The Biosynthetic Pathway of VOCs in ‘Nanguo’ Pear: Comparison
Please note this is a comparison between Version 1 by zhuoran zhang and Version 6 by Catherine Yang.
The biosynthesis of aroma volatiles in fruit mostly occurs during the late stage of fruit development. Fatty acids, amino acids, and carbohydrates in fruit serve as precursors of the aroma volatiles that form under the catalysis of various enzymes. The aroma volatiles of fruits are secondary metabolites, and various precursors synthesize aroma volatiles through different pathways. Based on the types of precursors, the metabolic pathways of aroma biosynthesis are divided into the fatty acid metabolic pathway, amino acid metabolic pathway, terpenoid metabolic pathway, and carbohydrate metabolic pathway among others. Volatile compounds (VOCs) may have antibacterial or anticancer activity. 果实中香气挥发物的生物合成多发生在果实发育后期。水果中的脂肪酸、氨基酸和碳水化合物是在各种酶的催化下形成的香气挥发物的前体。水果的香气挥发物是次生代谢产物,各种前体通过不同的途径合成香气挥发物。根据前体的类型,香气生物合成的代谢途径分为脂肪酸代谢途径、氨基酸代谢途径、萜类代谢途径和碳水化合物代谢途径等。
  • Aroma compounds
  • Pyrus ussuriensis
  • biosynthetic pathways

1. Fatty Acid Pathway脂肪酸途径

The primary aroma components of the ‘Nanguo’ pear are esters, which are biosynthesized by fatty acid metabolism (Figure

'南果'梨的主要香气成分是酯类,通过脂肪酸代谢生物合成(1

) [1]. The β-oxidation of fatty acids is the primary biosynthetic process, which provides alcohols and acyl-CoA molecules to form esters [2]. Studies have found that the aroma volatiles in intact fruits are produced by β-oxidation. When plant tissue cells are destroyed, aroma volatiles are produced through the lipoxygenase (LOX) pathway [3]. However, some studies have shown that as the fruits mature, the membrane permeability increases, which increases the activity of the LOX pathway in the intact fruit. At this time, the LOX pathway can substitute for β-oxidation [4].

)[1738]。脂肪酸的β氧化是主要的生物合成过程,它提供醇和酰基辅酶A分子形成酯[1]。研究发现,完整水果中的香气挥发物是通过β氧化产生的。当植物组织细胞被破坏时,通过脂氧合酶(LOX)途径产生香气挥发物[3]。然而,一些研究表明,随着果实的成熟,膜通透性增加,从而增加了完整果实中LOX途径的活性。此时,LOX途径可以替代β氧化[346]。

Figure 1. Metabolic pathway of volatile esters in fruit. (

水果中挥发性酯的代谢途径。(

A) Monosaccharide pathway. (

)单糖途径。(

B) Fatty acid pathway. (

)脂肪酸途径。(

C) CXE pathway. (

) CXE途径。(

D) Amino acid pathway. EMP: Embden-Meyerhof-Parnas; PDC: pyruvate dehydrogenase complex; FAD: fatty acid desaturase; LOX: lipoxygenase;9-HPOD: (10E,12Z)-9-hydroperoxy-10,12-oetadeeadienoic acid; 9-HPOT: (10E,12Z,15Z)-9-hydroperoxy-10,12,15-octadecatrienoic acid; 13-HPOD: (9Z,11E)-13-hydroperoxy-9,11-octadecadienoic acid, 13-HPOT: (9Z,11E,15Z)-13-hydroperoxy-9,11,15-octadecatrienoic acid; HPL: hydroperoxide lyase; ADH: alcohol dehydrogenase; AAT: alcohol acyl-CoA transferase; CXE: carboxylesterases.

In β-oxidation, acyl-CoA is reduced to aldehyde by acyl-CoA reductase, and the aldehyde is then reduced to alcohol by alcohol dehydrogenase (ADH) for alcohol acyltransferase (AAT) to produce esters [5]. The substrates of the LOX pathway are linolenic acid and linoleic acid, which can be obtained from free fatty acids under the action of fatty acid desaturase enzymes [6].

)氨基酸途径。EMP:恩布登-迈耶霍夫-帕纳斯;PDC:丙酮酸脱氢酶复合物;FAD:脂肪酸去饱和酶;LOX:脂氧合酶;9-羟甲基氧合酶:(10E,12Z)-9-氢过氧基-10,12-二烯酸;9-氢氟乙酸: (10E,12Z,15Z)-9-氢过氧基-10,12,15-十八碳三烯酸;13-氢氟丁烷酸:(9Z,11E)-13-氢过氧基-9,11-十八碳二烯酸,13-氢过氧基-9,11,15-十八碳二烯酸;HPL:过氧化氢裂解酶;ADH:醇脱氢酶;AAT:醇酰辅酶A转移酶;CXE:羧基酯酶。

Linolenic acid and linoleic acid are derived through the LOX pathway into unsaturated short-chain alcohols, aldehydes, and esters [7]. Hydroperoxide lyase (HPL) is a downstream enzyme of LOX, which catalyzes the cleavage of hydroperoxide, the reaction product of LOX, into short-chain aldehydes [8]. Plant HPL is divided into two isozymes based on the difference of substrate peroxy group position. 13-HPL catalyzes the cleavage of the 13-position peroxy to produce C

在β氧化中,酰基辅酶A被酰基辅酶A还原为醛,然后醛被醇脱氢酶(ADH)还原为醇,使醇酰基转移酶(AAT)产生酯[47]。LOX途径的底物是亚麻酸和亚油酸,可以在脂肪酸去饱和酶的作用下从游离脂肪酸中获得[37]。
亚麻酸和亚油酸通过LOX途径衍生成不饱和短链醇、醛和酯[48]。氢过氧化物裂解酶(HPL)是LOX的下游酶,可催化氢过氧化物(LOX的反应产物)裂解成短链醛[49]。植物HPL根据底物过氧基团位置的差异分为两种同工酶。13-HPL催化13位过氧树脂的裂解产生C

6 compounds, while 9-HPL cleaves the 9-position peroxy to form C

化合物,而 9-HPL 将 9 位过氧树脂切割形成 C

9 compounds [9]. Next, alcohol dehydrogenase (ADH) catalyzes the interconversion of aldehydes and the corresponding alcohols. Finally, AAT catalyzes the reaction of acyl-CoA with alcohols to produce a variety of esters [5]. The alcohols involved in the reaction can be produced by the LOX pathway or reduced by short-chain acids produced by β-oxidation [2]. In addition, the LOX pathway can also produce jasmonic acid and its derivatives. In the allene oxide synthase (AOS) branch of the LOX pathway, 13-hydroxyperoxylinolenic acid is converted into 12,13-epoxyoctadecatrienoic acid through AOS, and jasmonic acid is then produced through a series of reactions. Jasmonic acid can be converted into the volatile ester methyl jasmonate by jasmonic acid carboxyl methyltransferase [10].

化合物[50]。接下来,醇脱氢酶(ADH)催化醛和相应醇的相互转化。最后,AAT催化酰辅酶A与醇的反应产生多种酯[47]。参与反应的醇可以通过LOX途径产生,也可以由β氧化产生的短链酸还原[1]。此外,LOX途径还可以产生茉莉酸及其衍生物。在LOX途径的氧化丙二烯合酶(AOS)分支中,13-羟基过氧亚麻酸通过AOS转化为12,13-环氧十八碳三烯酸,然后通过一系列反应生成茉莉酸。茉莉酸可以通过茉莉酸羧基甲基转移酶转化为挥发性酯茉莉酸甲酯[51]。

2. Amino Acid Pathway of Ester Biosynthesis酯类生物合成的氨基酸途径

The amino acid metabolic pathway is also an important way to biosynthesize fruit aroma volatiles [11]. Aliphatic alcohols, aldehydes, and esters that contain branched chains can be biosynthesized through the amino acid metabolic pathway (Figure

氨基酸代谢途径也是生物合成水果香气挥发物的重要途径[52]。含有支链的脂肪醇、醛和酯可以通过氨基酸代谢途径进行生物合成(1)

[11]. A previous study found that the amino acids leucine, isoleucine, and valine could be the precursor of volatile alcohols, aldehydes, and esters in fruits, such as banana (Musa spp.), apple, strawberry (Fragaria × ananassa), and tomato (Solanum lycopersicum),[12]. In strawberries, alanine can also serve as the precursor for volatile ethyl esters, which can be produced by AAT [12]. Amino acids are converted to the corresponding α-keto acids by aminotransferases, which are the key intermediates to convert amino acids into volatiles. α-Keto acids are then converted to volatile aldehydes or acyl-CoA in the substrate of α-keto acid decarboxylase or α-keto dehydrogenase. Subsequently, aldehyde and acyl-CoA are converted to esters by ADH and AAT [13].

)[52]。先前的一项研究发现,氨基酸亮氨酸、异亮氨酸和缬氨酸可能是水果中挥发醇、醛和酯的前体,如香蕉(Musa spp.)、苹果、草莓(Fragaria × ananassa)和番茄(Solanum lycopersicum)[25354]。在草莓中,丙氨酸也可以作为挥发性乙酯的前体,挥发性乙酯可以通过AAT生产[54]。氨基酸通过转氨酶转化为相应的α酮酸,转氨酶是将氨基酸转化为挥发物的关键中间体。然后将α-酮酸转化为挥发性醛或酰基辅酶A,在α-酮酸脱羧酶或α-酮脱氢酶的底物中。随后,醛和酰基辅酶A通过ADH和AAT转化为酯[55]。

3. Carbohydrate Pathway碳水化合物途径

Carbohydrates are not only the energy source of fruit metabolism but also an important source of fruit flavor, which can act as precursors for the biosynthesis of aroma volatiles (Figure

碳水化合物不仅是水果代谢的能量来源,也是水果风味的重要来源,可以作为香气挥发物生物合成的前体(1

) [14]. Carbohydrates can be decomposed into pyruvate by the Embden-Meyerhof-Parnas (EMP) pathway, and acetyl-CoA can be produced under the action of pyruvate dehydrogenase complex (PDC), which can be involved in the fatty acid pathway and contribute to the formation of esters [2]. Another pathway is that pyruvate forms acetaldehyde under the action of PDC, then acetaldehyde is reduced to ethanol under the catalysis of ADH, and then the ester is synthesized, but the pathway has not been confirmed.

)[17]。碳水化合物可以通过Embden-Meyerhof-Parnas(EMP)途径分解成丙酮酸,乙酰辅酶A可以在丙酮酸脱氢酶复合物(PDC)的作用下产生,PDC可以参与脂肪酸途径并有助于酯的形成[1]。另一种途径是丙酮酸在PDC的作用下形成乙醛,然后在ADH的催化下将乙醛还原成乙醇,然后合成酯,但途径尚未得到证实。

Terpenoid Pathway萜类途径

Terpenoids are biosynthesized from acetyl-CoA and pyruvate provided by carbohydrates in plastids and the cytoplasm. Although fatty acid oxidation is one of the primary pathways for the production of acetyl-CoA, this process may have little to do with the formation of terpenoids because fatty acid oxidation occurs in the peroxisome [2]. Terpenoids are the most abundant secondary metabolites, which are the primary aroma volatiles of citrus and grapes (Vitis vinifera) [15]. The terpenoids in the ‘Nanguo’ pear are primarily derived from α-farnesene [16]. α-Farnesene is a sesquiterpene-like volatile that can be biosynthesized via the mevalonate pathway [11]. The MVA pathway is carried out in the cytoplasm MEP pathway is in the plastid [11]. The biosynthetic precursors of terpenoids are isopentenyl pyrophosphate (IPP) and dimethylallyl diphosphate (DMAPP) [17]. Its biosynthesis has two pathways, which include the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway and the mevalonate pathway (MVA) (Figure

萜类化合物由质体和细胞质中的碳水化合物提供的乙酰辅酶A和丙酮酸生物合成。虽然脂肪酸氧化是产生乙酰辅酶A的主要途径之一,但这一过程可能与萜类化合物的形成关系不大,因为脂肪酸氧化发生在过氧化物酶体中[1]。萜类化合物是最丰富的次生代谢产物,是柑橘和葡萄(葡萄)的主要香气挥发物[5657]。“南果”梨中的萜类化合物主要来源于α-法呢烯[19]。α-法呢烯是一种倍半萜样挥发物,可通过甲羟戊酸途径生物合成[52]。MVA途径在细胞质中进行,MEP途径在质体中进行[52]。萜类化合物的生物合成前体是异戊烯基焦磷酸盐(IPP)和二甲基烯丙基二磷酸酯(DMAPP)[5258]。其生物合成有两种途径,包括2-C-甲基-D-赤藓糖醇-4-磷酸(MEP)途径和甲羟戊酸途径(MVA)(图2

) [11]. The products of the MEP pathway are monoterpenes and diterpenes, and the products of the MVA pathway are sesquiterpenes and triterpenes [17]. Acetyl-CoA is catalyzed by an enzyme to produce isopentenyl pyrophosphate (IPP). IPP is catalyzed by IPP isomerase to produce DMAPP, and it is then catalyzed by an enzyme to produce geranyl pyrophosphate (GPP) and farnesyl pyrophosphate (FPP) [11]. FPP is catalyzed by an enzyme to synthesize α-farnesene.

)[52]。MEP途径的产物是单萜和二萜,MVA途径的产物是倍半萜和三萜[58]。乙酰辅酶A由酶催化产生异戊烯基焦磷酸盐(IPP)。IPP由IPP异构酶催化产生DMAPP,然后由酶催化产生香叶基焦磷酸盐(GPP)和法呢基焦磷酸盐(FPP)[52]。FPP在酶的催化下合成α-法呢烯。

Figure 2. Synthesis of terpenoid volatile organic compounds. AACT, acetyl-CoA acetyltransferase; HMG-CoA, hydroxymethylglutaryl-CoA; HMGS, HMG-CoA synthase; HMGR, hydroxymethylglutaryl-CoA; MVA, mevalonic acid; MVK, mevalonate kinase; MVP, mevalonate 5-phosphate; PMK, phosphomevalonate kinase; MVPP, phosphomevalonate kinase; FPP, farnesyl pyrophosphate; FPPS, FPP synthase; G3P, glyceraldehyde 3-phosphate; DXS, DXP synthase; DXP, 1-deoxy-d-xylulose 5-phosphate; DXR, 1-deoxy-d-xylulose 5-phosphate reductoisomerase; MEP, methylerythritol phosphate; CDP-ME, 4-diphosphocytidyl-2-C-methyl-d-erythritol; CDP-MEP, CDP-ME 2-phosphate; MECDP, 2-C-methyl-d-erythritol 2,4-cyclodiphosphate; MECPS, MECPD synthase; HDS, 4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase; HMBPP, (E)-4-hydroxy-3-methylbut-2-en-1-yl diphosphate; IDS, isopentenyl diphosphate synthase; IPP, isopentenyl pyrophosphate; IDI, isopentenyl pyrophosphate isomerase; DMAPP, dimethylallyl pyrophosphate; GGPP, geranylgeranyl pyrophosphate; GGPPS: GGPP synthase; TPS: terpene synthase; GPP, geranyl pyrophosphate; GPPS, GPP synthase; MCT, 2-C-methyl-d-erythritol 4-phosphate cytidylyltransferase.

Moreover, the aroma of fruits is also regulated by carboxylesterases (CXE), which is an esterase that hydrolyzes esters [18]. Studies have found that the CXE in tomato and peach (

萜类挥发性有机化合物的合成。AACT,乙酰辅酶A乙酰转移酶;HMG-辅酶A,羟甲基戊二酰辅酶A;HMGS, HMG-CoA 合酶;HMGR,羟甲基戊二酰辅酶A;MVA,甲羟戊酸;MVK,甲羟戊酸激酶;MVP,甲羟戊酸5-磷酸;PMK,磷酸戊酸激酶;MVPP,磷酸戊酸激酶;FPP,法呢基焦磷酸盐;FPPS,FPP合酶;G3P,甘油醛3-磷酸;DXS,DXP合酶;DXP, 1-脱氧-d-木酮糖 5-磷酸;DXR, 1-脱氧-d-木酮糖 5-磷酸还原异构酶;MEP,甲基赤藓糖醇磷酸酯;CDP-ME, 4-二磷酸胞苷基-2-C-甲基-d-赤藓糖醇;CDP-MEP, CDP-ME 2-磷酸;MECDP, 2-C-甲基-D-赤藓糖醇 2,4-环二磷酸;MECPS,MECPD合酶;HDS,4-羟基-3-甲基丁-2-烯-1-基二磷酸合酶;HMBPP, (E)-4-羟基-3-甲基丁-2-烯-1-基二磷酸酯;IDS,异戊烯基二磷酸合酶;IPP,异戊烯基焦磷酸盐;IDI,异戊烯基焦磷酸异构酶;二甲基烯丙基焦磷酸酯;GGPP,香叶基香叶基焦磷酸盐;GGPPS:GGPP合酶;TPS:萜烯合酶;GPP,香叶基焦磷酸盐;GPPS,GPP合酶;MCT,2-C-甲基-d-赤藓糖醇 4-磷酸胞苷基转移酶。
Moreover, the aroma of fruits is also regulated by carboxylesterases (CXE), which is an esterase that hydrolyzes esters [59]. Studies have found that the CXE in tomato and peach (

Prunus persica) can use acetate as a substrate [19][20]. In pears, the content of CXE decreased with the extension of storage time, which could increase the accumulation of volatile esters in pears by reducing the degradation of esters [21][22][23][24].

) can use acetate as a substrate [60,61]. In pears, the content of CXE decreased with the extension of storage time, which could increase the accumulation of volatile esters in pears by reducing the degradation of esters [8].