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Transketolase is an enzyme of both the pentose phosphate pathway in all organisms and the Calvin cycle of photosynthesis. It catalyzes two important reactions, which operate in opposite directions in these two pathways. In the first reaction of the non-oxidative pentose phosphate pathway, the cofactor thiamine diphosphate accepts a 2-carbon fragment from a 5-carbon ketose (D-xylulose-5-P), then transfers this fragment to a 5-carbon aldose (D-ribose-5-P) to form a 7-carbon ketose (sedoheptulose-7-P). The abstraction of two carbons from D-xylulose-5-P yields the 3-carbon aldose glyceraldehyde-3-P. In the Calvin cycle, transketolase catalyzes the reverse reaction, the conversion of sedoheptulose-7-P and glyceraldehyde-3-P to pentoses, the aldose D-ribose-5-P and the ketose D-xylulose-5-P.

The second reaction catalyzed by transketolase in the pentose phosphate pathway involves the same thiamine diphosphate-mediated transfer of a 2-carbon fragment from D-xylulose-5-P to the aldose erythrose-4-phosphate, affording fructose 6-phosphate and glyceraldehyde-3-P. Again, in the Calvin cycle exactly the same reaction occurs, but in the opposite direction. Moreover, in the Calvin cycle this is the first reaction catalyzed by transketolase, rather than the second.

In mammals, transketolase connects the pentose phosphate pathway to glycolysis, feeding excess sugar phosphates into the main carbohydrate metabolic pathways. Its presence is necessary for the production of NADPH, especially in tissues actively engaged in biosyntheses, such as fatty acid synthesis by the liver and mammary glands, and for steroid synthesis by the liver and adrenal glands. Thiamine diphosphate is an essential cofactor, along with calcium.

Transketolase is abundantly expressed in the mammalian cornea by the stromal keratocytes and epithelial cells and is reputed to be one of the corneal crystallins.^[1]

Species distribution

Transketolase is widely expressed in a wide range of organisms including bacteria, plants, and mammals. The following human genes encode proteins with transketolase activity:

TKT (transketolase)
TKTL1 (transketolase-like protein 1)
TKTL2 (transketolase-like protein 2)

Structure

The entrance to the active site for this enzyme is made up mainly of several arginine, histidine, serine, and aspartate side-chains, with a glutamate side-chain playing a secondary role. These side-chains, to be specific Arg359, Arg528, His469, and Ser386, are conserved within each transketolase enzyme and interact with the phosphate group of the donor and acceptor substrates. Because the substrate channel is so narrow, the donor and acceptor substrates cannot be bound simultaneously. Also, the substrates conform into a slightly extended form upon binding in the active site to accommodate this narrow channel.

Although this enzyme is able to bind numerous types of substrates, such as phosphorylated and nonphosphorylated monosaccharides including the keto and aldosugars fructose, ribose, etc., it has a high specificity for the stereoconfiguration of the hydroxyl groups of the sugars. These hydroxyl groups at C-3 and C-4 of the ketose donor must be in the D-threo configuration in order to correctly correspond to the C-1 and C-2 positions on the aldose acceptor.^[2] Also they stabilize the substrate in the active site by interacting with the Asp477, His30, and His263 residues. Disruption of this configuration, both the placement of hydroxyl groups or their stereochemistry, would consequently alter the H-bonding between the residues and substrates thus causing a lower affinity for the substrates.

In the first half of this pathway, His263 is used to effectively abstract the C3 hydroxyl proton, which thus allows a 2-carbon segment to be cleaved from fructose 6-phosphate.^[3] The cofactor necessary for this step to occur is thiamin pyrophosphate (TPP). The binding of TPP to the enzyme incurs no major conformational change to the enzyme; instead, the enzyme has two flexible loops at the active site that make TPP accessible and binding possible.^[2] Thus, this allows the active site to have a "closed" conformation rather than a large conformational change. Later in the pathway, His263 is used as a proton donor for the substrate acceptor-TPP complex, which can then generate erythrose-4-phosphate.

The histidine and aspartate side-chains are used to effectively stabilize the substrate within the active site and also participate in deprotonation of the substrate. To be specific, the His 263 and His30 side-chains form hydrogen bonds to the aldehyde end of the substrate, which is deepest into the substrate channel, and Asp477 forms hydrogen bonds with the alpha hydroxyl group on the substrate, where it works to effectively bind the substrate and check for proper stereochemistry. It is also thought that Asp477 could have important catalytic effects because of its orientation in the middle of the active site and its interactions with the alpha hydroxyl group of the substrate. Glu418, which is located in the deepest region of the active site, plays a critical role in stabilizing the TPP cofactor. To be specific, it is involved in the cofactor-assisted proton abstraction from the substrate molecule.^[2]

The phosphate group of the substrate also plays an important role in stabilizing the substrate upon its entrance into the active site. The tight ionic and polar interactions between this phosphate group and the residues Arg359, Arg528, His469, and Ser386 collectively work to stabilize the substrate by forming H-bonds to the oxygen atoms of the phosphate.^[2] The ionic nature is found in the salt bridge formed from Arg359 to the phosphate group.

Mechanism

The catalysis of this mechanism is initiated by the deprotonation of TPP at the thiazolium ring. This carbanion then binds to the carbonyl of the donor substrate thus cleaving the bond between C-2 and C-3. This keto fragment remains covalently bound to the C-2 carbon of TPP. The donor substrate is then released, and the acceptor substrate enters the active site where the fragment, which is bound to the intermediate α-β-dihydroxyethyl thiamin diphosphate, is then transferred to the acceptor.^[2]

Experiments have also been conducted that test the effect replacing alanine for the amino acids at the entrance to the active site, Arg359, Arg528, and His469, which interact with the phosphate group of the substrate. This replacement creates a mutant enzyme with impaired catalytic activity.^[2]

Role in disease

Transketolase activity is decreased in deficiency of thiamine, which in general is due to malnutrition. Two diseases are associated with thiamine deficiency: beriberi and Wernicke-Korsakoff syndrome. While no mutations could be demonstrated,^[4] there is an indication that thiamine deficiency leads to Wernicke-Korsakoff syndrome only in those whose transketolase has a reduced affinity for thiamine.^[5] In this way, the activity of transketolase is greatly hindered, and, as a consequence, the entire pentose phosphate pathway is inhibited.^[6]

Diagnostic use

Red blood cell transketolase activity is reduced in deficiency of thiamine (vitamin B₁), and may be used in the diagnosis of Wernicke's encephalopathy and other B₁-deficiency syndromes if the diagnosis is in doubt.^[7] Apart from the baseline enzyme activity (which may be normal even in deficiency states), acceleration of enzyme activity after the addition of thiamine pyrophosphate may be diagnostic of thiamine deficiency (0-15% normal, 15-25% deficiency, >25% severe deficiency).^[8]

References

^ Sax CM, Kays WT, Salamon C, Chervenak MM, Xu YS, Piatigorsky J (November 2000). "Transketolase gene expression in the cornea is influenced by environmental factors and developmentally controlled events". Cornea 19 (6): 833–41. doi:10.1097/00003226-200011000-00014. PMID 11095059.
^ ^a ^b ^c ^d ^e ^f Nilsson U, Meshalkina L, Lindqvist Y, Schneider G (January 1997). "Examination of substrate binding in thiamin diphosphate-dependent transketolase by protein crystallography and site-directed mutagenesis". J. Biol. Chem. 272 (3): 1864–9. doi:10.1074/jbc.272.3.1864. PMID 8999873.
^ Wikner C, Nilsson U, Meshalkina L, Udekwu C, Lindqvist Y, Schneider G (December 1997). "Identification of catalytically important residues in yeast transketolase". Biochemistry 36 (50): 15643–9. doi:10.1021/bi971606b. PMID 9398292.
^ McCool BA, Plonk SG, Martin PR, Singleton CK (January 1993). "Cloning of human transketolase cDNAs and comparison of the nucleotide sequence of the coding region in Wernicke-Korsakoff and non-Wernicke-Korsakoff individuals". J. Biol. Chem. 268 (2): 1397–404. PMID 8419340.
^ Blass JP, Gibson GE (1977). "Abnormality of a thiamine-requiring enzyme in patients with Wernicke-Korsakoff syndrome". N. Engl. J. Med. 297 (25): 1367–70. doi:10.1056/NEJM197712222972503. PMID 927453.
^ Cox, Michael; Nelson, David R.; Lehninger, Albert L (2005). Lehninger principles of biochemistry. San Francisco: W.H. Freeman. ISBN 0-7167-4339-6.
^ Smeets EH, Muller H, de Wael J (July 1971). "A NADH-dependent transketolase assay in erythrocyte hemolysates". Clin. Chim. Acta 33 (2): 379–86. doi:10.1016/0009-8981(71)90496-7. PMID 4330339.
^ Doolman R, Dinbar A, Sela BA (July 1995). "Improved measurement of transketolase activity in the assessment of "TPP effect"". Eur J Clin Chem Clin Biochem 33 (7): 445–6. PMID 7548453.

UpToDate Contents

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1. ウェルニッケ脳症 wernicke encephalopathy
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4. 水溶性ビタミンの概要 overview of water soluble vitamins
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English Journal

Aspirin acetylates multiple cellular proteins in HCT-116 colon cancer cells: Identification of novel targets.

Marimuthu S, Chivukula RS, Alfonso LF, Moridani M, Hagen FK, Bhat GJ.SourceDepartment of Biomedical Sciences, Texas Tech University Health Sciences Center, Amarillo, TX 79106, USA.
International journal of oncology.Int J Oncol.2011 Nov;39(5):1273-83. doi: 10.3892/ijo.2011.1113. Epub 2011 Jul 4.
Epidemiological and clinical observations provide consistent evidence that regular intake of aspirin may effectively inhibit the occurrence of epithelial tumors; however, the molecular mechanisms are not completely understood. In the present study, we determined the ability of aspirin to acetylate a
PMID 21743961

Two genes, rif15 and rif16, of the rifamycin biosynthetic gene cluster in Amycolatopsis mediterranei likely encode a transketolase and a P450 monooxygenase, respectively, both essential for the conversion of rifamycin SV into B.

Yuan H, Zhao W, Zhong Y, Wang J, Qin Z, Ding X, Zhao GP.SourceKey Laboratory of Synthetic Biology, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200032, China.
Acta biochimica et biophysica Sinica.Acta Biochim Biophys Sin (Shanghai).2011 Oct 10. [Epub ahead of print]
Amycolatopsis mediterranei produces an important antibiotic rifamycin, the biosynthesis of which involves many unusual modifications. Previous work suggested a putative P450 enzyme encoded by rif16 within the rifamycin biosynthetic gene cluster (rif) was required for the conversion of the intermedia
PMID 21986914

Japanese Journal

2P-1123 エリスリトール生産菌Trichosporonoides megachiliensis SN-124AのTransketolaseは浸透圧ストレス応答のキーエンザイム(3a発酵生理学,発酵工学,一般講演,代謝生理学・発酵生産,伝統の技と先端科学技術の融合)

田中洋輔,小山善幸,大倉哲也,荻原淳,加藤順,春見隆文
日本生物工学会大会講演要旨集平成22年度, 39, 2010-09-25
NAID 110008084490

Molecular Characterization of Two Genes with High Similarity to the Dihydroxyacetone Synthase Gene in the Methylotrophic Yeast Pichia methanolica

NAKAGAWA Tomoyuki,FUJIMURA Shuki,ITO Takashi,MATSUFUJI Yoshimi,OZAWA Shotaro,MIYAJI Tatsuro,NAKAGAWA Junichi,TOMIZUKA Noboru,YURIMOTO Hiroya,SAKAI Yasuyoshi,HAYAKAWA Takashi
Bioscience, biotechnology, and biochemistry 74(7), 1491-1493, 2010-07-23
… Upon phylogenetic analysis, PmDas1p fit into the DAS group with other DASs, while PmDlp1p was grouped with the DAS-like proteins (DLP) of non-methylotrophic yeasts and fungi, a branch of the phylogenetic tree independent of the DAS and transketolase (TK) groups. …
NAID 10027557611

Related Pictures

★リンクテーブル★

リンク元	「解糖系」「ビタミンB1」「代謝系」「トランスケトラーゼ」

「解糖系」

transketolase
Identifiers
Symbol	TKT
Entrez	7086
HUGO	11834
OMIM	606781
RefSeq	NM_001064
UniProt	P29401
Other data
EC number	2.2.1.1
Locus	Chr. 3 p14.3

Search
PMC	articles
PubMed	articles
NCBI	proteins

transketolase
Identifiers
EC number	2.2.1.1
CAS number	9014-48-6
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / EGO
Search
PMC	articles
PubMed	articles
NCBI	proteins

　　[★]

英: glycolytic pathway
同: エムデン-マイヤーホフ経路 Embden-Meyerhof pathway
関: 解糖、TCA回路

CH(OH)-CH(OH)-C(OH)H-CH(OH)-C(CH2OH)H-O- グルコース (OHが↓↓↑↓↑)
CH(OH)-CH(OH)-C(OH)H-CH(OH)-C(CH2O-PO3)H-O- グルコース-6-リン酸 (OHが↓↓↑↓↑)
C(CH2OH)OH-C(OH)H-CH(OH)-C(CH2O-PO3)H-O- フルクトース-6-リン酸
C(CH2O-PO3)OH-C(OH)H-CH(OH)-C(CH2O-PO3)H-O- フルクトース-1,6-リン酸

glucose
↓-hexokinase/glucokinase(liver)
glucose 6-phosphate
↓-phosphohexose isomerase
fructose 6-phosphate
↓-phosphofructokinase
fructose 1,6-bisphosphate
↓-aldolase
glyceraldehyde 3-phosphate
↓-glyceraldehyde-3-phosphate dehydrogenase
1,3-bisphosphoglycerate
↓-phosphoglycerate kinase →ATP
3-phosphoglycerate
↓-phosphoglyceate mutase
2-phosphoglycerate
↓-enolase
phosphoenolpyruvate
↓-pyruvate kinase → ATP
pyruvate
　－(pyruvate dehydrogenase)→acetyl-CoA
　－(pyruvate carboxylase)→oxaloacetate－(NADH+H+)→malate

解糖系の酵素 (first aid step1 2006 p.87, FASTEP1_87_酵素一覧.xls)

1	galactokinase	キナーゼ
2	galactose-1-phosphate uridyltransferase	転移酵素
3	hexokinase/glucokinase	キナーゼ
4	glucose-6-phosphatase	ホスファターゼ
5	glucose-6-phosphate dehydrogenase	脱水素酵素
6	transketolase
7	phosphofructokinase	キナーゼ
8	fructose-1,6-bisphosphatase	ホスファターゼ
9	fructokinase	キナーゼ	フルクトキナーゼ
10	aldolase B		アルドラーゼ
11	pyruvate kinase	キナーゼ	ピルビン酸キナーゼ
12	pyruvate dehydrogenase	脱水素酵素	ピルビン酸デヒドロゲナーゼ
13	HMG-CoA reductase	還元酵素	HMG-CoA還元酵素
14	pyruvate carboxylase	カルボキシラーゼ	ピルビン酸カルボキシラーゼ
15	PEP carboxykinase	キナーゼ	PEPカルボキシキナーゼ
16	citrate synthase	合成酵素	クエン酸合成酵素
17	α-ketoglutarate dehydrogenase	脱水素酵素	α-ケトグルタミン酸脱水素酵素
18	ornithine transcarbamylase	転移酵素	オルニチンカルバモイルトランスフェラーゼ