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Amino Acids: 2 Czech J. Food Sci. Vol. 24, No. 2: 45–58 Biosynthesis of Food Constituents: Amino Acids: 2. The Alanine-Valine-Leucine, Serine-Cysteine-Glycine, and Aromatic and Heterocyclic Amino Acids Groups – a Review JAN VELÍŠEK and KAREL CEJPEK Department of Food Chemistry and Analysis, Faculty of Food and Biochemical Technology, Institute of Chemical Technology Prague, Prague, Czech Republic Abstract VELÍŠEK J., CEJPEK K. (2006): Biosynthesis of food constituents: Amino acids: 2. The alanine-valine- leucine, serine-cysteine-glycine, and aromatic and heterocyclic amino acids groups – a review. Czech J. Food Sci., 24: 45–58. This review article gives a survey of principal pathways that lead to the biosynthesis of the proteinogenic amino acids of the alanine-valine-leucine group starting with pyruvic acid from the glycolytic pathway and serine-cysteine-glycine group starting with 3-phospho-D-glyceric acid from the glycolytic pathway. A survey is further given to the aromatic and heterocyclic amino acids (phenylalanine, tyrosine, tryptophan, histidine) starting with 3-phosphoenolpyruvic acid from the glycolytic pathway and D-erythrose 4-phosphate, an intermediate in the pentose phosphate cycle and Calvin cycle. Keywords: biosynthesis; amino acids; alanine; valine; leucine; serine; glycine; cysteine; phenylalanine; tyrosine; tryp- tophan; histidine 1 THE ALANINE, VALINE, AND LEUCINE GROUP Alanine is formed from pyruvic acid in a simple transamination reaction (Figure 2) catalysed by 1.1 Alanine alanine transaminase (EC 2.6.1.2) (SENGBUSCH). L-Alanine and the essential amino acids L-valine The transamination reactions are catalysed by and L-leucine have a common precursor, i.e. pyruvic transaminases dependent on the coenzyme pyri- acid from the glycolytic pathway (Figure 1). doxal 5'-phosphate (PLP). Glycolysis pyruvic acid L-alanine L-glutamic acid 2-oxoglutaric acid CH3 COOH H3C COOH L-leucine O EC 2.6.1.2 NH2 pyruvic acid L-alanine L-valine Figure 1 Figure 2 Partly supported by the Ministry of Education, Youth and Sports of the Czech Republic, Project MSM 6046137305. 45 Vol. 24, No. 2: 45–58 Czech J. Food Sci. 1.2 Valine synthase (EC 2.3.3.13, requires K+ ions), is the condensation of 3-methyl-2-oxobutanoic acid Valine is synthesised in a set of four reactions, with acetyl-CoA to (S)-2-isopropylmalic acid. The and an extension of the valine pathway results in next two steps are catalysed by 3-isopropylmalate leucine synthesis (Figure 3) (IUBMB 2003). The first dehydratase (EC 4.2.1.33). 2-Isopropylmaleic acid three enzymes involved in the biosynthesis of valine arises from 2-isopropylmalic acid by elimination and leucine, i.e. acetolactate synthase (EC 2.2.1.6), of water and addition of water to 2-isopropyl- ketol-acid reductoisomerase (EC 1.1.1.86), and di- maleic acid yields (2R,3S)-3-isopropylmalic acid. hydroxyacid-dehydratase (EC 4.2.1.9), also catalyse The enzyme also hydrates the product back to analogous reactions leading to the biosynthesis (S)-2-isopropylmalic acid, thus bringing about of isoleucine. 3-Methyl-2-oxobutanoic acid is the the interconversion between the two isomers. common precursor of both amino acids, valine and In the next step, 3-isopropylmalic acid is oxi- leucine. This carboxylic acid then yields valine dised to (S)-2-isopropyl-3-oxosuccinic acid by the by the action of the branched-chain-amino-acid NAD+-dependent 3-isopropylmalate dehydroge- transaminase (EC 2.6.1.42). nase (EC 1.1.1.85). The product decarboxylates spontaneously to 4-methyl-2-oxopentanoic acid 1.3 Leucine (4-oxoisocapronic acid), which is, in the last step, transformed to leucine by either branched-chain- Leucine is synthesised from 3-methyl-2-oxobu- amino-acid transaminase (EC 2.6.1.42) or more tanoic acid in six steps (Figure 3) (IUBMB 2003). specific leucine transaminase (EC 2.6.1.6) that The first step, catalysed by 2-isopropylmalate does not act on valine or isoleucine. CO2 NADPH + H NADP CH CH3 3 COOH H3C COOH H3C COOH H3C 2 OH OH O EC 2.2.1.6 O EC 1.1.1.86 OH pyruvic acid (S)-2-hydroxy-2-methyl-3-oxobutanoic acid (R)-2,3-dihydroxy-3-methylbutanoic acid EC 4.2.1.9 H2O H3C S CoA H2O L-glutamic acid 2-oxoglutaric acid COOH HS CoA + H O COOH O 2 CH3 CH3 H3C COOH H3C COOH COOH COOH H3C H C OH 3 CH3 EC 4.2.1.33 CH O 3 EC 2.3.3.13 EC 2.6.1.42 NH2 2-isopropylmaleic acid (S)-2-isopropylmalic acid 3-methyl-2-oxobutanoic acid L-valine H2O EC 4.2.1.33 NAD NADH + H CO2 L-glutamic acid 2-oxoglutaric acid COOH COOH H C COOH H3C COOH H3C COOH H3C COOH 3 EC 1.1.1.85 EC 2.6.1.42 CH NH CH3 OH CH3 O CH3 O 3 2 EC 2.6.1.6 (2R,3S)-3-isopropylmalic acid (S)-2-isopropyl-3-oxosuccinic acid 4-methyl-2-oxopentanoic acid L-leucine Figure 3 46 Czech J. Food Sci. Vol. 24, No. 2: 45–58 2 THE SERINE, GLYCINE, 2.2 Glycine AND CYSTEINE GROUP The major pathway of glycine biosynthesis is the 2.1 Serine direct transformation of L-serine, which is catalysed by PLP protein, i.e. serine hydroxymethyl trans- L-Serine is generated in a three-step reaction ferase (EC 2.1.2.1). The hydroxymethyl group of the from 3-phospho-D-glyceric acid, formed in the serine side chain is split off and this C1 unit is ac- glycolytic pathway, and becomes the precursor of cepted by tetrahydrofolic acid (H4PteGlu, R = ben- glycine and L-cysteine (Figure 4). zoyl glutamic acid residue) under the formation of 5,10-methylene-tetrahydrofolic acid (5,10-meth- Glycolysis 3-phospho-D-glyceric acid ylene-H4PteGlu) (Figure 6) (SENGBUSCH). 2.3 Cysteine Cysteine directly or indirectly provides sulfide for structural, regulatory, or catalytic purposes in glycine L-serine L-cysteine peptides, proteins, and numerous low molecular weight compounds such as methionine, gluta- Figure 4 thione, biotin, and thiamine. Biosynthesis of the cysteine -SH group needs sulfur and thus provides The first step of serine biosynthesis is the oxida- an exclusive entry of reduced sulfur into cellular tion of 3-phospho-D-glyceric acid at C-2 to 3-phos- metabolism. Plants take up sulfur as inorganic phopyruvic acid catalysed by 3-phosphoglycerate sulfate ions. After its uptake by roots, sulfate is dis- dehydrogenase (EC 1.1.1.95). This phosphorylated tributed into different organs and, to be assimilated, oxo-analogue of serine is transaminated by PLP-de- it has to be reduced in a process called assimila- pendent phosphoserine transaminase (EC 2.6.1.52) tory sulfate reduction performed in chloroplasts. in the second step yielding 3-phospho-L-serine. The first step in the sulfate assimilation pathway Finally, in the third step, phosphoserine is hydro- is catalysed by ATP sulfurylase (EC 2.7.7.4). This lysed by phosphoserine phosfatase (EC 3.1.3.3) to enzyme activates sulfate via an ATP-dependent serine (Figure 5) (SENGBUSCH). reaction that leads to the formation of adenylyl- NAD NADH + H L-glutamic acid 2-oxoglutaric acid H2O P COOH COOH COOH COOH PO PO PO HO O NH OH EC 1.1.1.95 EC 2.6.1.52 NH2 EC 3.1.3.3 2 3-phospho-D-glyceric acid 3-phosphopyruvic acid 3-phospho-L-serine L-serine Figure 5 10 R R OH H N OH N H N N COOH N 5 COOH N HO + + + H2O NH NH H N N N 2 H2N N N EC 2.1.2.1 2 2 H H L-serine H4PteGlu glycine 5,10-methylene-H4PteGlu Figure 6 47 Vol. 24, No. 2: 45–58 Czech J. Food Sci. NH2 NH2 N N N N OH OH 2 GSH G-S-S-G OH O S OPO CH N N CH 2 O OH HO PO 2 O N N O O OS + O OH EC 1.8.4.9 OH OH OH OH adenosine-5´-phosphosulfate sulfurous acid adenosine-5´-phosphate ATP EC 2.7.1.25 SH HS S S O O H H N N ADP NH R R R 2 R NH2 O O N N N N OH OH thioredoxin (reduced) thioredoxin (oxidized) OH O S OPO CH2 N N HO O OH PO CH2 O N N O O OS + O OH EC 1.8.4.8 O OH O OH O P OH O P OH OH OH adenosine-5´-phosphosulfate-3´-phosphate sulfurous acid adenosine-3´,5´-diphosphate Figure 7 sulfate (adenosine-5'-phosphosulfuric acid, APS) and plants. Sulfite is reduced to sulphide at the and inorganic diphosforic acid (PP). expense of oxidising three molecules of NADPH, To accomplish the incorporation of sulphur into by sulphite reductase (EC 1.8.7.1)1. biomolecules, specifically amino acids, sulphate Depending on the organism, there are two dif- in APS is transformed into sulphite and this into ferent ways by which sulfide is incorporated into a sulphide. This process may occur through two carbon backbone to produce L-cysteine (Figure 8). different pathways, depending on the organism. First, the physiological O-ester substrate O-ace- One of them involves phosforylation of APS by tylserine is synthesised from L-serine by serine adenosine 5'-phosphosulfate kinase (EC 2.7.1.25) acetyltransferase (EC 2.3.1.30). Sulfide is condensed using ATP to produce 3'-phosfoadenosine-5'-phos- with O-acetylserine by the PLP-dependent enzyme phosulfate (PAPS) and ADP. In the following re- O-acetylserine (thiol)-lyase (also called O-acetyl- action, PAPS reductase (EC 1.8.4.8) firstly reacts serine sulfhydrylase, EC 2.5.1.47) to form cysteine with reduced thioredoxin and then with PAPS to directly.
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