carbon atoms of FAs containing an even number are derived from acetyl CoA (偶数 脂肪酸中的全部 原子来自 偶数)脂肪酸中的全部 原子来自acetyl CoA 偶数 脂肪酸中的全部C原子来自

The next 3 steps reduce the keto group酮基 (-C=O) at 酮基 - C-3 (β-C atom) to a methylene group亚甲基 (-CH2-) 亚甲基 -
acetoacetyl-ACP is reduced to D-3-hydroxybutyryl-ACP羟丁 羟丁 酮脂酰-ACP 还原酶 还原酶) 酰 (byβ–ketoacyl-ACP reductase KR β–酮脂酰 酮脂酰
This reaction differs from the corresponding one in FA degradation in two respects: A. D rather L epimer is formed, and B. NADPH is the reducing agent (NAD+ in β-oxidation) This difference exemplifies general principle that NADPH is consumed in biosynthetic reactions, whereas NADH is generated in energyyielding reactions

D-3-hydroxybutyryl-ACP is dehydrated to form crotonyl-ACP (巴豆酰 巴豆酰-ACP) which is a trans-△2-enoyl-ACP 反式 △2-烯脂 反式-△ 烯脂 巴豆酰 △ 酰ACP(catalyzed by 3-hydroxyacyl-ACP dehydratase 3-羟脂 羟脂 乙酰乙酰(酮丁酰)→羟丁酰→烯丁酰(巴豆 酰ACP脱水酶 DH) 乙酰乙酰 酮丁酰 羟丁酰 烯丁酰 巴豆 脱水酶 酰)→丁(脂)酰 丁脂酰 crotonyl-ACP is reduced to butyryl-ACP丁酰 (by enoyl-ACP 丁酰 reductase ER 烯脂酰 烯脂酰-ACP 还原酶 NADPH is again 还原酶), reductant (FAD is oxidant in corresponding reaction in βoxidation)

In the second round of FA synthesis, butyrylACP condenses with malonyl-ACP to form a C6β-ketoacyl-ACP which is converted (through reduction, dehydration and reduction) into a C6acyl-ACP “酮” → “羟”→ “烯” → 脂酰 脂酰ACP (4 + 3- 酮 羟 烯 - 1= 6)

Stoichiometry of FA synthesis
8 acetyl CoA+ 7ATP+ 14NADPH+ 6H+→palmitate棕榈酸 14NADP++ 棕榈酸+ 棕榈酸 8CoA+ 6H2O+ 7ADP+ 7Pi (8 acetyl CoA ,7ATP and 14NADPH are consumed)

FAs are synthesized in eukaryotes by a multifuctional enzyme complex

Mammalian FA synthase (different from that of E. Coli and yeast) is a dimer of identical 260-kd subunits Each chain is folded into 3 domains joined by flexible regions
Domain 1, substrate entry and condensation unit底物 底物 进入和缩合域, 进入和缩合域 consists of acetyl transferase (AT), malonyl transferase (MT), andβ-ketoacyl synthase (or condensing enzyme CE) Domain 2, reduction unit还原域 contains the acyl 还原域, 还原域 carrier protein (ACP), β-ketoacyl reductase (KR), dehydratase (DH), and enoyl reductase (ER) Domain 3, palmitate release unit棕榈酸释放域 棕榈酸释放域 contains thioesterase (硫酯酶 硫酯酶TE) 硫酯酶

7 different catalytic sites are present on single polypeptide chain
many eukaryotic multienzyme complexes are multifunctional proteins in which different enzymes are linked covalently不同酶共价结 不同酶共价结 合形成多酶复合体(多功能蛋白) 合形成多酶复合体(多功能蛋白)

The advantages of this arrangement are:
synthesis of different enzymes is coordinated酶合成协调 酶合成协调 a multienzyme complex consisting of covalently joined enzymes is more stable than one formed by noncovalent attractions复合 复合 体结构稳定 intermediates can be efficiently handed from one active site to another without leaving the assembly反应中间物在活性位点间 反应中间物在活性位点间 高效传递 time for diffusion is markedly reduced and side reactions are minimized减少扩散时间和付作用 减少扩散时间和付作用

It seems likely that multifunctional enzymes such as FA synthase arose in eukaryotic evolution by exon shuffling (外显子改组 外显子改组) 外显子改组

The flexible phosphopantetheinyl unit of ACP carries substrate from one active site to another
CH3-CO-SCoA→CH3-CO-O-Ser-AT→CH3-CO-S-Cys-CE; -OOC-CH -CO-SCoA→-OOC-CH -CO-O-Ser-MT→ - 2 2 -OOC-CH - CO-S-ACP (another chain in dimer) 2 Translocation of the elongating FA chain between the Cys sulfhydryl group of CE and the phosphopantetheine (磷酸泛 磷酸泛 酰巯基乙胺) 酰巯基乙胺 sulfhydryl group of ACP is in Fig.24-16 The flexibility and 20A maximal length of phosphopantetheinyl-moiety are critical for function of this multienzyme complex
enzyme subunits need not undergo large structural rearrangements to interact with substrate substrate is on a long, flexible arm that can reach each of the numerous active sites

biotin and lipoamide are also on long, flexible arms in their multienzyme complexes

Citrate carries acetyl groups from mito to the cytosol for FA synthesis
Acetyl CoA must be transferred from mito to cytosol to synthesize FA Mito are not readily permeable to acetyl CoA. The barrier to acetyl CoA is bypassed by citrate, which carries acetyl groups across inner mito membrane柠檬酸作为乙酰基载体穿越 柠檬酸作为乙酰基载体穿越mito内膜 内膜 Acetyl CoA and OAA are transferred from mito to cytosol at the expense of an ATP ATP-citrate lyase(裂合 裂合
在细胞质中再切开citrate 酶)在细胞质中再切开 在细胞质中再切开

Sources of NADPH for FA synthesis
OAA formed in the transfer must be returned to mito mito is impermeable to OAA a series of bypass reactions are needed. Most important, these reactions generate much of NADPH needed for FA synthesis 为合成脂肪酸,将acetyl CoA由线粒体基质 为合成脂肪酸, 由线粒体基质 运至细胞质(通过柠檬酸)以及将OAA由细胞质运回 运至细胞质(通过柠檬酸)以及将 由细胞质运回 线粒体基质(通过丙酮酸)的过程中, 线粒体基质(通过丙酮酸)的过程中,形成还原力 为代价) NADPH(以消耗 个ATP和1个NADH为代价) (以消耗2个 和 个 为代价 OAA+ NADH + H+→malate+ NAD+ (细胞质 细胞质malate DH) 细胞质 malate+ NADP+→Pyr+ CO2+ NADPH (与NADP+连接 与 的malate酶) Pyr formed readily enters mito, where it is 酶 carboxylated to OAA Pyr+ CO2+ ATP+ H2O→OAA+ ADP+ Pi+ 2H+ (Pyr羧 羧
化酶) 化酶

The sum of the 3 reactions: NADP++ NADH+ ATP+ H2O→NADPH + NAD++ ADP+ Pi+ H+ (另外 在柠檬酸水解中 还消耗掉一个 另外,在柠檬酸水解中 另外 在柠檬酸水解中,还消耗掉一个
ATP)

Thus, one NADPH is generated for each acetyl CoA that is transferred from mito to cytosol Hence, 8 NADPH are formed when 8 acetyl CoA are transferred to cytosol for synthesis of palmitate additional 6 NADPH required for this process come from PPP合成一个棕榈酸所需的
14个NADPH中, 8个来自 将8个acetyl CoA从线粒体向细 个 个来自(将 个 中 个来自 从线粒体向细 胞质的)转运过程 个来自磷酸戊糖途径 转运过程, 个来自磷酸戊糖途径. 胞质的 转运过程 6个来自磷酸戊糖途径

Acetyl CoA carboxylase plays a key role in controlling FA metabolism
FA metabolism is stringently controlled FA synthesis is maximal when carbohydrate↑and energy↑, but FA↓; 当糖和能量多、脂肪 当糖和能量多、 酸少时,脂肪酸合成最多 酸少时 脂肪酸合成最多 Acetyl CoA carboxylase plays a key role in regulating FA metabolism
which catalyzes committed step in FA synthesis, the production of malonyl CoA (activated twocarbon donor)

The carboxylase is controlled by 3 global signals (glucagon胰高血糖素 epineph-rine 胰高血糖素, 胰高血糖素 肾上腺素, 胰岛素) 肾上腺素 and insulin胰岛素 that reflect 胰岛素 overall needs of organism
Insulin stimulates FA synthesis by activating carboxylase glucagon and epinephrine have reverse effect Citrate activates the enzyme胰岛素和柠檬酸激活乙
酰辅酶A羧化酶 酰辅酶 羧化酶

Palmitoyl CoA and AMP lead to its inhibition肾
上腺素,胰高血糖素,棕榈酸, 上腺素,胰高血糖素,棕榈酸,AMP等则抑制此酶。 等则抑制此酶

Acetyl CoA carboxylase is switched off by phosphorylation. 磷酸化使之失活
Modification of a single Ser residue by an AMP-activated protein kinase converts carboxylase into an inactive form phosphoryl group on inhibited carboxylase is removed by protein phosphatase 2A (PP2A) some catalytic hormones switch off FA synthesis by keeping carboxylase in inactive phosphorylated state Insulin, by contrast, stimulates the enzyme 胰岛素激活PP2A→ (maybe, it activates PP2A)胰岛素激活 胰岛素激活 使之去磷酸化→使之激活 使之激活→刺激脂肪酸合成 使之去磷酸化 使之激活 刺激脂肪酸合成

Acetyl CoA carboxylase is allosterically stimulated by citrate
In starvation, level of free FA rises because adipose-cell lipase is stimulated by hormones such as epinephrine and glucagon Insulin, in contrast, inhibits lipolysis

Long-term control is mediated by changes in rate of synthesis and degradation of enzymes participating in FA synthesis

Elongation and unsaturation of FAs are carried out by accessory enzyme systems辅助酶系 辅助酶系
In eukaryotes, longer (than palmitate) FAs are formed by elongation reactions catalyzed by enzymes on cytosolic face of ER membrane (or microsome微粒体 微粒体 systems)内质网膜胞质 外”)侧 内质网膜胞质(“外 侧 内质网膜胞质
which add two-carbon units sequentially to carboxyl end of both saturated and unsaturated FAs (Malonyl CoA is the two-carbon donor)在饱和或非饱 在饱和或非饱 和脂肪酸羧基端加入两碳单位(以丙二酰 以丙二酰CoA作为二碳供体 作为二碳供体) 和脂肪酸羧基端加入两碳单位 以丙二酰 作为二碳供体 and introduce double bonds into long-chain acyl CoAs (C≤9) 即ω<7(从甲 < 从甲 基数起)的双键不能靠自身合成 只能靠从食物获得), 也即真核生物(向十六 的双键不能靠自身合成(只能靠从食物获得 基数起 的双键不能靠自身合成 只能靠从食物获得 也即真核生物 向十六 个碳以上的长链脂酰CoA引入双键时 只能向第九号碳原子或小于第九号 引入双键时)只能向第九号碳原子或小于第九号 个碳以上的长链脂酰 引入双键时 羧基碳为一号)引入双键 碳原子 (羧基碳为一号 引入双键 羧基碳为一号 引入双键.
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH

ω1

(不能 不能) 不能

(能) 能

C1

Mammals lack enzyme to introduce double bonds at carbon atoms beyond C-9 in FA chain Hence, mammals cannot synthesize linoleate亚油酸 亚油酸 and linolenate亚麻酸 which are two essential fatty 亚麻酸, 亚麻酸 acids (so they must be supplied in the diet) palmitoleate棕榈油酸(16:1 cis△9) ω-7 棕榈油酸( 棕榈油酸 △ oleate油酸(18:1 cis△9)ω-9 油酸( 油酸 △ linoleate亚油酸(18:2 cis△9△12)ω-6 亚油酸( 亚油酸 △ linolenate亚麻酸(18:3 cis△9△12△15)ω-3(ω<7) 亚麻酸( 亚麻酸 △ < (不能靠自身合成 不能靠自身合成) 不能靠自身合成

Arachidonate (20:4,花生四烯酸 derived from 花生四烯酸) 花生四烯酸 linoleate亚油酸 is the major precursor of 亚油酸 several classes of signal molecules
prostaglandins (前列腺素 前列腺素) 前列腺素 prostacyclins (前列环素 前列环素) 前列环素 thromboxanes (凝血噁烷 凝血噁烷) 凝血噁烷 leukotrienes (白三烯 白三烯) 白三烯

Eicosanoid (类二十烷酸 类二十烷酸) 类二十烷酸 hormones are derived from polyunsaturated FAs

they are local hormones (局部激素 局部激素). 局部激素 脂肪酸→不饱和脂肪酸 不饱和脂肪酸→局部激素 脂肪酸 不饱和脂肪酸 局部激素

Aspirin inhibits the synthesis of prostaglandins前列 前列 腺素 by acetylating the cyclooxygenase (使环加氧 使环加氧 酶乙酰化)
Aspirin inhibits synthesis of prostaglandins by inactivating prostaglandin synthase Specifically, aspirin(i.e.acetylsalictlate乙酰水扬酸 irreversibly 乙酰水扬酸) 乙酰水扬酸 inhibits cyclooxygenase activity of this enzyme by acetylating a specific Ser -OH Aspirin is a potent anti-inflammatory agent because it blocks the first step in synthesis of prostaglandins前列腺素是一类具高度生物学活性
的脂质, 涉及炎症反应(作为炎症介质可使 血管舒张, 血管收缩, 产生催产素活性, 作为炎症介质可使: 的脂质 涉及炎症反应 作为炎症介质可使 血管舒张 血管收缩 产生催产素活性 支气 管扩张和收缩). 阿司匹林涉及此类物质的合成酶(使该酶失活 所以是抗炎症药物. 使该酶失活), 管扩张和收缩 阿司匹林涉及此类物质的合成酶 使该酶失活 所以是抗炎症药物

Aspirin is antithrombotic (抗凝剂 because it blocks formation of 抗凝剂) 抗凝剂 thromboxane A2凝血噁烷 凝血噁烷(TXA2), a potent aggregator (凝集体 凝集体) 凝血噁烷 凝集体 of blood platelets血小板 阿司匹林阻断凝血噁烷 血小板 阿司匹林阻断凝血噁烷A2—一种血小板凝集体的形 一种血小板凝集体的形
成,所以它用作抗凝剂 所以它用作抗凝剂


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